Amino acid composition of organic matter associated with carbonate and non-carbonate sediments

Geochimica et Cosmochimica Acta Vol. 42. pp. 1131 lo 13X 0 Pergamon Press Ltd.1978.Printed inGreatBritain

Amino acid composition of organic matter associated with carbonate and non-carbonate sediments* PAUL W. CARTERand RICHARD M. MITTERER Program in Geosciences, The University of Texas at Dallas, P.O. Box 688. Richardson, TX 75080, U.S.A. (Received

13 April

1977:

accepted

in

revised

form

6 April

1978)

Abstract-Amino acids comprise from I5 to 36% by weight of humic substances from carbonate and non-carbonate sediments. Humic and fulvic acids extracted from carbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids, aspartic and glutamic acid. Humic substances from non-carbonate sediments have a distinctly different amino acid composition consisting primarily of glycine and alanine. Amino acid analyses of various molecular weight fractions of fulvic acids extracted from carbonates show that lower molecular weight fractions have appreciably higher relative abundances of the acidic amino acids compared to higher molecular weight fractions. Based on typical values for carboxyl group content in humic substances, acidic amino acids may be a significant contributor of these functional groups. Carbonate surfaces appear to selectively adsorb aspartic acid-enriched organic matter while non-carbonates do not have this property. INTRODUCTION

ALKALINEextractable organic matter, operationally termed humic substances, constitutes the bulk of extractable organic matter in soils, marine sediments and seawater. Although humic substances in soils have been a subject of much study, primarily concerned with structure and functional group content, marine humic substances have received lesser attention. Various investigators have noted the presence of amino acid compounds in humic substances. Apparently, as much as IO-SO% of the humic substances in recent sediments and soils is acid hydrolyzable and consists largely of proteins and other nitrogenous compounds (BREMMER,1955; RASHID, 1972). Despite this abundance, however, conspicuously little attention has been directed to the amino acid composition of marine humic substances or to the adsorption of this organic matter by mineral surfaces. In particular, humic substances may play an important role in the precipitation and dissolution of calcium carbonate in the shallow marine environment. Organic matter may occur in carbonate sediments as particulate organic matter or it may be directly associated with mineral grains, either as an adsorbed fraction on grain surfaces or as an internal skeletal matrix. The role of adsorbed organic matter is widely presumed to be that of an inhibitory agent in carbonate precipitation (CHAVE, 1965; CHAVEand SUES, 1970). However, there are some data to suggest that organic matter adsorbed onto sediment grains does not entirely play the passive role of merely coating a carbonate grain with an unreactive sheath. SUESS (1970, 1973) demonstrated that carbonate grains preferentially adsorb surface-active organic matter from seawater and SUESSand F~TTERER(1972) precipitated *Contribution No. 330, Program in Geosciences. The University of Texas at Dallas.

artificial ooids in the presence of humic extracts from sediments. Organic matter incorporated within natural ooids was shown to contain proteinaceous material and the composition of this organic matter is strikingly similar to that of skeletal carbonates with a predominance of aspartic acid (MITTERER, 1968, 1972; TRICHET,1968).

The purpose of this study is to examine the amino acid composition of organic matter associated with modern carbonate sediments. Because alkaline extractable organic matter may comprise a significant portion of adsorbed organics, humic substances from both carbonate and non-carbonate sediments are included. In addition, amino acid analyses together with organic carbon, carbonate carbon and nitrogen were performed on various grain sizes of two sediments, one essentially a pure carbonate and the other predominantly carbonate but with a significant noncarbonate component in the finer fractions. MATERIALS AND METHODS Sediment

