- Email: [email protected]
Bacteriohopanepolyol signatures of cyanobacterial and methanotrophic bacterial populations recorded in a geothermal vent sinter
Organic Geochemistry 39 (2008) 1020–1023
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Bacteriohopanepolyol signatures of cyanobacterial and methanotrophic bacterial populations recorded in a geothermal vent sinter Robert A. Gibson a,*, Helen M. Talbot a, Gurpreet Kaur b, Richard D. Pancost b, Bruce Mountain c a b c
School of Civil Engineering and Geosciences, Drummond Building, Newcastle University, Newcastle-upon-Tyne, NE1 7RU, UK Organic Geochemistry Unit, The Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand
a r t i c l e
i n f o
Article history: Received 21 September 2007 Accepted 10 April 2008 Available online 20 April 2008
a b s t r a c t The distribution of bacteriohopanepolyols (BHPs) in a modern terrestrial geothermal silica sinter has been employed as a proxy for bacterial populations and bacterially mediated processes occurring within the geothermal ecosystem. Analysis of samples from Opaheke Pool hot spring in the Taupo Volcanic Zone (North Island, New Zealand) revealed the presence of a suite of BHPs relating to cyanobacteria and Type I and Type II methanotrophic bacteria. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Modern terrestrial geothermal areas provide a natural laboratory with which to investigate the interplay between biochemical, chemical and physical processes and modern extremophilic bacterial communities. Understanding these processes provides a means of investigating how geological and biogeochemical processes have affected the evolution and diversification of life on Earth. Distributions of ‘intact’ membrane lipids preserved in successions of geothermal sinter provide measurable indicators of precursor biota indigenous to the corresponding geothermal system (Table 1). Bacteriohopanepolyols (BHPs) are the biological precursors of hopanoids, hopanoic acids and hopanes widespread throughout the sedimentary record (e.g. Ourisson and Albrecht, 1992). Synthesised exclusively by bacteria, non-methylated BHPs are produced by a great many different groups of organisms inhabiting ‘meso-environmental’ conditions (e.g. Rohmer et al., 1984) and are considered to be produced and regulated in response to a
* Corresponding author. Tel.: +44 (0) 191 222 6605; fax: +44 (0) 191 222 5233. E-mail address: [email protected] (R.A. Gibson). 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.04.014
changing extracellular physical or chemical parameter, i.e. change in pH or temperature, or desiccation (e.g. Kannenburg and Poralla, 1999; Poralla et al., 2000; Joyeux et al., 2004). The relationship between the structural complexity of a BHP and its physiological function is not well understood (e.g. Rashby et al., 2007); however, the number and nature of functional groups contained within the molecule can be indicative of certain groups of organisms and therefore provide a useful tool when assessing bacterial populations in environmental matrices. Certain species of cyanobacteria and methanotrophic bacteria have been shown to produce diagnostic, nonmethylated BHPs (e.g. Talbot et al., 2001, 2008). Type I methanotrophs are the only known source of non-methylated, hexa-functionalised BHPs containing an amino group at C-35 (Ia; see Appendix). Likewise, Type II methanotrophic bacteria have been shown to simultaneously synthesise significant quantities of aminobacteriohopanetriol (Ib) and aminobacteriohopanetetrol (Ic; e.g. Talbot et al. 2001). Therefore, the absence of methylated BHPs does not necessarily indicate the absence of cyanobacterial or methanotrophic bacterial populations (e.g. Coolen et al., 2008). Interpretation of the distribution of ‘intact’ BHP components is therefore important when assessing modern bacterial populations by proxy.
