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Effect of gut transit and mound deposit on soil organic matter transformations in the soil feeding termite: A review§
Eur. J. Soil Biol. 36 (2000) 117−125 © 2000 Éditions scientifiques et médicales Elsevier SAS. All rights reserved S116455630001058X/REV
Effect of gut transit and mound deposit on soil organic matter transformations in the soil feeding termite: A review§ Alain Brauman* IRD (ex-Orstom), Laboratory of soil microbiology, Centre ISRA-IRD de Bel-Air, BP 1386, Dakar, Sénégal
Received 10 November 1999; accepted 2 October 2000
Abstract − Even if termites are often considered as a pest due to the damage they cause to agriculture and architecture, they contribute to the soil humification process in the tropics. This impact on the soil organic matter humification process is due to the most important feeding habit in terms of species diversity, the soil feeding termites (∼1 200 species). Unlike other termites, their diet is not based on lignocellulosic plant degradation, but on the consumption of the mineral-containing horizons for the acquisition of nutrients. They are mostly distributed in humid forest or savannah equatorial zone. High structure and compartment with steep radial and axial gradients of O2, H2 and pH characterize their gut and create a patchy biotope. Furthermore, the humic compounds ingested are submitted, during a sequential transit, to different chemical (alkaline hydrolysis) and microbial degradation processes (fermentation, anaerobic respiration and mineralization). During this gut transit, the soil organic matter is strongly modified in terms of nature (organic matter concentration, fulvic and humic acid ratio) and organization (formation of organo-mineral complexes with clay). The soil organic matter ingested is further included as faeces in the nest and the galleries which, as a whole, constitutes the termitosphere. Compared to the control soil, the soil organic matter in the termitosphere is more stable and protected from the intense mineralization, which occurs in the tropics. These shifts of the organic matter into long turnover pool generated by the termite gut transit and deposition in the termitosphere indicate that besides the earthworm, the soil feeding termite has a positive impact on the overall organic matter cycling in the tropics. © 2000 Éditions scientifiques et médicales Elsevier SAS termites / gut microbiota / soil organic matter / humification
1. INTRODUCTION “The soil in many areas of Africa was altered, soaked with saliva, and worked by the termites, the pedological consequences are very significant”. This statement of P.P. Grassé [38], a famous French zoologist, illustrates the ecological importance of termites in the various processes of soil formation in the tropics. The termites have indeed a considerable impact on the soil morphology; ascent of deep horizons, formation of subsurface horizons [56] on soil structures (soil aeration, porosity, aggregation [33, 37]) and texture (selection of fine particles [2]). They are nevertheless especially known for their capacities of degradation of lignocellulosic organic matter [20] due to the presence of a specific microbiota, which could be exogenous § Paper presented at the 16th World Congress of Soil Science, 20–26 August 1998, Montpellier, France. * Correspondence and reprints: fax +221 832 16 75. E-mail address: [email protected] (A. Brauman).
(symbiotic nest fungus; [49]) or endogenous, (gut microbiota; [19]). This microbial symbiosis helps them degrade vegetable matter in its various states (wood and plants) for xylophagous termites and to strongly polymerize soil for soil feeding termites (SFT). Their influence on organic matter (o.m.) turnover in tropical areas are thus determining: in tropical forest, they could consume half of the vegetal litter [25] and in some savannahs, they can consume up to 49 % of the grass [41]. However termite impact on organic matter turnover is not restricted to plant organic matter, half of the 2 200 termite species now referenced [30] thrive on the humic compounds of the soil and contribute to the soil humification process. The recent estimates of their biomass in different parts of Asian and African forests (up to 10 000 individuals, [29, 30]) evidence the major impact they have on soil biological and physical processes. Despite their ecological importance, they are not much studied mostly due to their presence in remote areas, their difficulty in surviving in laboratory conditions and their lack of
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economical interest. Nevertheless, the density reached by these termites implies that most of the higher horizons of the soil in some tropical areas are deeply altered by the passage through their digestive tracts. The introduced organic matter is given back to the ecosystem via faeces deposit included in the nest and the galleries forming the termitosphere [36]. After a quick overview of the biology and gut physiology of the SFT, I will review the data acquired in the soil organic matter transformation (SOM) through gut transit and its further evolution within the termitosphere. The role of the SFT in the SOM cycling in the tropics will be discussed.
2. BIOLOGY OF SOIL FEEDING TERMITE
according to Garnier-Sillam and Harry [35], SFT select within the soil horizon the rich and poorly polymerized SOM as shown by carbon enrichment of termite faeces and the presence of fresh SOM in the anterior part of the gut. Moreover, in laboratory conditions, the SFT species tested are able to differentiate a soil rich in o.m. [57]. The foraging activities are mainly located in the surface horizon (0–15 cm) of the soil where the plant material is incorporated. The species seems to be distributed in soil microsites along a gradient of humification, which varies from very degraded wood to very humified SOM [27, 29]. Yapi [57] suggests that this SFT distribution is realized according to the size of the species; small-sized species live in the higher layer rich in o.m., whereas larger-sized species are located in the deeper layers of the soil, low in o.m.
2.1. Generality The soil feeding termite could be considered as the only social insect living and feeding in soil. As other termites, their society is characterized by the presence of social castes: sexual and neutral castes such as workers and soldiers [38]. They belong to the higher termite family, which encompass 75 % of the termite species (Termitidae) and live in the tropical area. The evolution from lower to higher termites is characterized by the loss of the ability to host gut symbiotic cellulolytic flagellates and the diversification of diet (litter, soil, plant feeders [20]). The natural habitat of SFT is the tropical humid forest where they reach their maximum biomass [29, 30]. Their diversity and number in soil seem to correlatively decline with rainfall level [16]. Their repartition is also conditioned by the o.m. quality and/or anthropization effect as they disappear when a tropical forest, where they represent 80 % of the 114 species recorded, is brought into cultivation [27, 29]. Thus, if their relationship to the quality of the organic matter still needs to be elucidated, SFT seem to represent a good indicator of soil disturbance. They are mainly found in the subsurface horizon (0 to 10 cm) and are easily recognizable by the dark colour of their abdomen. Most of the SFT, especially the subfamily of Apicotermitinae, form diffuse and small colonies in soils and their physiology and soil impact is globally unknown. Thanks to the available data, this review will only consider soil impact of genuine soil feeders mostly belonging to the Cubitermes clade [26], which form large colonies, build epigeal mounds and thrive on soil without recognizable vegetable material compounds.
2.2. Characteristic of the initial food The quantity of soil ingested by SFT is very variable between species (between 2.76 and 9.1 g·d–1 [4, 16]). The question of food selection by SFT remains in fact controversial. The heterogeneousness of the gut contents was evidenced by a microscopic analysis of different termite species [53], but it does not show a preference for plant cells over mineral soil. In contrast,
3. ORGANIC MATTER TRANSFORMATION DURING GUT TRANSIT 3.1. Anatomical and physiological characteristics of the digestive tract The anatomy and physiology of the digestive gut are far more complex in the soil feeding termites than in the other termite feeding guilds. The anatomy of the SFT has already been well described (for review see [4, 45]) and our description will be limited to some general features, important in terms of soil organic transformations. Their gut is highly compartmentalized in five sections as shown in figure 1 and also characterized by an increase of the length and volume of the paunch allowing a sequential transit of long duration (36 to 48 h [4]). The use of micro-sensors in different gut sections carried out by Brune and co-workers [22, 28, 52] has clearly demonstrated that during this transit, the organic matter is submitted to different physical and chemical environments mostly due to pH, oxygen and hydrogen pressure variations (figure 1). The high alkaline level of the paunch, first demonstrated by Bignell and Eggleton [5] is the most important, in terms of pH level, ever recorded in an animal gut [22]. This alkaline treatment of the organic matter ingested begins (figure 1) in the anterior part of the hindgut, slightly decreases in the main paunch and only returns to more neutral conditions in the last part of the gut [5, 22]. Thus whereas the high pH level is restricted to the anterior part of the gut in other xylophagous termites [23] and other herbivorous insects tested [32], it is only in SFT that the alkaline environment appears to be a general feature for most of the major hindgut compartments [22]. The use of micro-electrodes [28, 52, 55] have also very nicely demonstrated the presence of radial steep gradients of oxygen and hydrogen allowing the maintenance in the same gut compartments of anoxic and oxic conditions respectively in the centre and the gut periphery. The presence of an axial H2 and CH4, and to a less extent O2 profiles (figure 1)
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Figure 1. Physicochemical characteristics of the difference gut section of Cubitermes sp. determined by micro-electrodes measurements from reference [48]. The gut morphology does not respect the real scale for illustration purpose. C, crop; M, midgut; ms, mixed segment; P1 to 5, proctodeal segments 1 to 5, respectively. The pH values are average luminal pH determined on intact guts. The H2 and O2 partial pressure were measured at the gut centre.