samples

Carbonate sediment samples were obtained from the following localities in the Florida Keys and Florida Bay region: (I) Whale Harbor; (2) Cross Bank; (3) Coupon Bight; (4) Hawk Channel; and (5) Bahia Honda Beach. The sediment distribution in the region has been presented by GINSBURG (1956). The samples were collected by grab sampling the upper 1&20cm of bottom sediment. Sediment samples were stored in plastic bags and kept cool until they could be frozen. Additional grab samples of carbonate sediment were obtained from the West Flower Garden reef complex (offshore Texas) in the Gulf of Mexico (EDWARDS. 1971). These sediments consist predominantly of coral debris with a significant terrigenous content in the finer grain sizes. Therefore, sufficient differences in sediment composition should exist between coarse and fine-grained fractions. In addition. non-carbonate sediments were collected from Mexico Beach, Florida. These consist predominantly of clean, well-sorted. quartz beach sand.

1231

P. W. CARTERand

I232

Wet-sieving was used to separate grains larger than 35 pm. Standard settling separation in distilled water was used for smaller grains. Size-fractionated samples were airdried and stored in a desiccator. The amino acid composition as well as organic carbon, nitrogen and carbonate carbon were determined for these size fractions. Analytical procedures Extraction pf hurnic substances. Humic pfus fulvic substances were extracted with 0.1 N NaOH under nitrogen for 24 hr. The ratio of sediment to NaOH was maintained at approx I :lO fur atI samples. Constant agitation was provided by phcing the extraction vessels on a shaker at 30 cycles per min. Humic acids were precipitated from the extract by acidification to pH 2 and purified by repeated solution, precipitation and centrifugation, and finally freeze-dried. The fulvic acid remaining in solution was dialyzed and freeze-dried. 0rganic matrix material from Bahamas ooids was obtained by dissolving approx 100 grams of ooids in dilute HCl. The solution was adjusted to pH 7 and dialyzed by ultrafiltration as described below. Carhon. Both organic carbon and carbonate carbon analyses were obtained on a custom-made analyzer similar to that described by BUSH(1972). In general, about 200 mg of dried sediment was required for organic carbon analysis and only about SOmg for carbonate carbon. Nitrogm. Approximate& l@fmg OF dried sediment was weighed and analyzed for nitrogen as ammonia by an ion snecific electrode fOrion Model No. 95-10) after dinestion in concentrated sulfuric acid with a selenium catalyst. The electrode manufacturer’s instructions were followed in the measurement of nitrogen as ammonia. Amino acids. Amino acid analyses were abtained on a custom-DDE analyzer similar to that described by HIRE (1949). For total hydrolysis, approx 10 mg of humic material was weiahed and hvdrolvzed for 22 hr with 6 N HCl. All hydrolyies were cairied *out at 110°C in sealed tubes under nitrogen. Sediment samples were dissolved in sufficient 12 N HCI to yield a finat concentration of 6 N. After hydrolysis. calcium ions were removed by precipitation with HF. M&c&r we&&r~~tja~atjan~ An uhraf%ration system was used to separate dissotved organic matter (fulvic acids) into various molecular weight fractions using a variety of membrane filters (Burr, 1971; Sknnt, 1976). These membranes do not yield quantitative fractionation of the dis-

R. M. MITTERER soived organic matter on the basis of absolute molecular weights but their retentive capacity is suitable to separate proteins into reasonably accurate molecular weight ranges. Membrane filters also have the advantage of dialyzing dissolved organics without serious loss of low molecular weight material. An Amicon mode) 402 Ultrafiltration Systern was employed using UM-05, UM-2, PM-10 and XM-50 ultrafilters. The fractionation procedure was begun with the membrane having the highest molecular weight cut-off to avoid membrane clogging.