1021
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023 Table 1 Abundances of bacteriohopanepolyols (lg g dry sinter 1)
2.2. Extraction and analysis in
Opaheke
Pool
sinter
BHP
Structure
lg g dry sinter
Aminobacteriohopanepentol 3-Methyl aminobacteriohopanepentol Aminobacteriohopanetetrol Aminobacteriohopanetriol 2-Methyl aminobacteriohopanetriol 2-Methyl bacteriohopanepentol Bacteriohopanetetrol 2-Methyl bacteriohopanetetrol
Ia IIIa Ic Ib IIb IIe Id IId
0.15 0.05 0.04 13.68 0.17 0.11 4.26 0.06
The freeze-dried, powdered sample (ca. 3 g) was extracted according to a modified Bligh and Dyer method as described previously for sediments (e.g. Talbot et al. 2008; Coolen et al., 2008). Extracts were acetylated as described previously (e.g. Talbot et al., 2008) prior to analysis using atmospheric pressure chemical ionisation-high performance liquid chromatography-multistage mass spectrometry with a ThermoFinnigan LCQ ion trap (e.g. Talbot et al., 2003, 2007a). A semi-quantitative estimate of BHP abundance was calculated from the characteristic base peak ion peak areas of individual BHPs in mass chromatograms (Fig. 1) relative to the m/z 345 ([M+H-CH3COOH]+) base peak area response of acetylated 5a-pregnane3b,20b-diol as internal standard. Averaged relative response factors (from a suite of five acetylated authentic BHP standards) were used to adjust the BHP peak areas relative to that of the internal standard, where BHPs containing one or more N atoms give an average response approximately 12 times that of the standard and compounds with no N atoms give a response approximately 8 times that of the standard.
1
Here, following on from recent reports of bacteriohopanepolyols and hopanoids in geothermal sinters (Talbot et al. 2005; Pancost et al. 2005, 2006), we present data for a suite of ‘intact’ BHP signatures indicative of bacterial populations indigenous to a contemporary geothermal system and relevant to important evolutionary biogeochemical processes. 2. Experimental 2.1. Sample
3. Results
The sinter sample was collected from the Opaheke Pool hot spring (OP;Taupo Volcanic Zone, North Island, New Zealand). The water temperature of the spring is 98 °C and the pH 7.2. The sample discussed here is an inactive sinter collected ca. 50 cm from the pool edge.
Studies of the BHP distribution of the sample from the OP hot spring revealed an array of known and novel compounds, including cyanobacterial markers and BHP
Ia m/z 830
1.2
m/z 844
Relative Intensity
m/z 772
IIIa
0.4
Ic
0.02
m/z 727
IIe Ib
m/z 714
1 100
m/z 728
IIb
0.7
m/z 655
Id
21
m/z 669
IId
0.3
Retention Time Fig. 1. Partial mass chromatograms (15–27 min) showing bacteriohopanepolyol distribution in Opaheke Pool sinter sample; m/z values refer to derivatised (peracetylated) base peak ion (see Appendix for structures).
1022
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023
signatures corresponding to Type I and Type II methaneoxidising bacteria. 3.1. Possible cyanobacterial BHP signatures in geothermal vent sinters A total of five BHP structures (Ib, Id, IIb, IId, IIe), all of which have been found in cultured cyanobacteria (e.g. Talbot et al., 2008 and references therein), were identified in the sinter material (Fig. 1). Bacteriohopanepentol (Ie) and 2-methyl bacteriohopanetetrol (IIe) are known to be synthesised by Nostoc cyanobacteria (Bisseret et al., 1985). The presence of two non-methylated tetrafunctionalised BHPs (Ib, Id), known to be biosynthesised by a plethora of bacterial species including cyanobacteria (e.g. Rohmer, 1993 and references therein) in environmental samples, together with the more diagnostic C-2 methylated homologues (IIb, IId), can be used to infer the presence of a cyanobacterial population. It should be noted, however, that IId has recently also been reported from two strains of an anoxygenic phototroph (Rashby et al., 2007). 3.2. Signatures of methanotrophic bacteria in geothermal vent sinters The BHP distribution in the sinter included compounds relating to Type I methane-oxidising bacteria (e.g. Talbot et al., 2001; Ia and IIIa). The presence of the 3-methylhopanoids in sediments can be indicative of an oxic–anoxic transition in aquatic systems (e.g. Farrimond et al., 2004; Blumenberg et al., 2007); however, the full implications of the presence of 3-methylaminobacteriohopanepentol (IIIa) and aminobacteriohopanepentol (Ia) in this geothermal setting have yet to be assessed. Also detected in OP sinter was aminobacteriohopanetetrol (Ic), a compound produced by both Type I and Type II methanotrophs (e.g. Talbot et al., 2001). Due to the absence of a C-3 methylated counterpart, indicative of Type I methanotrophs, the presence of this lipid is tentatively assigned to a Type II methanotrophic precursor (e.g. Talbot et al., 2001 and references therein).