was only demonstrated in SFT showing that their gut is far from being a simple homogenized anaerobic bioreactor [21]. The SFT digestive tract must be seen as a highly structured and compartmentalized reactor characterized by both radial and axial steep gradients. The SOM can thus be degraded according to various metabolic pathways (fermentation, anaerobic respiration, mineralization) depending on its position in the digestive tract.
diversity of these feeding guilds, the nutritional strategy could be different within the group or subfamily. Nevertheless, because of its further impact in the soil humification process, the exact nature of the o.m. assimilated during the gut transit by the SFT remains one of the most important challenging question, which should be investigated further.
3.2. Digestive enzyme
One major termite characteristic shared by the SFT is the presence of a significant and varied digestive microbiota located at the level of their posterior digestive tract [8, 50]. In the xylophagous species, most of this microbial population consists of new species that have not yet been cultured [3, 46]. The atypical gut environment and the gut soil content of the SFT do not allow the study of their gut microflora diversity with traditional microbial techniques. This partly explains why the overall composition of the SFT gut microflora remains globally unknown. Nevertheless, the data previously obtained and the recent concomitant development of molecular techniques and the use of fine physiological tools begin to shed some light on the structure and distribution of the gut microflora which could be summarized as follows: — A high density of bacteria (108 to 109 cells·mL–1 gut content; [8]). Compared to other feeding guilds, they have a lower density of carbohydrate fermenting bacteria [50]. As in the xylophagous species, the lactic acid bacteria constitutes half of the total carbohydrate utilizing bacteria [3]. — The level domain profile (Archaea, Bacteria and Eucarya) of the microbial community study with 16S rRNA oligonucleotides probes shows a good correlation with the termite diet [15, 18]. — Compared to the other diet, the SFT gut microbial community is characterized by a high level of active Archaea methanogen micro-organisms [14, 55]. Consequently, the SFT are, among the different termite diets, the highest methane producers [14]. — Compared to the telluric microbiota, the digestive bacterial population is characterized by an increase of unbranched filamentous bacteria, which represents up to 10 % of the biovolume of the digestive tract [6, 7, 9].
An interesting hypothesis elaborated by Noirot [45] has suggested that to adapt to the soil feeding mode, the termites were obliged to change their strategy radically. Indeed, if the decomposition of plant polysaccharides (cellulose, hemicellulose) is based on the hydrolysis of poly- and oligo-osides using hydrolases (cellulases, hemicellulases, cellobiase, etc.) of various origins, the decomposition of lignin depends on more varied strategies where hydrolases do not need to be present. In a comparative study on the polyosidasic activities between various higher termite diets, Rouland et al. [48, 51] have confirmed the hypothesis showing that compared to other diets, soil feeders have the lowest activities (figure 2). Only amylasic activities (figure 2) seem significant in some termite species [57]. The comparative analysis between initial food and faeces does not show degraded polysaccharides [16]. This absence of hydrolytic enzymes is not limited to carbohydrates. Moreover, a recent work [44] has evidenced the same pattern in six SFT species for most of the enzymes implied in the decomposition of lignin (lignin peroxidases, Mn peroxidases, pyranasoses oxidases, laccases, esterases). All these enzymes have been recorded in fungus growing termites [43]. All these measurements were realized with gut homogenates, which mixed all the microsites present in these highly structured guts. Further studies are thus needed to assess the reality of the in situ hydrolytic enzyme activities in the different SFT gut compartments. However the data are now consistent enough to assume that some genuine soil feeders do not depend on the degradation of plant carbohydrates and to a less extent lignin compounds for their nutrition. We could presume that due to the
3.3. Gut microbiota
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Figure 2. Osidasic activities of midgut and hindgut of several species of soil-feeding termites from reference [49]. 1. Crenetermes albotarsalis; 2. Cubitermes speciosus; 3. Thoracotermes macrothorax; 4. Basidentitermes potens; 5. Astratotermes sp. Specific values (S.A.) were expressed as µg glucose equivalent per min per mg of proteins.
— The microbial distribution, structure and density vary within the different gut compartments. The alkaline first proctodeal segment (P1) contains a lower density of micro-organisms [8] and host, with the paunch (P3), the hydrogen producing bacteria as formate producers and/or lactic acid bacteria [3, 55]. The filamentous bacteria and the hydrogen consuming micro-organisms (methanogenic and homoacetogens) have colonized the posterior part (P4a and P4b) of the hindgut [7, 55]. The filamentous bacteria (putatively actinomycetes bacteria) are attached to the gut wall by cuticular spines and constitute a ‘bacterial’ filter for the transit o.m. [9]. — The intestinal transit time seems to activate the endogenous microbial activity; in Cubitermes speciosus the metabolic activity is 200-fold higher in the paunch than in the control soil [10]. The absence of polyosidasic enzymes in SFT and the presence of mono- and poly-aromatic compounds in the soil have generated the search for bacteria able to metabolize the aromatic structures in termites [12–14]. Studied SFT have indeed a digestive flora able to mineralize or partially ferment the aromatic cycles; this characteristic is however not specific to this mode [24, 42]. The recent possibility to invest in situ gut metabolism with the simultaneous use of different micro-techniques such as micro-injections of
labelled compounds and micro-electrodes techniques [24, 52, 55] allows a putative scenario of the organic matter anaerobic digestion in the SFT gut centre to be established. After a quick transit in the foregut and midgut, the soil humic compounds are submitted, at the beginning of the hindgut, to a drastic alkaline hydrolysis breaking them in smaller units (proteins, aromatic compounds, carbohydrate monomers, etc.), which are degraded in the paunch (P3) by fermentative bacteria such as lactic acid bacteria [3]. As a result of the fermentative process, hydrogen is produced at a high level. Schmitt-Wagner and Brune [52] have suggested that H2 could diffuse across the epithelia into the adjacent posterior gut segments where it is further used by hydrogen-consuming bacteria in mainly methane by methanogenic micro-organisms and to a lesser extent acetate by homoacetogenic bacteria whose presence has been previously shown [12, 50, 55]. Finally, in this posterior part, the o.m. is probably also submitted to partial mineralization by passing through the bacterial filter formed by filamentous bacteria (putatively actinomycetes) [6], as these micro-organisms are well known for their ability to degrade recalcitrant compounds [47]. Another evidence of the participation of the gut microbiota to SOM degradation is the high level of δ-13C in soil feeder tissues which seem to come from
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Table I. Organic matter characterization in the different parts of the termitosphere of Thoracotermes macrothorax (from [33]).
