Carbon isotope effects associated with mixed-acid fermentation of saccharides by Clostridium papyrosolvens

In anoxic environments, microbial fermentation is the first metabolic process in the path of organic matter degradation. Since little is known about carbon isotope fractionation during microbial fermentation, we studied mixed-acid fermentation of different saccharides (glucose, cellobiose, and cellulose) in Clostridium papyrosolvens. The bacterium was grown anaerobically in batch under different growth conditions, both in pure culture and in co-culture with Methanobacterium bryantii utilizing H₂/CO₂ or Methanospirillum hungatei utilizing both H₂/CO₂ and formate. Fermentation products were acetate, lactate, ethanol, formate, H₂, and CO₂ (and CH₄ in methanogenic co-culture), with acetate becoming dominant at low H₂ partial pressures. After complete conversion of the saccharides, acetate was ¹³C-enriched (α_sacc/ac = 0.991-0.997), whereas lactate (α_sacc/lac = 1.001-1.006), ethanol (α_sacc/etoh = 1.007-1.013), and formate (α_sacc/form = 1.007-1.011) were ¹³C-depleted. The total inorganic carbon produced was only slightly enriched in ¹³C, but was more enriched, when formate was produced in large amounts, as ¹²CO₂ was preferentially converted with H₂ to formate. During biomass formation, ¹²C was slightly preferred (α_sacc/biom ≈ 1.002). The observations in batch culture were confirmed in glucose-limited chemostat culture at growth rates of 0.02-0.15 h⁻¹ at both low and high hydrogen partial pressures. Our experiments showed that the carbon flow at metabolic branch points in the fermentation path governed carbon isotope fractionation to the accumulated products. During production of pyruvate, C isotopes were not fractionated when using cellulose, but were fractionated to different extents depending on growth conditions when using cellobiose or glucose. At the first catabolic branch point (pyruvate), the produced lactate was depleted in ¹³C, whereas the alternative product acetyl-CoA was ¹³C enriched. At the second branch point (acetyl-CoA), the ethanol formed was 15.6-18.6‰ depleted in ¹³C compared to the alternative product acetate. At low hydrogen partial pressures, as normally observed under environmental conditions, fermentation of saccharides should mainly result in the production of acetate that is only slightly enriched in ¹³C (<3‰).     

Fractionation of stable carbon isotopes during anaerobic production and degradation of propionate in defined microbial cultures

In many anoxic environments propionate is, after acetate, the second most important fermentation product, being degraded further to finally result in CH₄ production. In principle, isotope discrimination can be used to assess the path of organic matter degradation to acetate, CO₂ and CH₄. However, nothing is known about the isotope fractionation in primary and secondary fermentation steps involving propionate, although it is an important precursor of acetate. We therefore studied the degradation of propionate with a syntrophic coculture of Syntrophobacter fumaroxidans and Methanobacterium formicicum. The isotope enrichment factor for propionate degradation to acetate, CO₂ and CH₄ was almost negligible (ε_prop 0.9‰). The fermentative production of propionate was studied in cultures with Opitutus terrae growing on pectin, xylan and starch. These polysaccharides were fermented to acetate, succinate, propionate, H₂ and CO₂. While the δ¹³C value of the initially produced propionate was similar to that of the organic substrates (ca. −28 to −25‰), the δ¹³C value of the other fermentation products was higher. The δ¹³C values of all products generally decreased during the course of fermentation. Finally, a small depletion in ¹³C (ca. 6‰) with respect to the organic substrate was observed for propionate, while the other fermentation products where slightly enriched. Overall, stable carbon isotope discrimination was small during both fermentative production and consumption of propionate in the anaerobic microbial cultures, so that propionate turnover probably only marginally affects isotope fractionation during anaerobic degradation of organic matter.     