Values for organic carbon, total nitrogen and carbonate carbon for various grain sizes of Hawk Channel (Florida Keys) and West Flower Garden reef (Texas shelf) sediments are listed in Table 1. A significant aspect of these sediments is the change in carbonate content in different grain sizes. In grain sizes greater than lOOpm, both sediments are predominantly CaCO, as evidenced by the CO2 values. In grain sizes less than loOnm, however, carbonate CO2 decreases to Iess than looi, in Flower Garden sediments whereas in Hawk Channel sediments the decrease is much less. In Flower Garden sediments the large decrease in CO2 values in grain sizes less than 1OOpm reflects the influence of continental debris, siliceous bioclastic material and. to some extent, an increase in organic matter. Sediments from Hawk Channel do not exhibit such a decrease primarily because of the lesser influence of terrigenous sediment. The organic carbon and totai nitrogen of Hawk Channef sediments ranges between 0.5 and 1.3% and between 1%05and O.XE& respectively, depending on grain size. For the same grain sizes, carbon and nitrogen contents of sediments from the West Flower Garden reef complex ranges between 0.3 and 1.6% and between 0.035 and 0.14%. respectively. The in-

Table 1. Organic carbon, nitrogen and carbonate carbon in various grain size fractions of Hawk Channel and Flower Garden sediments

_ Sample

Grai;)S$zc

Nawk Channel

F1~~a~de~

Shelf)

~2

2-20 20-36. 36-73 73-102 102-153 153-202 202-427 - __ _ 427-602 >602

Organic carbon (%I :*z

1:21

0.73 0.62 0.58

x_.__ ii::;

Nltrogen (%I

8.49

$06: ok6 0.057 _.__. 0.052 0.053

9:8Z 9.82 9.81 9.62

35.2 36.0 38.2 40.5 41.5 41.8 41.5 41.9 42.0 41.8

13.01 ;::ii 10.00 8.25 8.12

IF!: 13:o 21.4 36.5 41.0

8.61

42.0

0,188

%i

20-36 36-73 73-102 102-153

1.98 1.56 0.98 0.30 0.33 0.30

0.153 0.122 0.083 Il*x::

153-202

0.31

0.036

0.26 It%

Carbonate carbmfC02)

8.72 9.03 9.24 f*!!

0.212

~2 2-20

202-427 427-W .6@2

C/N

0:037 0.031 6.037 0.036

8.39 7.67 8.57

42.3 42.1 41.9

1233

Organic matter-sediments associations Table 2. Amino acid composition, in residues per thousand, of various grain size classes Amino AcIda

Grain Sire (uml

ASPC

THR

SER

GLU'

PRO

GLV

ALA

68 ;: 67

101 ;:, 96

17 24 10 21

165 202 165 211

;: 1::

66 57

111 110

45 36

219 209

tX

60 62

100 106

40 44

214 203

::

::

E97

45 53

193 191 194

ii 91

39

15 17 11 21 24

282 265 251 283 293

145 147 132 121 107

VAL

MET

ILE

LEU

TVR

PHE

LVS

AR6

Totalb (uW91

Hawk Channel 210 10-20 20-35 35-73 73-102 102-202 202-253 253-427 427-602 ~602

209 194 182 180 186 169 206 220 228 227 221

70

106

47

30 79.73

29 75.20 38 73.96 33 4G.00 31 24 26 31 23 29 22

39.50 28.68 17.32 16.18 11.04 12.09 11.36

13 30 30 38 40 52 31 27 32 30 27

15.67 13.98 12.51 10.63 5.84 6.93 6.50 6.88 5.80 4.73 5.25

Flower Garden ;'10

63

10-20

::

20-35 35-73 73-102 101-202 202-253 253-427 427-602 a602

!! 191 246 238 270 270 269

:: 82 88

ii

:: 36

43

61

1::

34

193

94

59 64

120

31 36

170 172

;:

z;:

K

27 28

155 153

1::

53

117

56

144

59 56 i5f it 39 34 ii 32

96

*Amino acid abbreviations are: ASP, aspartic acid; THR, threonine; SER. serine; GLU, giutamic acid; PRO, prohne; isoleucine;LEU, Ieucine; TYR, tyrosine; PHE, phenyl&nine; LYS, lysine; ARG, arginine. bpmoles amino acids per gram of sediment. ‘Includes asparagine and glutamine.