Soil fractions Control soil Worked over soil* Mound Faeces
o.m. (%)
Total C (%)
Total N (%)
C/N
Ash (%)
Fulvic A. (g/100 g)
Humic A. (g/100 g)
Fulvic A/Humic A.H
6.2 9.4 8.3 17.2
2.79 4.67 4.01 7.85
0.21 0.45 0.38 1.08
13.3 10.3 10.5 7.26
93.7 88.5 91.7 82.8
1.08 1.58 1.28 2.98
0.58 1.46 1.48 2.11
1.86 1.08 0.86 1.41
* Soil sampled in the first 5 cm from a 6-m radius from the nest.
the assimilation through the termite gut of SOM modified by bacteria [54]. Thus the digestive tract of the SFT studied with its particular conditions (significant metabolic flow, alkaline pH, micro-spatialisation of the microbiota, simultaneous presence of oxic and anoxic conditions) represents a unique and complex biotope where the soil organic matter is submitted, all along the gut transit, to diverse environments and micro-organisms. This specific environment seems to shelter very specialized bacteria with restricted metabolic profiles [17] and could be a new source of unusual micro-organisms with interesting metabolic activities on SOM.
4. ORGANIC MATTER TRANSFORMATION IN THE TERMITOSPHERE 4.1. Characteristic of the soil organic matter after gut transit As underlined before, the change of soil organic matter during gut transit remains unclear. The use of spectroscopic techniques (13C-NMR) in a SFT species (Thoracotermes macrothorax [39]) has evidenced little change in the distribution of 13C after gut transit. However, by studying the same species, GarnierSillam [34, 36] has shown that the faeces are 3-fold richer in SOM than in control soil. This enrichment is mainly due to an increase in nitrogen (5-fold; table I). The author attributes this enrichment to food selection of rich organic compound in soil. The important level of nitrogen in the faeces does not come from bacterial nitrogen fixation as demonstrated by the high level of δ-15N of soil feeder tissues [54]. This increase in nitrogen faeces content came from the augmentation of combined and residual nitrogen, which is a sign of the important microbial activity occurring during the gut transit [36]. The faeces are noticeably depleted in alpha amine nitrogen coming from proteins associated with the humic compound [35]. These points must be underlined as, besides carbohydrates and lignin, the humic compounds also contain a significant amount of nitrogen, (3.5–5 %) where a significant part of it consists of plant and animal protein [40]. The alkaline hydrolysis of the humic compound ingested could lead to the liberation of a large part of this nitrogen, which could further be degraded by the gut microflora. This hypothesis is also supported by the analysis in mass
spectrometry of the intestinal contents which have shown a significant degradation of complexes proteintannin on the level of the posterior digestive tract [4]. Therefore, the contribution of nitrogen compound as protein in termite nutrition needs further investigation. Compared to the control soil (table I), the humic ratio is slightly higher due to an enrichment in fulvic acid. Consequently, the SOM in the SFT faeces seems more recently formed (or less polymerized) than in the control soil [35]. The origin of this change is unclear: whether it comes from a selection of a more recent o.m. by the termite or from the depolymerization process due to the alkaline hydrolysis of the o.m. remains uncertain. The faeces analysis [35] reveal a deep re-organization during gut transit of the organomineral soil complexes. The vegetable fragments degraded by the microbial degradation during the gut transit forms, with the soil clay particles, organomineral complexes which are more stable than the initial ingested humus. Therefore, the gut transit in SFT stabilizes the SOM and protects it from the rapid turnover of organic matter, which occurs in the tropics [36].
4.2. Soil organic matter in the termitosphere 4.2.1. Texture and physicochemical characteristics of the nest The analysis of soil nest texture has been assayed in different termite nest species coming from different biotopes (humid forest, savannah; table II). These texture analyses indicate that SFT mounds are built with materials coming mainly from the surface horizon [2, 37]. The high level of faeces deposit as building cement have important consequences on the chemical nest environment as it has been demonstrated in T. macrothorax nests [35, 36]. It increases the level of exchangeable bases (Ca, Mg, K, Na; [22, 39]). This enrichment in bi- and trivalent cations in the mound is important as it favourably influences the stability of the argilo-humic complexes in the mound. The ammoniacal nitrogen level is 2.5 higher in the nest wall than in the control soil, unlike the nitrate level, which is 10-fold lower in the nest wall. GarnierSillam and Harry’s [35] hypothesis shows that these differences could come from an inhibition of the nitrification process within the nest. These will differ-
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Table II. Physicochemical characteristics of some soil feeding termite mounds. C., Cubitermes; P., Procubitermes; Cr., Crenetermes; T., Thoracotermes; N., Noditermes; sav., savannah; f., forest; nd., not determined; M., mound; C.S., control soil; T.S., termitosphere soil.
Species
Location
C. oculatus
sav.
C. severus
sav.
C. severus
f
P. aburiensis
f
C. fungifaber Cr. albotarsalis N. lamanianus T. macrothorax P. niapuensis
f f f f f
Samples
M. C.S. M C.S. M. T.S. M. T.S. M. M. M. M. M. C.S.
Texture (%)
Organic matter
P
References –1
Clay
Limon
Sand
Total C
Total N
(µg·g )
19.8 10.9 23.4 18.3 31.2 32.3 33.7 38.9 46.8 38.7 48.2 33.7 40.7 38.4
19.6 12.1 52.0 47.0 31.9 26.4 26.7 21.9 33.5 30.0 35.6 29.2 36.9 28.0
60.6 77.3 24.6 34.8 36.9 41.3 39.7 39.9 19.7 31.3 16.2 37.1 22.4 33.6
1.7 0.8 2.5 1.5 4.3 4.2 5.2 5.0 5.6 3.3 4.4 4.0 4.2 2.8
0.15 0.06 0.29 0.14 0.42 0.33 0.69 0.52 0.45 0.22 0.41 0.36 0.41 1.9
15.4 6.3 1.9 0.2 nd nd nd nd 25.0 12.0 27.0 24.0 23.0 0.7
entiate the SFT mound from the typical clay mound of the fungus growing species where an active mineralization of organic nitrogen was shown [1]. The phosphorus concentration could be 35-fold higher in the mound wall than in the control soil (table II); according to Wood [56], this difference comes from the release, due to the alkaline pH, of phosphorus during the intestinal transit time. These physicochemical characteristics of the mound could have a very positive effect on the fertility of the tropical soils particularly low in phosphorus and exchangeable bases. By taking into account the aggregate distribution of SFT mound and their limited impact (20 cm under the nest, 50 m2 around the nest), these changes could be seen as essentially limited [56]. However, as a majority of the SFT species live within the soil in diffuse structures whose global characteristics are ignored, all conclusions on the SFT impacts on soil fertility remain speculative.
4.2.2. Organic composition of the nest The nest is composed of soil and faeces playing the role of cement [56]. The mound enrichment in carbon and nitrogen (table II) results from the faeces deposit, richer in o.m. as previously demonstrated. However, the level of this enrichment seems to depend mostly on the SOM content of the control soil. The mound enrichment in fine particles and SOM really becomes significant in the poor sandy soil of savannahs (table II), whereas we only notice a small or no enrichment in the organic rich humid forest soil. The organic distribution is in fact heterogeneous according to the different nest fractions (table II); more recent constructions (worked-over soil, faeces) are richer in o.m. than the mound [11]. On the other hand, the faeces evolution in the mound is correlated with an increase of the refractory o.m. as humic acid (table I) and residual refractory nitrogen [36]. According to Garnier-Sillam and Harry [35], the incorporation of
[56] [56] [2] [2] [35] [35] [35] [35] [35] [35]
the organo-mineral micro-aggregates formed during the gut transit into the mound stabilizes the humic compounds recently formed and increases the structural stability of the nest walls. Ultrastructural analysis of the external wall of Thoracotermes macrothorax nests has revealed the absence of living bacteria [33]. In contrast, recent studies [31] indicated that the close related forest and savannah mounds of Cubitermes constitute an active microbial reservoir, with a different community density, composition and activity. These authors have suggested that the mound microbiota could be involved in SOM degradation after the gut transit. The internal wall of the nest could be consumed by termites and therefore constitute an external rumen.