4. Discussion BHPs containing an additional methyl on the A ring (II, III) provide characteristic features with which to investigate both present and past bacterial populations. An abundance of C-2 methylated hopanoids in ancient sediments is used as a proxy for aerobic metabolism in ancient ecosystems as BHPs methylated at C-2 are diagnostic of cyanobacteria (e.g. Talbot et al., 2008) with corresponding diagenetic products known to survive in sediments for up to 2.7 billion years (Brocks et al., 1999; Summons et al., 1999). BHPs containing an additional methyl at C-3 are known to be synthesised by methanotrophic bacteria (Ia and IIIa) and some acetic acid bacteria (e.g. IIIf and IIIg; e.g. Talbot et al., 2007b). Type I methanotrophs are the only known source of C-35 amino, hexa-functionalised and C-3 methyl-
ated, hexa-functionalised BHPs. The presence of these compounds and corresponding diagenetic derivatives in environmental matrices is used as an indicator of aerobic methanotrophic metabolism and can be used to infer palaeoenvironmental and palaeo-redox conditions in ancient sediments. The variety of structures detected in sinter material from OP shows the potential for BHPs as biomarkers for bio- and geochemical processes occurring in situ within a geothermal environment. However, the assignment of BHPs to a specific biological source is complicated, as interpretations of bacterial sources are limited to those bacteria which have been tested for production of BHPs. The majority of these organisms are aerobic and, although BHPs have been detected in a number of bacterial cultures grown under microaerophilic and anaerobic conditions (e.g. Blumenberg et al., 2006 and references therein), BHPs and hopanoids are still generally regarded as markers for oxic depositional conditions. The presence of cyanobacterial and, in particular, methanotrophic bacterial BHP signatures in sinters from OP suggests that sinter formation occurs at the oxic/anoxic boundary and is consistent with the findings of Mountain et al. (2003). Ongoing analysis of the distribution, abundance and preservation of bacterial lipids in sinters from Opaheke pool, and other geothermal areas from the Taupo Volcanic Zone, has shown that there remains a significant source of novel precursor membrane lipids indicative of uncharacterised organisms, undetermined BHP producers or novel biochemical pathways. The presence of BHPs in modern sinters is assumed to be indicative of biota colonising the waters and the vicinity of the geothermal pool. The long term fate of these intact lipids in sinter material has yet to be determined; however, the extent of molecular complexity retained upon deposition and incorporation into the silica matrix seems to promote preservation of organic material.
5. Conclusions Analysis of the distribution of bacterial membrane lipids, bacteriohopanepolyols (BHPs) in geothermal silica sinter has shown the presence of diagnostic signatures of cyanobacterial and methanotrophic bacterial populations. This, to our knowledge, is the first report of methylated BHPs and only the second report of non-methylated, polyfunctionalised BHPs (Talbot et al., 2005) preserved in geothermal silica sinters, which seem to enhance preservation, and would indicate that distributions of lipids in sinter depositions provide a useful palaeobiological tool. Acknowledgements The authors would like to thank the NERC for funding (RAG) and the Science Research Infrastructure Fund (SRIF) from HEFCE for funding the purchase of the Thermo Electron Finnigan LCQ Ion Trap Mass Spectrometer. We thank two anonymous reviewers for helpful comments and suggestions that helped to improve the manuscript.
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023
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Appendix Structures referred to in the text. Note: compounds are analysed as peracetate derivatives but parent compounds are shown for simplicity.