GLY, glycine; ALA, alanine; VAL, valine; MET, methionine; ILE,

in organic carbon with decreasing grain size is similar for both Hawk Channel and Flower Garden sediments. Such an increase could be attributed to co-sedimentation of particulate organic matter with small mineral grains or to enhanced surface adsorption of organic matter because of the greater surface area of fine sediment grains. SUES!+(1973) demonstrated that the increase in organic carbon and nitrogen with decreasing grain size is linearly related to the surface area of mineral grains. He further noted that carbonate grains are capable of adsorbing about 1.2 mg C and 0.175 mg N for every square meter of carbonate surface. Suess’ data for organic carbon and nitrogen for various grain sizes of carbonate sediment are similar to those reported here. However, this does not preclude the possibility that particulate organic matter could contribute significantly to the finer grain sizes. The relationship between carbon and nitrogen has often been used as an approximation of the nature of organic matter. Generally, organic matter with a low carbon-to-nitrogen ratio is considered to be relatively undegraded and rich in protein. On this basis, it appears from data in Table 1 that, with decreasing grain size, organic matter associated with Hawk Channel sediment becomes slightly more proteinaceous (C/N decreases) while organic matter associated with Flower Garden sediment becomes less proteinaceous or is more degraded (C/N increases). crease

Amino acids Amino acids comprise between 11.4 and 79.7 mic-

romoles of amino acids per gram bM/g) of Hawk Channel sediment and between 5.25 and 15.7pM/g of Flower Garden sediment, depending on the grain size (Table 2). Larger grain sizes contain the lowest concentration of total amino acids and finer grain sizes contain the highest. In the very large grain sizes the composition reflects that of the skeletal fraction. Molluscan debris is the primary constituent of the coarser sediment from Hawk Channel, whereas coral debris is predominant in the coarser sediment from Flower Garden. The increase in amino acid content with decreasing grain size, although observed for both sediments, is much greater for the pure carbonate sample. This increase is not an indigenous feature of the skeletal materials but is, rather, related to the increase in organic carbon content of these sediments. As the organic carbon increases with decreasing grain size it seems reasonable to expect the amino acid content to do likewise. However, the situation is much more complex. In grain sizes above approx lOOpm, both the total amino acid content of the sediment and the proportion of organic carbon as amino acids are relatively constant. For finer grain sizes in Hawk Channel, the proportion of organic carbon as amino acids increases. Conversely, in the finer grain sizes in Flower Garden the proportion decreases. Hence, the amino acid concentrations are in agreement with carbon and nitrogen values and further support the suggestion that organic matter associated with finegrained carbonates is more proteinaceous than organic matter associated with non-carbonates. A striking feature of these amino acid analyses is

1234

P. W. CARTER and R.

the different relative concentrations of amino acids in different grain size fractions (Table 2). Aspartic and glutamic acid are predominant in the larger grain sizes in both sediments, which is a reflection of the skeletal organic matrix of these larger grains. The relative concentration of amino acids in Flower Garden sediment varies considerably depending on the grain size. Most significant. perhaps, is the dramatic decrease in the acidic amino acids. aspartic and glutamic acid. with decreasing grain size. At approximately 100 pm there is a rather abrupt change from aspartic acid-rich organic matter to organic matter characterized by an amino acid composition dominated by glycine. The acidic amino acids decrease from about 39% to loo/, of the total amino acids in Flower Garden whiIe giycine and alanine, which comprise only about 23% of the total amino acids in the largest grain size, increase to 43% in the finest grain size. These results are in marked contrast with the relative distribution of amino acids in Hawk Channel sediment where the acidic amino acids predominate throughout and there is no significant trend with grain size. Evidently, the decrease in the relative abundance of aspartic acid in FIower Garden sediment coincides with the decrease in carbonate content. Hence. not only is the organic matter associated with carbonates more proteinaceous than organic matter associated with non-carbonates, but it also has a distinctly different amino acid composition. Furthermore. it appears that even within the same Table 3. Amino