5. CONCLUSION If success in biotope adaptation could be measured by species diversity and density, then the SFT represent with the earthworms the most successful soil fauna. This review has underlined the remarkable adaptation of the SFT to the most challenging biotope in terms of food resources; the tropical soil. These trophic successes seem to come mainly from the evolution of their digestive tract, which allows a long and sequential transit in which the o.m. is sequentially degraded in various microsites (anoxic, oxic) by different microbiota (lactic acid bacteria, actinomycetes). Therefore, the action of SFT on o.m. cycling could be seen as an intense humification process which begins during the gut transit by an alkaline ‘explosion’ of the macromolecules producing less refractory oligomers which are further partially degraded by the microbiota in the different gut compartments. The oligomers produced are re-organized with the mineral soil fraction and have formed stable clay-humic complexes, which are further incorporated as micro-aggregates in
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the galleries and nest forming the termitosphere. At this stage, we still do not know whether the o.m. is shifted to long turnover pools or is submitted to a fermentation process occurring in the internal wall of the mound. However, this still speculative scenario of SOM evolution by SFT demonstrates that unlike other termite feeding guilds which mineralize the o.m. ingested, the soil feeding termite, by protecting the o.m. from mineralization, has a positive influence on the overall o.m. dynamics in the tropics. This review has also underlined that a lot of important and basics questions remain open: — Do the SFT select rich organic matter in the soil they have ingested? — What kind of compounds are preferentially degraded during the gut transit? — To what extent does the digestive microbiota participate in termite nutrition? — What is the rule of the nest, a simple shelter or a external rumen where digestion of refractory compounds continues? — What is the impact of termite nest erosion on overall soil fertility? These questions will only be resolved by an interdisciplinary approach, which will include soil scientists, microbiologists, insect physiologists and ethologists. With such an approach, we will be able to determine whether the SFT represent a new paradigm of soil adaptation or not.
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[8] Bignell D.E., Oskarsson H., Anderson J.M., Distribution and abundance of bacteria in the gut of a soil-feeding termite Procubitermes aburiensis (Termitidae, Termitinae), J. Gen. Microbiol. 117 (1980) 393–403. [9] Bignell D.E., Oskarsson H., Anderson J.M., Association of actinomycetes with soil-feeding termites: a novel symbiotic relationship? Z. Bakt. Mikrobiol. Hyg. (Suppl.) 11 (1981) 206–210. [10] Brauman A., Étude du métabolisme bactérien de termites supérieurs aux régimes alimentaires différenciés, Ph.D. thesis, université Aix-Marseille-I, Marseille, 1989. [11] Brauman A., Fall S., Influence des termites humivores sur le compartiment organique et microbien des termitières, Paper presented at the 16th International Congress of Soil Sciences, Montpellier, 1998. [12] Brauman A., Kane M.D., Labat M., Breznak J.A., Hydrogen metabolism by termites guts microbes, in: Belaich J.P., Bruschi M. (Eds.), Biochemistry of Strict Anaerobes Involved in Interspecies Hydrogen Transfer, Plenum Press, New York, 1989, pp. 369–371. [13] Brauman A., Labat M., Garcia J.L., Preliminary studies on the gut microbiota of the soil feeding termite: Cubitermes speciosus, in: Lesel R. (Ed.), Microbiology of Poecilotherms, Elsevier Science, Amsterdam, 1990, pp. 73–77. [14] Brauman A., Kane M.D., Labat M., Breznak J.A., Genesis of acetate and methane by gut bacteria of nutritionally diverse termites, Science 257 (1992) 1387. [15] Brauman A., Doré J., Eggleton P., Bignell D., Kane M.D., Analyse comparée de la structure des communautés microbiennes du tube digestif des termites en fonction de leurs régimes alimentaires, Paper presented at the Coll. UIEIS, 1997, Créteil, France. [16] Brauman A., Bignell D.E., Tayasu I., Soil-feeding termites: Biology, microbial associations and digestive mechanisms, in: Abe T., Bignell D.E., Higashi M. (Eds.), Termites: Evolution, Sociality, Symbiosis, Ecology, Kluwer Academic Publishers, Dordrecht, 2000, pp. 233–259. [17] Brauman A., Müller J.A., Garcia J.-L., Brune A., Schink B., Fermentative degradation of 3-hydroxybenzoate in pure culture by a novel strictly anaerobic bacterium, Sporotomaculum hydroxybenzoicum gen. nov., sp. nov., Int. J. Syst. Bacteriol. 48 (1998) 215–221. [18] Brauman A., Doré J., Eggleton P., Bignell D., Kane M.D., Molecular phylogenetic profiling of microbial communities in guts of termites with different feeding habits, FEMS Microb. Ecol. (2000) (in press). [19] Breznak J.A., Intestinal microbiota of termites, and other xylophagous insects, Ann. Rev. Entomol. 36 (1982) 323–343. [20] Breznak J.A., Brune A., Role of microorganisms in the digestion of lignocellulose by termites, Annu. Rev. Entomol. 39 (1994) 453–487. [21] Brune A., Termite gut: the world smallest bioreactor, Trends Biotechnol. 16 (1998) 16–21.
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[22] Brune A., Kühl M., pH profiles of the extremely alkaline hindguts of soil-feeding termites (Isoptera: Termitidae) determined with microelectrodes, J. Insect Physiol. 42 (1996) 1121–1127. [23] Brune A., Emerson D., Breznak J.A., The termites gut microflora as an oxygen sink: Microelectrode determination of oxygen and pH gradients in guts of lower and higher termites, Appl. Environ. Microbiol. 61 (1995) 2681–2687. [24] Brune A., Miambi E., Breznack J.A., Role of oxygen and the intestinal microflora in the metabolism of lignin-derived phenylpropanoids and other monoaromatic compounds by termites, Appl. Environ. Microbiol. 61 (1995) 2688–2695. [25] Collins N.M., Termites populations and their role in litter removal in Malaysian rain forests, in: Sutton S.L., Werger M.J.A., Chadwick A.C. (Eds.), Tropical Rain Forest: Ecology and Management, Blackwell Scientific Publications, Oxford, 1983, pp. 311–325. [26] Collins N.M., Termites, in: Leith H., Werger M.J.A. (Eds.), Tropical Rain Forest Ecosystems, Elsevier Science Publishers, Amsterdam, 1989, pp. 455–471. [27] Davies R.G., Eggleton P., Dibog L., Lawton J.H., Bignell D.E., Brauman A., Hartmann C., Nunes L., Holt J., Rouland C., Successional response of a tropical termite assemblage to experimental habitat perturbation, J. Appl. Ecol. 36 (1999) 946–963. [28] Ebert A., Brune A., Hydrogen concentration profiles at the oxic-anoxic interface: a microsensor study of the hindgut of the wood-feeding lower termite Reticulitermes flavipes (Kollar), Appl. Environ. Microbiol. 63 (1997) 4039–4046. [29] Eggleton P., Homathevi R., Jeeva D., Jones D.T., Davies R.G., Maryati M., The species richness of termites (Isoptera) in primary and regenerating lowland dipterocarp forest in Sabah, east Malaysia, Ecotropica 3 (1997) 119–128. [30] Eggleton P., Bignell D.E., Sands W.A., Waite B., Wood T.G., Lawton J.H., The species richness of termites (Isoptera) under differing levels of forest disturbance in the Mbalmayo Forest Reserve, southern Cameroon, J. Trop. Ecol. 11 (1995) 85–98. [31] Fall S., Hamelin J., Rouland C., Chotte J.L., Lensi R., Nazaret S., Brauman A., Microbial diversity and activity in soil-feeding termites nest in tropical arid soil, Poster presented at the Microbial Diversity, 1999, Chicago, Illinois, USA. [32] Felton G.W., Duffey S.S., Reassessment of the role of gut alkalinity and detergency in insect herbivory, J. Chem. Ecol. 17 (1991) 1821–1836. [33] Garnier-Sillam E., Biologie et rôle des termites dans les processus d’humification des sols forestiers du Congo, Ph.D. thesis, université Paris-XII, Créteil, 1987. [34] Garnier-Sillam E., Comparative physico-chemical properties of soil-feeding Thoracotermes macrothorax and fungus-growing Macrotermes mülleri termite mounds, Biogeochemistry 48 (1991) 7–13. [35] Garnier-Sillam E., Harry M., Distribution of humic compounds in mounds of soil-feeding termites: its influence on soil structural stability, Insectes Sociaux 42 (1995) 167–185.