Guest Associate Editor—R. Harvey References Bisseret, P., Zundel, M., Rohmer, M., 1985. Prokaryotic triterpenoids 2. 2bMethylhopanoids from Methylobacterium organophilum and Nostoc muscorum, a new series of prokaryotic triterpenoids. European Journal of Biochemistry 150, 29–34. Blumenberg, M., Kruger, M., Nahaus, K., Talbot, H.M., Opperman, B.I., Pape, T., Michaelis, W., 2006. Biosynthesis of hopanoids by sulphatereducing bacteria (genus Desulfovibrio). Environmental Microbiology 8, 1220–1227. Blumenberg, M., Seifert, R., Michaelis, W., 2007. Aerobic methanotrophy in the oxic-anoxic transition zone of the Black Sea water column. Organic Geochemistry 38, 84–91. Brocks, J.J., Logan, G.A., Buick, R., Summons, R.E., 1999. Archaean molecular fossils and the early rise of the eukaryotes. Science 285, 1033–1036. Coolen, M.J.L, Talbot, H.M, Abbas, B.A, Ward, C., Schouten, S., Volkman, J.K, Sinninghe Damsté, J.S., 2008. Sources for sedimentary bacteriohopanepolyols as revealed by 16S rDNA stratigraphy. Environmental Microbiology, doi:10.1111/j.1462-2920.2008.01601.x. Farrimond, P., Talbot, H.M., Watson, D.F., Schulz, L.K., Wilhelms, A., 2004. Methylhopanoids: molecular indicators of ancient bacteria and a petroleum correlation tool. Geochimica et Cosmochimica Acta 68, 3873–3882. Joyeux, C., Fouchard, S., Llopiz, P., Neunlist, S., 2004. Influence of the temperature and the growth phase on the hopanoids and fatty acids content of Frateuria aurantia (DSMZ 6220). FEMS Microbiology Ecology 47, 371–379. Kannenburg, E.L., Poralla, K., 1999. Hopanoid biosynthesis and function in bacteria. Naturwissenschaften 86, 168–176. Mountain, B.W., Benning, L.G., Boerema, J.A., 2003. Experimental studies on New Zealand hot spring sinters: rates of growth and textural development. Canadian Journal of Earth Sciences 40, 1643–1667. Ourisson, G., Albrecht, P., 1992. Geohopanoids 2: the most abundant natural product on Earth? Accounts of Chemical Research 25, 398–402. Pancost, R.D., Pressley, S., Coleman, J.M., Benning, L.G., Mountain, B.W., 2005. Lipid biomolecules in silica sinters: indicators of microbial diversity. Environmental Microbiology 7, 66–77. Pancost, R.D., Pressley, S., Coleman, J.M., Talbot, H.M., Kelly, S.P., Farrimond, P., Schouten, S., Benning, L., Mountain, B.W., 2006. Composition and implications of diverse lipids in New Zealand geothermal sinters. Geobiology 4, 71–92.
Poralla, K., Muth, G., Hartner, T., 2000. Hopanoids are formed during the transition from substrate to aerial hyphae in Streptomyces coelicolor A3(2). FEMS Microbiology Letters 189, 93–95. Rashby, S.E., Sessions, A.L., Summons, R.E., Newman, D.K., 2007. Biosynthesis of 2-methylbacteriohopanepolyols by an anoxygenic phototroph. Proceedings of the National Academy of Sciences of the USA 104, 15099–15104. Rohmer, M., Bouvier-Navé, P., Ourisson, G., 1984. Distribution of hopanoid triterpenes in prokaryotes. Journal of General Microbiology 130, 1137–1150. Rohmer, M., 1993. The biosynthesis of triterpenoids of the hopane series in the Eubacteria: a mine of new enzyme reactions. Pure and Applied Chemistry 65, 1293–1298. Summons, R.E., Jahnke, L.L., Hope, J.M., Logan, G.A., 1999. 2Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400, 554–557. Talbot, H.M., Watson, D.F., Murrell, J.C., Farrimond, P., 2001. Analysis of intact bacteriohopanepolyols from methanotrophic bacteria by reversed phase high performance liquid chromatography – atmospheric pressure chemical ionisation – mass spectrometry. Journal of Chromatography A 921, 175–185. Talbot, H.M., Summons, R.E., Janhke, L.L., Farrimond, P., 2003. Characteristic fragmentation of bacteriohopanepolyols during atmospheric pressure chemical ionisation liquid chromatography/ ion trap mass spectrometry. Rapid Communications in Mass Spectrometry 17, 2788–2796. Talbot, H.M., Farrimond, P., Schaeffer, P., Pancost, R.D., 2005. Bacteriohopanepolyols in hydrothermal vent biogenic silicates. Organic Geochemistry 36, 663–672. Talbot, H.M., Rohmer, M., Farrimond, P., 2007a. Rapid structural elucidation of composite bacterial hopanoids by atmospheric pressure chemical ionisation liquid chromatography/ion trap mass spectrometry. Rapid Communications in Mass Spectrometry 21, 880–892. Talbot, H.M., Rohmer, M., Farrimond, P., 2007b. Structural characterisation of unsaturated bacterial hopanoids by atmospheric pressure chemical ionisation liquid chromatography/ion trap mass spectrometry. Rapid Communications in Mass Spectrometry 21, 1613–1622. Talbot, H.M., Summons, R.E., Jahnke, L.L., Cockell, C.S., Rohmer, M., Farrimond, P., 2008. Cyanobacterial bacteriohopanepolyol signatures from cultures and natural environmental settings. Organic Geochemistry 39, 232–263.