M. MITERER

depositional environment there exists a basic difference in the organic matter associated with the carbonate and non-carbonate sediments. HARE (1972) obtained similar results for the carbonate and noncarbonate fractions of a deep sea core. Adsorption of organic matter onto carbonate surfaces as noted by Su~ss (1973) may be a reasonable explanation for the trends observed in these sediments. The fact that the organic carbon consists of in~easin~y higher relative proportions of amino acids with decreasing grain size is suggestive of seiective adsorption of amino acid-enriched organic matter by carbonates. In addition, selective association of organic matter enriched in the acidic amino acids is indicated since these amino acids are largely responsible for this increase. The opposite trend observed for fine-grained fractions from Flower Garden suggests that non-carbonates do not have the abiiity to selectively adsorb acidic amino acid-enriched organic matter. Thus, these data support the previous suggestion that aspartic acid-enriched organic matter is selectiveiy associated with carbonate grains (MITTERER, 1972). Amino acid composition acid (FA)

of humic acid (HA) and fulvic

In an effort to learn more about the composition of the extractable surface component of the organic matter and to compare its composition with that obtained from the total sediment, HA and FA,

acid composition. in residues per thousand. of humic acids (HA) and fulvic acids (FA) Amino Acid

ASPb

THR

SER

200

53 70 49

MET

Total (un/~)

GLUb PRO

GLY

ALA

VAL

60

117

2;

y;

28 19 30

125 131

110 95 91

77 76 SR

20 10 19

2.605 1.533 2.028

:: 19 23 la

x33 1:330 2.687 1.919

ILE

LEU

TYR

PHE

LYS

ARG

Carbonate mud Hawk Channel HA Hawk Channel FA Whale Harbor HA Whale Harbor FA Coupon Bight HA Coupon Bight FA Cross Bank HA Cross Bank FA

99 45 it23 ii Iii

:z 255 174 223 205 289

85 39 46 43

73 49 58 36

101 120 127 146

25 25 28 24

155 144 147 135

106 87 88 79

64 63 59 70

267

49 56

65 52

112 144

20 23

113 135

95 119

09 69

50 32

92 34

15 12

2.741 1.178

143 129 133 113

59 54 89 65

60 82 79 66

101 86 129 130

36 19 29 28

238 247 120 135

93 118 105 99

37 61 56 68

39 22 50 58

64: 103 102

17 38 15 35

1.719 1.489 1.871 1.770

128 146

72 58

74 67

95 109

34 30

177 180

126 119

72 64

:z

67:

42 52

0.448 0.326

127 165

72 57

61 77

126 137

33 21

139 126

93 104

64 55

79 54

21 19

2.708 1.832

ii

85 ii

Carbonate sand Bahia Honda HA flahiaHonda FA Plant debris Hawk Channel HA Hawk Channel FA Thalassla HA Thalassla FA Non-carbonate mud Pausacaco

Pausacaco

Pond H& Pond FA

Non-carbonate sand Mexico Beach HA W=exicoBeach FA

“ymoks of amino acids per mg of HA or FA. “Inctudes asparagine and g~utamin~.