[36] Garnier-Sillam E., Villemin G., Toutain F., Renoux J., Formation de micro-agrégats organo-minéraux dans les fèces de termites, C. R. Acad. Sci. 301 (1985) 213–218. [37] Garnier-Sillam E., Braudeau E., Tessier D., Rôle des termites sur le spectre poral des sols forestiers tropicaux: cas de Thoracotermes macrothorax et de Macrotermes mülleri (Sjöstedt), (Macrotermitinae), Insectes Sociaux 38 (1991) 397–412. [38] Grassé P.P., Terminologia, vol. II, Fondation des sociétés - Constructions, Masson, Paris, 1984. [39] Hopkins D.W., Chudek J.A., Bignell D.E., Frouz J., Wenster E.A., Lawson T., Application of 13C-NMR to investigate the transformations and biodegradation of organic materials by wood-and soil-feeding termites, and a coprophagous litter-dwelling dipteran larva, Biodegradation (1998) (in press). [40] Kononova M.M., Soil Organic Matter, its Nature its Role in Soil Formation and in Soil Fertility, Pergamon Press, London, 1961. [41] Lepage M.G., L’impact des populations récoltantes de Macrotermes michaëlsoni dans un écosystème semi aride. II - La nourriture récoltée, comparaison avec les grands herbivores, Insectes Sociaux 28 (1981) 309–319. [42] Miambi E., Brauman A., Could monoaromatic compounds be a source of energy for symbiotic gut microflora of higher termite with different feeding guilds? Paper presented at the International Union of Social Insect, 1994, Paris. [43] Mora P., Rouland C., Comparison of hydrolytic enzymes produced during growth on carbohydrate substrates by Termitomyces associates of Pseudacanthotermes spiniger and Microtermes subhyalinus (Isoptera: Termitidae), Sociobiology 26 (1994) 39–53. [44] Mora P., Lattaud C., Rouland C., Recherche d’enzymes intervenant dans la dégradation de la lignine chez plusieurs espèces de termites à régimes alimentaires différents, Proceeding of the French section of UIEIS, 11, 1998, pp. 77–80. [45] Noirot C., From wood to humus feeding an important trend in termite ecolution, Biol. Evol. Social Insects (1992) 107–119. [46] Ohkuma M., Kudo T., Phylogenetic diversity of the intestinal bacterial community in the termite Reticulitermes speratus, Appl. Environ. Microbiol. 62 (1996) 461–468. [47] Pasti M.B., Pometto III A.L., Nuti M.P., Crawford D.L., Lignin-solubilizing ability of actinomycetes isolated from termite (Termitidae) gut, Appl. Environ. Microbiol. 56 (1990) 2213–2218. [48] Rouland C., Symbiosis with fungi, in: Abe T., Brignell D.E., Higashi M. (Eds.), Termites: Evolution, Sociality, Symbiosis, Ecology, Kluwer Academic Publishers, Dordrecht, 2000, pp. 289–305. [49] Rouland C., Chararas C., Renoux J., Étude comparée des osidases de trois termites africains à régime alimentaire different, C. R. Acad. Sci. 302 (1986) 341–345. [50] Rouland C., Brauman A., Labat M., Lepage M., Nutritional factors affecting methane emission from termites, Chemosphere 26 (1993) 617–622.
A. Brauman / Eur. J. Soil Biol. 36 (2000) 117–125 [51] Rouland C., Brauman A., Labat M., Niambi E., Les émissions de méthane par les termites en forêt tropicale, Échanges forêt-atmosphère en milieu tropical humide, UNESCO Editon, pp. 97–106. [52] Schmitt-Wagner D., Brune A., Hydrogens profiles and localization of methanogenic activities in the highly compartmentalized hindgut of soil feeding higher termites, Appl. Environ. Microbiol. 65 (1999) 4490–4496. [53] Sleaford F., Bignell D.E., Eggleton P., A pilot analysis of gut contents in termites from the Mbalmayo Forest Reserve, Cameroon, Ecol. Entomol. 21 (1996) 279–288.
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[54] Tayasu I., Abe T., Eggleton P., Bignell D.E., Nitrogen and carbon isotope ratio in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding, Ecol. Entomol. 22 (1997) 343–351. [55] Tholen A., Andreas B., Localization and in situ activities of homoacetogenic bacteria in the highly compartmentalized hindgut of soil feeding termite, Appl. Environ. Microbiol. 65 (1999) 4497–4505. [56] Wood T.G., Termites and the soil environment, Biol. Fertil. Soils 6 (1988) 228–236. [57] Yapi A., Biologie, écologie et métabolisme digestif de quelques espèces de termites humivores de savane, Ph.D. thesis, université d’Abidjan, Abidjan, 1991.
Effect of gut transit and mound deposit on soil organic matter transformations in the soil feeding termite: A review§ Alain Brauman* IRD (ex-Orstom), Laboratory of soil microbiology, Centre ISRA-IRD de Bel-Air, BP 1386, Dakar, Sénégal
Received 10 November 1999; accepted 2 October 2000
Abstract − Even if termites are often considered as a pest due to the damage they cause to agriculture and architecture, they contribute to the soil humification process in the tropics. This impact on the soil organic matter humification process is due to the most important feeding habit in terms of species diversity, the soil feeding termites (∼1 200 species). Unlike other termites, their diet is not based on lignocellulosic plant degradation, but on the consumption of the mineral-containing horizons for the acquisition of nutrients. They are mostly distributed in humid forest or savannah equatorial zone. High structure and compartment with steep radial and axial gradients of O2, H2 and pH characterize their gut and create a patchy biotope. Furthermore, the humic compounds ingested are submitted, during a sequential transit, to different chemical (alkaline hydrolysis) and microbial degradation processes (fermentation, anaerobic respiration and mineralization). During this gut transit, the soil organic matter is strongly modified in terms of nature (organic matter concentration, fulvic and humic acid ratio) and organization (formation of organo-mineral complexes with clay). The soil organic matter ingested is further included as faeces in the nest and the galleries which, as a whole, constitutes the termitosphere. Compared to the control soil, the soil organic matter in the termitosphere is more stable and protected from the intense mineralization, which occurs in the tropics. These shifts of the organic matter into long turnover pool generated by the termite gut transit and deposition in the termitosphere indicate that besides the earthworm, the soil feeding termite has a positive impact on the overall organic matter cycling in the tropics. © 2000 Éditions scientifiques et médicales Elsevier SAS termites / gut microbiota / soil organic matter / humification
1. INTRODUCTION “The soil in many areas of Africa was altered, soaked with saliva, and worked by the termites, the pedological consequences are very significant”. This statement of P.P. Grassé [38], a famous French zoologist, illustrates the ecological importance of termites in the various processes of soil formation in the tropics. The termites have indeed a considerable impact on the soil morphology; ascent of deep horizons, formation of subsurface horizons [56] on soil structures (soil aeration, porosity, aggregation [33, 37]) and texture (selection of fine particles [2]). They are nevertheless especially known for their capacities of degradation of lignocellulosic organic matter [20] due to the presence of a specific microbiota, which could be exogenous § Paper presented at the 16th World Congress of Soil Science, 20–26 August 1998, Montpellier, France. * Correspondence and reprints: fax +221 832 16 75. E-mail address: [email protected] (A. Brauman).