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Bacteriohopanepolyol signatures of cyanobacterial and methanotrophic bacterial populations recorded in a geothermal vent sinter Robert A. Gibson a,*, Helen M. Talbot a, Gurpreet Kaur b, Richard D. Pancost b, Bruce Mountain c a b c
School of Civil Engineering and Geosciences, Drummond Building, Newcastle University, Newcastle-upon-Tyne, NE1 7RU, UK Organic Geochemistry Unit, The Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand
a r t i c l e
i n f o
Article history: Received 21 September 2007 Accepted 10 April 2008 Available online 20 April 2008
a b s t r a c t The distribution of bacteriohopanepolyols (BHPs) in a modern terrestrial geothermal silica sinter has been employed as a proxy for bacterial populations and bacterially mediated processes occurring within the geothermal ecosystem. Analysis of samples from Opaheke Pool hot spring in the Taupo Volcanic Zone (North Island, New Zealand) revealed the presence of a suite of BHPs relating to cyanobacteria and Type I and Type II methanotrophic bacteria. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Modern terrestrial geothermal areas provide a natural laboratory with which to investigate the interplay between biochemical, chemical and physical processes and modern extremophilic bacterial communities. Understanding these processes provides a means of investigating how geological and biogeochemical processes have affected the evolution and diversification of life on Earth. Distributions of ‘intact’ membrane lipids preserved in successions of geothermal sinter provide measurable indicators of precursor biota indigenous to the corresponding geothermal system (Table 1). Bacteriohopanepolyols (BHPs) are the biological precursors of hopanoids, hopanoic acids and hopanes widespread throughout the sedimentary record (e.g. Ourisson and Albrecht, 1992). Synthesised exclusively by bacteria, non-methylated BHPs are produced by a great many different groups of organisms inhabiting ‘meso-environmental’ conditions (e.g. Rohmer et al., 1984) and are considered to be produced and regulated in response to a
* Corresponding author. Tel.: +44 (0) 191 222 6605; fax: +44 (0) 191 222 5233. E-mail address: [email protected] (R.A. Gibson). 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.04.014
changing extracellular physical or chemical parameter, i.e. change in pH or temperature, or desiccation (e.g. Kannenburg and Poralla, 1999; Poralla et al., 2000; Joyeux et al., 2004). The relationship between the structural complexity of a BHP and its physiological function is not well understood (e.g. Rashby et al., 2007); however, the number and nature of functional groups contained within the molecule can be indicative of certain groups of organisms and therefore provide a useful tool when assessing bacterial populations in environmental matrices. Certain species of cyanobacteria and methanotrophic bacteria have been shown to produce diagnostic, nonmethylated BHPs (e.g. Talbot et al., 2001, 2008). Type I methanotrophs are the only known source of non-methylated, hexa-functionalised BHPs containing an amino group at C-35 (Ia; see Appendix). Likewise, Type II methanotrophic bacteria have been shown to simultaneously synthesise significant quantities of aminobacteriohopanetriol (Ib) and aminobacteriohopanetetrol (Ic; e.g. Talbot et al. 2001). Therefore, the absence of methylated BHPs does not necessarily indicate the absence of cyanobacterial or methanotrophic bacterial populations (e.g. Coolen et al., 2008). Interpretation of the distribution of ‘intact’ BHP components is therefore important when assessing modern bacterial populations by proxy.