103 91

Organic matter-sediments associations extracted from a variety of both carbonate and noncarbonate sediments, were analyzed for amino acids. Because plant debris was observed to comprise a visible portion of the carbonate mud, HA and FA from plant debris were also analyzed for amino acids. The amino acid composition of these samples is tabulated in Table 3. Except for the comparatively low values for non-carbonate mud, all values for total amino acid content are quite similar, varying only from 1.4 to 2.7pM/mg of humic material. On a dry weight basis, amino acids comprise 15-20% of fulvic acids and 27-36x of humic acids from carbonate sediments. These values are only slightly higher than those reported for humic substances from soils (PIPER and POSNER, 1968) and marine sediments (RASHID, 1972). It is of interest to note that, on the average, humic acids contain half again as much amino acid material compared to corresponding fulvic acids. The reason for this is unclear. The amino acid composition of adsorbed humic substances is strikingly different in carbonate sediments compared to non-carbonate sediments. The acidic amino acids comprise about 30 and 40% of the total amino acids in HA and FA, respectively, from carbonate sediments. The amino acid distribution, particularly that for fulvic acid, is similar to that reported for the organic matter occurring in both biogenie and non-biogenic marine carbonates (MITTERER, 1968, 1971, 1972). It could be argued, of course, that humic material extracted from carbonate sediments is heavily influenced by the internal proteinaceous matrix. This is considered unlikely since the matrix has been shown to be relatively inaccessible to comparable NaOH treatment (MITTERER, 1972). This amino acid distribution is not observed for the FA from non-carbonates. Acidic amino acids comprise only about 25% of the total amino acids in extracts from non-carbonate sediments and plant debris. The distribution of amino acids in the non-carbonate and plant fractions is similar to that reported by JONES and VALLENTYNE (1960), DEGENSet al. (1964), DEGENS (1970X EMERYet al. (1964) and MOPPERand DEGENS (1972) for marine sediments, by SIEGEL and DEGENS (1966), DEGENS(1970) and LEE and BADA (1975) for seawater, and by RASHID(1972) for marine humic substances. Evidently, carbonate sediments contain adsorbed humic substances with an amino acid composition very similar to those of calcifying organic matrices, while non-carbonates contain adsorbed organic matter with a lesser aspartic acid content. Experimental evidence for this selective adsorption is presented by CAR’IER(1978). The relative concentrations of individual amino acids differ considerably between HA and FA. The most striking difference is the greater relative abundance of acidic amino acids in FA compared to HA from both carbonate and non-carbonate sediments. Only in plant debris is the relative abundance of acidic amino acids greater in HA than in FA. Plant debris represents relatively fresh organic matter and

1235

the amino acid composition of HA and FA primarily reflects the plant biochemistry. These results suggest a more fundamental difference between HA and FA in contrast to the view that HA is simply a larger, more highly condensed version of FA (NISSENBAIJM and KAPLAN, 1972). We suggest that HA, at least in recent carbonate sediments, may represent only partially degraded biopolymers, whereas FA represents organic matter that is further degraded or hydrolyzed. Conversion of larger, more insoluble proteinaceous organic molecules to smaller, more soluble compounds is accomplished by hydrolysis of peptide bonds. The aspartic acid peptide bond is more easily hydrolyzed than are most other peptide bonds (PARTRIDGE and DAVIS, 1950; SCHULTZ et al., 1962). Hence, partial hydrolysis of the HA of plant debris may result in a lower molecular weight fraction (FA) enriched in acidic amino acids and a residue, of larger molecular weight (HA), relatively depleted in acidic amino acids. Amino acid composition of various molecular weight fractions

Ultrafiltration of fulvic acid solutions using a variety of membrane filters separated some of the extracts into a range of molecular weight fractions. The relative distribution of amino acids in these molecular weight fractions is illustrated in Fig. 1. In all three samples, the acidic amino acids show a pronounced relative increase with decreasing molecular weight while the other amino acids, with the exception of lysine, show a decrease. The total amino acid content is similar for all molecular weight fractions (Fig. 2). The only previous report of amino acid distributions in different molecular weight fractions of humic substances is by PIPER and POSNER (1968). They found that aspartic acid was about twice as abundant in lower molecular weight material compared to the larger molecular weight fraction from a soil humic acid. The fractionation results are in agreement with those presented above regarding the difference between the aspartic acid content of humic acid and fulvic acid (Table 3). Apparently the extractable humic material of marine sediments consists of a mixture of organic compounds with a variety of molecular weights ranging from about SOOMW to greater than 50,000 MW. Although these fractions have different amino acid compositions there is a trend toward increasing acidic amino acid content with decreasing molecular weight. This trend, if present in organic matter from other sedimentary environments, has broad implications with regard to the reactivity of low molecular weight organic matter. Recent studies have revealed that the major oxygen-containing functional groups, and in particular carboxyl groups, are concentrated in low molecular weight organic matter (RASHIDand KING, 1971). Evidence of the greater reactivity of low molecular weight organic matter has been provided by studying