(symbiotic nest fungus; [49]) or endogenous, (gut microbiota; [19]). This microbial symbiosis helps them degrade vegetable matter in its various states (wood and plants) for xylophagous termites and to strongly polymerize soil for soil feeding termites (SFT). Their influence on organic matter (o.m.) turnover in tropical areas are thus determining: in tropical forest, they could consume half of the vegetal litter [25] and in some savannahs, they can consume up to 49 % of the grass [41]. However termite impact on organic matter turnover is not restricted to plant organic matter, half of the 2 200 termite species now referenced [30] thrive on the humic compounds of the soil and contribute to the soil humification process. The recent estimates of their biomass in different parts of Asian and African forests (up to 10 000 individuals, [29, 30]) evidence the major impact they have on soil biological and physical processes. Despite their ecological importance, they are not much studied mostly due to their presence in remote areas, their difficulty in surviving in laboratory conditions and their lack of
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economical interest. Nevertheless, the density reached by these termites implies that most of the higher horizons of the soil in some tropical areas are deeply altered by the passage through their digestive tracts. The introduced organic matter is given back to the ecosystem via faeces deposit included in the nest and the galleries forming the termitosphere [36]. After a quick overview of the biology and gut physiology of the SFT, I will review the data acquired in the soil organic matter transformation (SOM) through gut transit and its further evolution within the termitosphere. The role of the SFT in the SOM cycling in the tropics will be discussed.
2. BIOLOGY OF SOIL FEEDING TERMITE
according to Garnier-Sillam and Harry [35], SFT select within the soil horizon the rich and poorly polymerized SOM as shown by carbon enrichment of termite faeces and the presence of fresh SOM in the anterior part of the gut. Moreover, in laboratory conditions, the SFT species tested are able to differentiate a soil rich in o.m. [57]. The foraging activities are mainly located in the surface horizon (0–15 cm) of the soil where the plant material is incorporated. The species seems to be distributed in soil microsites along a gradient of humification, which varies from very degraded wood to very humified SOM [27, 29]. Yapi [57] suggests that this SFT distribution is realized according to the size of the species; small-sized species live in the higher layer rich in o.m., whereas larger-sized species are located in the deeper layers of the soil, low in o.m.
2.1. Generality The soil feeding termite could be considered as the only social insect living and feeding in soil. As other termites, their society is characterized by the presence of social castes: sexual and neutral castes such as workers and soldiers [38]. They belong to the higher termite family, which encompass 75 % of the termite species (Termitidae) and live in the tropical area. The evolution from lower to higher termites is characterized by the loss of the ability to host gut symbiotic cellulolytic flagellates and the diversification of diet (litter, soil, plant feeders [20]). The natural habitat of SFT is the tropical humid forest where they reach their maximum biomass [29, 30]. Their diversity and number in soil seem to correlatively decline with rainfall level [16]. Their repartition is also conditioned by the o.m. quality and/or anthropization effect as they disappear when a tropical forest, where they represent 80 % of the 114 species recorded, is brought into cultivation [27, 29]. Thus, if their relationship to the quality of the organic matter still needs to be elucidated, SFT seem to represent a good indicator of soil disturbance. They are mainly found in the subsurface horizon (0 to 10 cm) and are easily recognizable by the dark colour of their abdomen. Most of the SFT, especially the subfamily of Apicotermitinae, form diffuse and small colonies in soils and their physiology and soil impact is globally unknown. Thanks to the available data, this review will only consider soil impact of genuine soil feeders mostly belonging to the Cubitermes clade [26], which form large colonies, build epigeal mounds and thrive on soil without recognizable vegetable material compounds.
2.2. Characteristic of the initial food The quantity of soil ingested by SFT is very variable between species (between 2.76 and 9.1 g·d–1 [4, 16]). The question of food selection by SFT remains in fact controversial. The heterogeneousness of the gut contents was evidenced by a microscopic analysis of different termite species [53], but it does not show a preference for plant cells over mineral soil. In contrast,
3. ORGANIC MATTER TRANSFORMATION DURING GUT TRANSIT 3.1. Anatomical and physiological characteristics of the digestive tract The anatomy and physiology of the digestive gut are far more complex in the soil feeding termites than in the other termite feeding guilds. The anatomy of the SFT has already been well described (for review see [4, 45]) and our description will be limited to some general features, important in terms of soil organic transformations. Their gut is highly compartmentalized in five sections as shown in figure 1 and also characterized by an increase of the length and volume of the paunch allowing a sequential transit of long duration (36 to 48 h [4]). The use of micro-sensors in different gut sections carried out by Brune and co-workers [22, 28, 52] has clearly demonstrated that during this transit, the organic matter is submitted to different physical and chemical environments mostly due to pH, oxygen and hydrogen pressure variations (figure 1). The high alkaline level of the paunch, first demonstrated by Bignell and Eggleton [5] is the most important, in terms of pH level, ever recorded in an animal gut [22]. This alkaline treatment of the organic matter ingested begins (figure 1) in the anterior part of the hindgut, slightly decreases in the main paunch and only returns to more neutral conditions in the last part of the gut [5, 22]. Thus whereas the high pH level is restricted to the anterior part of the gut in other xylophagous termites [23] and other herbivorous insects tested [32], it is only in SFT that the alkaline environment appears to be a general feature for most of the major hindgut compartments [22]. The use of micro-electrodes [28, 52, 55] have also very nicely demonstrated the presence of radial steep gradients of oxygen and hydrogen allowing the maintenance in the same gut compartments of anoxic and oxic conditions respectively in the centre and the gut periphery. The presence of an axial H2 and CH4, and to a less extent O2 profiles (figure 1)
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Figure 1. Physicochemical characteristics of the difference gut section of Cubitermes sp. determined by micro-electrodes measurements from reference [48]. The gut morphology does not respect the real scale for illustration purpose. C, crop; M, midgut; ms, mixed segment; P1 to 5, proctodeal segments 1 to 5, respectively. The pH values are average luminal pH determined on intact guts. The H2 and O2 partial pressure were measured at the gut centre.
was only demonstrated in SFT showing that their gut is far from being a simple homogenized anaerobic bioreactor [21]. The SFT digestive tract must be seen as a highly structured and compartmentalized reactor characterized by both radial and axial steep gradients. The SOM can thus be degraded according to various metabolic pathways (fermentation, anaerobic respiration, mineralization) depending on its position in the digestive tract.
diversity of these feeding guilds, the nutritional strategy could be different within the group or subfamily. Nevertheless, because of its further impact in the soil humification process, the exact nature of the o.m. assimilated during the gut transit by the SFT remains one of the most important challenging question, which should be investigated further.