1021
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023 Table 1 Abundances of bacteriohopanepolyols (lg g dry sinter 1)
2.2. Extraction and analysis in
Opaheke
Pool
sinter
BHP
Structure
lg g dry sinter
Aminobacteriohopanepentol 3-Methyl aminobacteriohopanepentol Aminobacteriohopanetetrol Aminobacteriohopanetriol 2-Methyl aminobacteriohopanetriol 2-Methyl bacteriohopanepentol Bacteriohopanetetrol 2-Methyl bacteriohopanetetrol
Ia IIIa Ic Ib IIb IIe Id IId
0.15 0.05 0.04 13.68 0.17 0.11 4.26 0.06
The freeze-dried, powdered sample (ca. 3 g) was extracted according to a modified Bligh and Dyer method as described previously for sediments (e.g. Talbot et al. 2008; Coolen et al., 2008). Extracts were acetylated as described previously (e.g. Talbot et al., 2008) prior to analysis using atmospheric pressure chemical ionisation-high performance liquid chromatography-multistage mass spectrometry with a ThermoFinnigan LCQ ion trap (e.g. Talbot et al., 2003, 2007a). A semi-quantitative estimate of BHP abundance was calculated from the characteristic base peak ion peak areas of individual BHPs in mass chromatograms (Fig. 1) relative to the m/z 345 ([M+H-CH3COOH]+) base peak area response of acetylated 5a-pregnane3b,20b-diol as internal standard. Averaged relative response factors (from a suite of five acetylated authentic BHP standards) were used to adjust the BHP peak areas relative to that of the internal standard, where BHPs containing one or more N atoms give an average response approximately 12 times that of the standard and compounds with no N atoms give a response approximately 8 times that of the standard.
1
Here, following on from recent reports of bacteriohopanepolyols and hopanoids in geothermal sinters (Talbot et al. 2005; Pancost et al. 2005, 2006), we present data for a suite of ‘intact’ BHP signatures indicative of bacterial populations indigenous to a contemporary geothermal system and relevant to important evolutionary biogeochemical processes. 2. Experimental 2.1. Sample
3. Results
The sinter sample was collected from the Opaheke Pool hot spring (OP;Taupo Volcanic Zone, North Island, New Zealand). The water temperature of the spring is 98 °C and the pH 7.2. The sample discussed here is an inactive sinter collected ca. 50 cm from the pool edge.
Studies of the BHP distribution of the sample from the OP hot spring revealed an array of known and novel compounds, including cyanobacterial markers and BHP
Ia m/z 830
1.2
m/z 844
Relative Intensity
m/z 772
IIIa
0.4
Ic
0.02
m/z 727
IIe Ib
m/z 714
1 100
m/z 728
IIb
0.7
m/z 655
Id
21
m/z 669
IId
0.3
Retention Time Fig. 1. Partial mass chromatograms (15–27 min) showing bacteriohopanepolyol distribution in Opaheke Pool sinter sample; m/z values refer to derivatised (peracetylated) base peak ion (see Appendix for structures).
1022
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023
signatures corresponding to Type I and Type II methaneoxidising bacteria. 3.1. Possible cyanobacterial BHP signatures in geothermal vent sinters A total of five BHP structures (Ib, Id, IIb, IId, IIe), all of which have been found in cultured cyanobacteria (e.g. Talbot et al., 2008 and references therein), were identified in the sinter material (Fig. 1). Bacteriohopanepentol (Ie) and 2-methyl bacteriohopanetetrol (IIe) are known to be synthesised by Nostoc cyanobacteria (Bisseret et al., 1985). The presence of two non-methylated tetrafunctionalised BHPs (Ib, Id), known to be biosynthesised by a plethora of bacterial species including cyanobacteria (e.g. Rohmer, 1993 and references therein) in environmental samples, together with the more diagnostic C-2 methylated homologues (IIb, IId), can be used to infer the presence of a cyanobacterial population. It should be noted, however, that IId has recently also been reported from two strains of an anoxygenic phototroph (Rashby et al., 2007). 3.2. Signatures of methanotrophic bacteria in geothermal vent sinters The BHP distribution in the sinter included compounds relating to Type I methane-oxidising bacteria (e.g. Talbot et al., 2001; Ia and IIIa). The presence of the 3-methylhopanoids in sediments can be indicative of an oxic–anoxic transition in aquatic systems (e.g. Farrimond et al., 2004; Blumenberg et al., 2007); however, the full implications of the presence of 3-methylaminobacteriohopanepentol (IIIa) and aminobacteriohopanepentol (Ia) in this geothermal setting have yet to be assessed. Also detected in OP sinter was aminobacteriohopanetetrol (Ic), a compound produced by both Type I and Type II methanotrophs (e.g. Talbot et al., 2001). Due to the absence of a C-3 methylated counterpart, indicative of Type I methanotrophs, the presence of this lipid is tentatively assigned to a Type II methanotrophic precursor (e.g. Talbot et al., 2001 and references therein).