1236

\fJl

JI

300

200

too

AS

A

(b)

)E

’

ABCDE

ABC DE

THR

SE R

Al 3CDE GILU

ABCDE

AEICDE

ABCCi

GLY

A LA

VAL

ILE

i

LEU

LYS

ARG

F

1 E

(cl

ABC0

-

ABCDE ALA

AECDE VAL

AWEE

ABCDE LEU

APCDE LYS

AiEIGE

300-

200-

100 -

BCDE UA

Fig. 1. Histograms of the relative amino acid composition, in residues per thousand, of various fulvic acid molecular weight fractions of organic matter from carbonate sediments: (la) Coupon Bight mud; (lb) Bahia Honda Beach sand; (Ic) organic matrix from Bahamian ooids. Molecular weight range: (A) total; (B) 500-1000; (C) lOO@-10,000;(D) 10,000-50,000: (E) > 50,000.

1237

Organic matter-sediments associations .

*W,HAMIAN

to concomitant increases in the relative abundances of acidic amino acids.

OOLITES

ii =COUPON BIGHT MUD Ir ‘BAHIA HONDA REACH SAN0

SUMMARY AND CONCLUSIONS -4__

ct-----

---__

)--__-__.

--b-------*______p 2:‘--- --__

-*__

-+-----

____ -1

01

0

1

1

!500-loo0 MOLECULAR

Fig.

2.

I

IOOO-lop30

J

‘x%cm

l0,ooo-5cu3oo

WEIGHT

FRACTION

Total amino acid content of fulvic acid molecular weight fractions.

the association of various metals with different molecular weight fractions (RASHID,1971;ANDRENand HARRIS, 197.5). For example, the lower molecular weight fraction of aquatic humus contains 5-20 times more Ga2+ compared to the higher molecular weight fraction (GJESING, 1970). Our results for the relative enrichment of aspartic and glutamic acid in lower molecular weight organic matter suggest a possible correlation between the acidic amino acid content and carboxyl group content. Typical values for carboxy1 group content in high and low mol~lar weight fractions along with carboxyl group contribution from acidic amino acids are listed in Table 4. Although the data are not directly comparable, it appears that acidic amino acids may account for a minimum of 15-30% of the total carboxyl groups present in recent humic substances. Differences observed in carboxyl group content in lower molecular weight fractions are probably due, at least in part,

Ammo acid analyses of various grain sizes of two sediments, one consisting of essentially pure carbonate and the other carbonate plus a si~ifi~nt non~ar~nate component, demonstrate that carbonates contain a distinctly different type of organic matter compared to non-carbonates. Organic matter associated with fine-grained carbonate sediment has an ammo acid composition similar to that in skeletal carbonates, consisting primarily of aspartic and glutamic acid, and with a high proportion of the organic carbon as amino acids. Organic matter associated with the non-carbonate fraction of these sediments has an amino acid composition consisting primarily of glycine and alanine with a comparatively lower proportion of the organic carbon as amino acids. In general, humic substances extracted from both carbonate and non-carbonate sources contain from 15-36x amino acids by dry weight. Humic acids contain, on the average, half again as much amino acid material compared with corresponding fulvic acids from the same source. Humic substances from earbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids. Humic substances from non~rbonate sediments have a distinctly different amino acid composition, comprised primarily of glycine and afanine. Amino acid analyses of various molecular fractions of fulvic acids from carbonate sediments show that the lower molecular weight organic matter contains appreciably higher relative abundances of the acidic amino acids and lower relative abundances of the neutral amino acids, compared to higher molecular weight organic matter. Plant debris, non-carbonate sediments and seawater all have a lower relative aspartic acid content