3.2. Digestive enzyme
One major termite characteristic shared by the SFT is the presence of a significant and varied digestive microbiota located at the level of their posterior digestive tract [8, 50]. In the xylophagous species, most of this microbial population consists of new species that have not yet been cultured [3, 46]. The atypical gut environment and the gut soil content of the SFT do not allow the study of their gut microflora diversity with traditional microbial techniques. This partly explains why the overall composition of the SFT gut microflora remains globally unknown. Nevertheless, the data previously obtained and the recent concomitant development of molecular techniques and the use of fine physiological tools begin to shed some light on the structure and distribution of the gut microflora which could be summarized as follows: — A high density of bacteria (108 to 109 cells·mL–1 gut content; [8]). Compared to other feeding guilds, they have a lower density of carbohydrate fermenting bacteria [50]. As in the xylophagous species, the lactic acid bacteria constitutes half of the total carbohydrate utilizing bacteria [3]. — The level domain profile (Archaea, Bacteria and Eucarya) of the microbial community study with 16S rRNA oligonucleotides probes shows a good correlation with the termite diet [15, 18]. — Compared to the other diet, the SFT gut microbial community is characterized by a high level of active Archaea methanogen micro-organisms [14, 55]. Consequently, the SFT are, among the different termite diets, the highest methane producers [14]. — Compared to the telluric microbiota, the digestive bacterial population is characterized by an increase of unbranched filamentous bacteria, which represents up to 10 % of the biovolume of the digestive tract [6, 7, 9].
An interesting hypothesis elaborated by Noirot [45] has suggested that to adapt to the soil feeding mode, the termites were obliged to change their strategy radically. Indeed, if the decomposition of plant polysaccharides (cellulose, hemicellulose) is based on the hydrolysis of poly- and oligo-osides using hydrolases (cellulases, hemicellulases, cellobiase, etc.) of various origins, the decomposition of lignin depends on more varied strategies where hydrolases do not need to be present. In a comparative study on the polyosidasic activities between various higher termite diets, Rouland et al. [48, 51] have confirmed the hypothesis showing that compared to other diets, soil feeders have the lowest activities (figure 2). Only amylasic activities (figure 2) seem significant in some termite species [57]. The comparative analysis between initial food and faeces does not show degraded polysaccharides [16]. This absence of hydrolytic enzymes is not limited to carbohydrates. Moreover, a recent work [44] has evidenced the same pattern in six SFT species for most of the enzymes implied in the decomposition of lignin (lignin peroxidases, Mn peroxidases, pyranasoses oxidases, laccases, esterases). All these enzymes have been recorded in fungus growing termites [43]. All these measurements were realized with gut homogenates, which mixed all the microsites present in these highly structured guts. Further studies are thus needed to assess the reality of the in situ hydrolytic enzyme activities in the different SFT gut compartments. However the data are now consistent enough to assume that some genuine soil feeders do not depend on the degradation of plant carbohydrates and to a less extent lignin compounds for their nutrition. We could presume that due to the
3.3. Gut microbiota
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Figure 2. Osidasic activities of midgut and hindgut of several species of soil-feeding termites from reference [49]. 1. Crenetermes albotarsalis; 2. Cubitermes speciosus; 3. Thoracotermes macrothorax; 4. Basidentitermes potens; 5. Astratotermes sp. Specific values (S.A.) were expressed as µg glucose equivalent per min per mg of proteins.
— The microbial distribution, structure and density vary within the different gut compartments. The alkaline first proctodeal segment (P1) contains a lower density of micro-organisms [8] and host, with the paunch (P3), the hydrogen producing bacteria as formate producers and/or lactic acid bacteria [3, 55]. The filamentous bacteria and the hydrogen consuming micro-organisms (methanogenic and homoacetogens) have colonized the posterior part (P4a and P4b) of the hindgut [7, 55]. The filamentous bacteria (putatively actinomycetes bacteria) are attached to the gut wall by cuticular spines and constitute a ‘bacterial’ filter for the transit o.m. [9]. — The intestinal transit time seems to activate the endogenous microbial activity; in Cubitermes speciosus the metabolic activity is 200-fold higher in the paunch than in the control soil [10]. The absence of polyosidasic enzymes in SFT and the presence of mono- and poly-aromatic compounds in the soil have generated the search for bacteria able to metabolize the aromatic structures in termites [12–14]. Studied SFT have indeed a digestive flora able to mineralize or partially ferment the aromatic cycles; this characteristic is however not specific to this mode [24, 42]. The recent possibility to invest in situ gut metabolism with the simultaneous use of different micro-techniques such as micro-injections of
labelled compounds and micro-electrodes techniques [24, 52, 55] allows a putative scenario of the organic matter anaerobic digestion in the SFT gut centre to be established. After a quick transit in the foregut and midgut, the soil humic compounds are submitted, at the beginning of the hindgut, to a drastic alkaline hydrolysis breaking them in smaller units (proteins, aromatic compounds, carbohydrate monomers, etc.), which are degraded in the paunch (P3) by fermentative bacteria such as lactic acid bacteria [3]. As a result of the fermentative process, hydrogen is produced at a high level. Schmitt-Wagner and Brune [52] have suggested that H2 could diffuse across the epithelia into the adjacent posterior gut segments where it is further used by hydrogen-consuming bacteria in mainly methane by methanogenic micro-organisms and to a lesser extent acetate by homoacetogenic bacteria whose presence has been previously shown [12, 50, 55]. Finally, in this posterior part, the o.m. is probably also submitted to partial mineralization by passing through the bacterial filter formed by filamentous bacteria (putatively actinomycetes) [6], as these micro-organisms are well known for their ability to degrade recalcitrant compounds [47]. Another evidence of the participation of the gut microbiota to SOM degradation is the high level of δ-13C in soil feeder tissues which seem to come from
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Table I. Organic matter characterization in the different parts of the termitosphere of Thoracotermes macrothorax (from [33]).
Soil fractions Control soil Worked over soil* Mound Faeces
o.m. (%)
Total C (%)
Total N (%)
C/N
Ash (%)
Fulvic A. (g/100 g)
Humic A. (g/100 g)
Fulvic A/Humic A.H
6.2 9.4 8.3 17.2
2.79 4.67 4.01 7.85
0.21 0.45 0.38 1.08
13.3 10.3 10.5 7.26
93.7 88.5 91.7 82.8
1.08 1.58 1.28 2.98
0.58 1.46 1.48 2.11
1.86 1.08 0.86 1.41
* Soil sampled in the first 5 cm from a 6-m radius from the nest.
the assimilation through the termite gut of SOM modified by bacteria [54]. Thus the digestive tract of the SFT studied with its particular conditions (significant metabolic flow, alkaline pH, micro-spatialisation of the microbiota, simultaneous presence of oxic and anoxic conditions) represents a unique and complex biotope where the soil organic matter is submitted, all along the gut transit, to diverse environments and micro-organisms. This specific environment seems to shelter very specialized bacteria with restricted metabolic profiles [17] and could be a new source of unusual micro-organisms with interesting metabolic activities on SOM.