4. Discussion BHPs containing an additional methyl on the A ring (II, III) provide characteristic features with which to investigate both present and past bacterial populations. An abundance of C-2 methylated hopanoids in ancient sediments is used as a proxy for aerobic metabolism in ancient ecosystems as BHPs methylated at C-2 are diagnostic of cyanobacteria (e.g. Talbot et al., 2008) with corresponding diagenetic products known to survive in sediments for up to 2.7 billion years (Brocks et al., 1999; Summons et al., 1999). BHPs containing an additional methyl at C-3 are known to be synthesised by methanotrophic bacteria (Ia and IIIa) and some acetic acid bacteria (e.g. IIIf and IIIg; e.g. Talbot et al., 2007b). Type I methanotrophs are the only known source of C-35 amino, hexa-functionalised and C-3 methyl-
ated, hexa-functionalised BHPs. The presence of these compounds and corresponding diagenetic derivatives in environmental matrices is used as an indicator of aerobic methanotrophic metabolism and can be used to infer palaeoenvironmental and palaeo-redox conditions in ancient sediments. The variety of structures detected in sinter material from OP shows the potential for BHPs as biomarkers for bio- and geochemical processes occurring in situ within a geothermal environment. However, the assignment of BHPs to a specific biological source is complicated, as interpretations of bacterial sources are limited to those bacteria which have been tested for production of BHPs. The majority of these organisms are aerobic and, although BHPs have been detected in a number of bacterial cultures grown under microaerophilic and anaerobic conditions (e.g. Blumenberg et al., 2006 and references therein), BHPs and hopanoids are still generally regarded as markers for oxic depositional conditions. The presence of cyanobacterial and, in particular, methanotrophic bacterial BHP signatures in sinters from OP suggests that sinter formation occurs at the oxic/anoxic boundary and is consistent with the findings of Mountain et al. (2003). Ongoing analysis of the distribution, abundance and preservation of bacterial lipids in sinters from Opaheke pool, and other geothermal areas from the Taupo Volcanic Zone, has shown that there remains a significant source of novel precursor membrane lipids indicative of uncharacterised organisms, undetermined BHP producers or novel biochemical pathways. The presence of BHPs in modern sinters is assumed to be indicative of biota colonising the waters and the vicinity of the geothermal pool. The long term fate of these intact lipids in sinter material has yet to be determined; however, the extent of molecular complexity retained upon deposition and incorporation into the silica matrix seems to promote preservation of organic material.
5. Conclusions Analysis of the distribution of bacterial membrane lipids, bacteriohopanepolyols (BHPs) in geothermal silica sinter has shown the presence of diagnostic signatures of cyanobacterial and methanotrophic bacterial populations. This, to our knowledge, is the first report of methylated BHPs and only the second report of non-methylated, polyfunctionalised BHPs (Talbot et al., 2005) preserved in geothermal silica sinters, which seem to enhance preservation, and would indicate that distributions of lipids in sinter depositions provide a useful palaeobiological tool. Acknowledgements The authors would like to thank the NERC for funding (RAG) and the Science Research Infrastructure Fund (SRIF) from HEFCE for funding the purchase of the Thermo Electron Finnigan LCQ Ion Trap Mass Spectrometer. We thank two anonymous reviewers for helpful comments and suggestions that helped to improve the manuscript.
R.A. Gibson et al. / Organic Geochemistry 39 (2008) 1020–1023
1023
Appendix Structures referred to in the text. Note: compounds are analysed as peracetate derivatives but parent compounds are shown for simplicity.
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