Table 4. Carboxyl group content in high and low molecular weight fractions of various humic substances High mol. wt. - Low mol. wt. Carboxyl

groups

(met/g)

Cariaco Trench HA"

2.0

-

4.0

Lagoonal HA"

2.0

-

4.5

Scotfan Shelf ILAb

2.0

-

3.0

Acidic amino acfd carboxy gmups (meq/g)d Bahia Honda FAc Coupon

Sfght FAc

Bahamian oolftes FA'

0.5

”

0.6

0.5

-

0.7

0.6

-

1.0

‘RASHID (1971).

‘RASHID and KING ‘This paper.

(1971).

dRepresents total aspartic acid pius glutamic acid carboxyi groups not involved in peptide linkage.

P. W. CARTERand R. M. MITTERER

1238

than the organic matter associated with carbonates. Therefore, the obvious sources of the aspartic acidrich organic matter, whether indirectly from seawater or perhaps dire&y from plant debris, are deficient in this fraction. Although apparently only a minor component in the marine environment, the aspartic acid-rich fraction nevertheless ia adsorbed to carbonate surfaces in significant amounts. Carbonate surfaces appear to selectively adsorb aspartic acidenriched organic matter, while non-carbonates do not have this property. This selectivity is probably a consequence of the structural similarity between carbonate anions and carboxyl groups. Acknowledgemenr-This work was supported by NSF Grant GA-30695. We are grateful to Dr. W. L. ORR for providing the Pausacaco Pond sediment. REFERENCES ANDRENA. W. and HARRW R. C. (1975) Observations on the association between mercury and organic matter dissoived in natural waters. Geochim. Cosmochim. Acra 39, 12.53-1257.

BLATSW. F. (1971) Membrane partition chromato~aphy: a tool for fractionation of protein mixtures. Agr. Food Chem. 19. 589-594. BREMMERJ. M. (19.55) The chemical nature of humic nitrogen. J. Agric. Sci. 46, 247-256. BUSH P. R. (1972) A rapid method for the determination of carbonate carbon and organic carbon. Chem. Geol. 6. 59-62. CARTERP. W. (1978) Adsorption of amino acid-containing organic matter by calcite and quartz. Geochim. Cosmochim. Acta 42. lug-1242. CHAVEK. E. (1965) Carbonates: association with organic matter in surfa& seawater. Science 148, 1723-1724: CRAVE K. E. and SUESS E. (1970) Calcium carbonate saturation in seawater: effects of dissolved organic matter. Litnnof. Oceanogr. IS, 633-637. DEGENS E. T. (1970) Molecular nature of nitrogenous compounds in seawater and recent sediments. In Organic matter in Natural Waters (editor D. W. Hood), pp. 77-106. Inst. Mar. Sci.. U. of Alaska. Pub. No. 1. DEGENSE. T., REUTERJ. H. and SHAW K. N. F. (1964) Biochemical compounds in offshore California sediments and sea waters. Geocbim. Casmoch~in. Acta 28. 45-66. EDWARDSG. S. (1971) Geology of the West Flower Garden Bank. TAMU-SG-71-215. Texas A & M Universitv. Sea Grant Program. EMERYK. 0.. STII-J C. and SALTMAN P. (1964) Amino acids in basin sediments. J. Sediment Petrol 34. 433-437. GINSBURGR. N. (1956) Environmental relationships of grain size, Florida carbonate sediments. B&t. Am. Assoc. Petrol.

Geo~~~s?s 40, 2384-2427.

GJE~~~NG E. T. (1970) Ultrafiltration of aquatic humus. $5.

& Tech.‘4.

Env.

437-438.

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