4. ORGANIC MATTER TRANSFORMATION IN THE TERMITOSPHERE 4.1. Characteristic of the soil organic matter after gut transit As underlined before, the change of soil organic matter during gut transit remains unclear. The use of spectroscopic techniques (13C-NMR) in a SFT species (Thoracotermes macrothorax [39]) has evidenced little change in the distribution of 13C after gut transit. However, by studying the same species, GarnierSillam [34, 36] has shown that the faeces are 3-fold richer in SOM than in control soil. This enrichment is mainly due to an increase in nitrogen (5-fold; table I). The author attributes this enrichment to food selection of rich organic compound in soil. The important level of nitrogen in the faeces does not come from bacterial nitrogen fixation as demonstrated by the high level of δ-15N of soil feeder tissues [54]. This increase in nitrogen faeces content came from the augmentation of combined and residual nitrogen, which is a sign of the important microbial activity occurring during the gut transit [36]. The faeces are noticeably depleted in alpha amine nitrogen coming from proteins associated with the humic compound [35]. These points must be underlined as, besides carbohydrates and lignin, the humic compounds also contain a significant amount of nitrogen, (3.5–5 %) where a significant part of it consists of plant and animal protein [40]. The alkaline hydrolysis of the humic compound ingested could lead to the liberation of a large part of this nitrogen, which could further be degraded by the gut microflora. This hypothesis is also supported by the analysis in mass
spectrometry of the intestinal contents which have shown a significant degradation of complexes proteintannin on the level of the posterior digestive tract [4]. Therefore, the contribution of nitrogen compound as protein in termite nutrition needs further investigation. Compared to the control soil (table I), the humic ratio is slightly higher due to an enrichment in fulvic acid. Consequently, the SOM in the SFT faeces seems more recently formed (or less polymerized) than in the control soil [35]. The origin of this change is unclear: whether it comes from a selection of a more recent o.m. by the termite or from the depolymerization process due to the alkaline hydrolysis of the o.m. remains uncertain. The faeces analysis [35] reveal a deep re-organization during gut transit of the organomineral soil complexes. The vegetable fragments degraded by the microbial degradation during the gut transit forms, with the soil clay particles, organomineral complexes which are more stable than the initial ingested humus. Therefore, the gut transit in SFT stabilizes the SOM and protects it from the rapid turnover of organic matter, which occurs in the tropics [36].
4.2. Soil organic matter in the termitosphere 4.2.1. Texture and physicochemical characteristics of the nest The analysis of soil nest texture has been assayed in different termite nest species coming from different biotopes (humid forest, savannah; table II). These texture analyses indicate that SFT mounds are built with materials coming mainly from the surface horizon [2, 37]. The high level of faeces deposit as building cement have important consequences on the chemical nest environment as it has been demonstrated in T. macrothorax nests [35, 36]. It increases the level of exchangeable bases (Ca, Mg, K, Na; [22, 39]). This enrichment in bi- and trivalent cations in the mound is important as it favourably influences the stability of the argilo-humic complexes in the mound. The ammoniacal nitrogen level is 2.5 higher in the nest wall than in the control soil, unlike the nitrate level, which is 10-fold lower in the nest wall. GarnierSillam and Harry’s [35] hypothesis shows that these differences could come from an inhibition of the nitrification process within the nest. These will differ-
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Table II. Physicochemical characteristics of some soil feeding termite mounds. C., Cubitermes; P., Procubitermes; Cr., Crenetermes; T., Thoracotermes; N., Noditermes; sav., savannah; f., forest; nd., not determined; M., mound; C.S., control soil; T.S., termitosphere soil.
Species
Location
C. oculatus
sav.
C. severus
sav.
C. severus
f
P. aburiensis
f
C. fungifaber Cr. albotarsalis N. lamanianus T. macrothorax P. niapuensis
f f f f f
Samples
M. C.S. M C.S. M. T.S. M. T.S. M. M. M. M. M. C.S.
Texture (%)
Organic matter
P
References –1
Clay
Limon
Sand
Total C
Total N
(µg·g )
19.8 10.9 23.4 18.3 31.2 32.3 33.7 38.9 46.8 38.7 48.2 33.7 40.7 38.4
19.6 12.1 52.0 47.0 31.9 26.4 26.7 21.9 33.5 30.0 35.6 29.2 36.9 28.0
60.6 77.3 24.6 34.8 36.9 41.3 39.7 39.9 19.7 31.3 16.2 37.1 22.4 33.6
1.7 0.8 2.5 1.5 4.3 4.2 5.2 5.0 5.6 3.3 4.4 4.0 4.2 2.8
0.15 0.06 0.29 0.14 0.42 0.33 0.69 0.52 0.45 0.22 0.41 0.36 0.41 1.9
15.4 6.3 1.9 0.2 nd nd nd nd 25.0 12.0 27.0 24.0 23.0 0.7
entiate the SFT mound from the typical clay mound of the fungus growing species where an active mineralization of organic nitrogen was shown [1]. The phosphorus concentration could be 35-fold higher in the mound wall than in the control soil (table II); according to Wood [56], this difference comes from the release, due to the alkaline pH, of phosphorus during the intestinal transit time. These physicochemical characteristics of the mound could have a very positive effect on the fertility of the tropical soils particularly low in phosphorus and exchangeable bases. By taking into account the aggregate distribution of SFT mound and their limited impact (20 cm under the nest, 50 m2 around the nest), these changes could be seen as essentially limited [56]. However, as a majority of the SFT species live within the soil in diffuse structures whose global characteristics are ignored, all conclusions on the SFT impacts on soil fertility remain speculative.
4.2.2. Organic composition of the nest The nest is composed of soil and faeces playing the role of cement [56]. The mound enrichment in carbon and nitrogen (table II) results from the faeces deposit, richer in o.m. as previously demonstrated. However, the level of this enrichment seems to depend mostly on the SOM content of the control soil. The mound enrichment in fine particles and SOM really becomes significant in the poor sandy soil of savannahs (table II), whereas we only notice a small or no enrichment in the organic rich humid forest soil. The organic distribution is in fact heterogeneous according to the different nest fractions (table II); more recent constructions (worked-over soil, faeces) are richer in o.m. than the mound [11]. On the other hand, the faeces evolution in the mound is correlated with an increase of the refractory o.m. as humic acid (table I) and residual refractory nitrogen [36]. According to Garnier-Sillam and Harry [35], the incorporation of
[56] [56] [2] [2] [35] [35] [35] [35] [35] [35]
the organo-mineral micro-aggregates formed during the gut transit into the mound stabilizes the humic compounds recently formed and increases the structural stability of the nest walls. Ultrastructural analysis of the external wall of Thoracotermes macrothorax nests has revealed the absence of living bacteria [33]. In contrast, recent studies [31] indicated that the close related forest and savannah mounds of Cubitermes constitute an active microbial reservoir, with a different community density, composition and activity. These authors have suggested that the mound microbiota could be involved in SOM degradation after the gut transit. The internal wall of the nest could be consumed by termites and therefore constitute an external rumen.
5. CONCLUSION If success in biotope adaptation could be measured by species diversity and density, then the SFT represent with the earthworms the most successful soil fauna. This review has underlined the remarkable adaptation of the SFT to the most challenging biotope in terms of food resources; the tropical soil. These trophic successes seem to come mainly from the evolution of their digestive tract, which allows a long and sequential transit in which the o.m. is sequentially degraded in various microsites (anoxic, oxic) by different microbiota (lactic acid bacteria, actinomycetes). Therefore, the action of SFT on o.m. cycling could be seen as an intense humification process which begins during the gut transit by an alkaline ‘explosion’ of the macromolecules producing less refractory oligomers which are further partially degraded by the microbiota in the different gut compartments. The oligomers produced are re-organized with the mineral soil fraction and have formed stable clay-humic complexes, which are further incorporated as micro-aggregates in
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the galleries and nest forming the termitosphere. At this stage, we still do not know whether the o.m. is shifted to long turnover pools or is submitted to a fermentation process occurring in the internal wall of the mound. However, this still speculative scenario of SOM evolution by SFT demonstrates that unlike other termite feeding guilds which mineralize the o.m. ingested, the soil feeding termite, by protecting the o.m. from mineralization, has a positive influence on the overall o.m. dynamics in the tropics. This review has also underlined that a lot of important and basics questions remain open: — Do the SFT select rich organic matter in the soil they have ingested? — What kind of compounds are preferentially degraded during the gut transit? — To what extent does the digestive microbiota participate in termite nutrition? — What is the rule of the nest, a simple shelter or a external rumen where digestion of refractory compounds continues? — What is the impact of termite nest erosion on overall soil fertility? These questions will only be resolved by an interdisciplinary approach, which will include soil scientists, microbiologists, insect physiologists and ethologists. With such an approach, we will be able to determine whether the SFT represent a new paradigm of soil adaptation or not.
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