Adsorption of short-chain organic acids onto nearshore marine sediments

The adsorption of acetate, butyrate, lactate, and stearate was measured using a clastic mud from Cape Lookout Bight N.C. (CLB), a lateritic muddy sand from Kahana Stream, Oahu, Hawaii (KS), and a fine carbonate sand from Waimanalo Beach, Oahu, (WB). Partition coefficients (K_d, moles adsorbed per g of solid phase/moles dissolved per ml of porewater) ranged from 10^2.3 to ≤10^−3.0, and displayed the following trends: CLB > KS > WB, and stearate > acetate ∼- butyrate > lactate. The percent adsorption of the sediment organic acid pools showed similar trends: stearate, 99%; acetate, 9–23%; butyrate, 5–23%; lactate, ≤0.2–7%. These results reflected the relatively nonpolar nature of the sand surfaces in WB and KS sediments, and the polarities of the organic acids. K_d was approximately constant for each organic acid-sediment combination over a dissolved organic acid concentration range of 10⁷, using concentrations between 1M and 10⁻¹⁴ M. This constancy over a wide porewater concentration range suggested that adsorption was not limited by the availability of surface adsorption sites.     

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH)₂) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H₂PO₄⁻ > catechol ≥ HCO₃⁻ > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO₄³⁻, CO₃²⁻, F⁻, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO₄²⁻ and B(OH)₄⁻ with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation.The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01-0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.     

Biodegradation of C-labeled low molecular organic acids using three biometer methods

Low molecular weight organic acids (LMWOA) are produced in soil by various biological and chemical processes and can exhibit substantial metal complexing and dissolution capacity. The reactivity of these compounds in the soil environment is dependent on their non-complexed concentration in the soil solution. Adsorption of LMWOA has been shown to reduce their concentration in the soil solution; however, little is known about the reduction of LMWOA concentration due to microbial degradation. To examine the extent of microbial degradation in reducing LMWOA concentration in the soil solution, three-biometer methods were used: a soil biometer flask, an in-situ field biometer and a soil column biometer. Four soil horizons were used with each method. To each soil sample, 2.0×10⁻⁶ moles of organic acid containing 3.7×10⁴ Bq total activity was applied. The ¹⁴C-radiolabeled aliphatic and aromatic acids studied included oxalic, malonic, succinic, and phthalic acid. Evolved ¹⁴CO₂ was trapped in 0.5 mol l⁻¹ NaOH and measured using liquid scintillation counting. Labeled acids degraded rapidly within the first 5 days for the Ap1, Ap2, and BA horizons, with a generally slower rate of ¹⁴CO₂ evolution being observed for the Bt1 horizon. The % degradation of labeled acid was substantially greater for the soil biometer flask method, compared to the field and soil column biometer methods. The average % degradation for the soil biometer flask was 67% for all soil horizons and organic acids, compared to 14% for the field biometer and 13% for the soil column biometer. Results indicate that substantial microbial degradation of organic acids can occur within a relatively short time period and the biometer method selected can influence the % acid degraded. Based on primary results, the soil column biometer method better approximated microbial degradation under field conditions, as evaluated using the field biometer.     

Fluctuation in proteinaceous labile organic matter verified with degradation rate constants of terrestrial biochemical indicators

Biogenic amino acids, taken as representative of organic matter, were analyzed to determine the apparent degradation rate constant in boreal terrestrial sediment. Age determination using ¹⁴C dating gave two rate constants: the initial degradation rate constant for glycine (k_{GLY 1}), the simplest amino acid, was 1.5 × 10⁻³ yr⁻¹ (r = 0.97) until about 2200 yr BP. After the inflection point, the rate constant k_{GLY 2} was 9.1 × 10⁻⁵ yr⁻¹ (r = 0.73). The degradation of amino acids in the labile organic matter in the sediment was markedly affected by rapid processes. After the inflection point, the rate constant profiles for sub-surface amino acids were shown to have discontinuous relationships with sediment age. One pattern which emerged in the vertical distribution is that the biogenic amino acid degradation rate constant k was far greater in the labile organic matter phase than that in the refractory organic matter over the past 10,000 years.     

Amino acid biogeo- and stereochemistry in coastal Chilean sediments

The spatial distribution of total hydrolysable amino acids (THAA) and amino acid enantiomers (d- and l-forms) was investigated in sediments underlying two contrasting Chilean upwelling regions: at ∼23 °S off Antofagasta and at ∼36 °S off Concepción. The contribution of amino acids to total organic carbon (%T_AAC: 7-14%) and total nitrogen (%T_AAN: 23-38%) in surface sediments decreased with increasing water depth (from 126 to 1350 m) indicating that organic matter becomes increasingly decomposed in surface sediments at greater water depth. Changes in the ratio between the protein amino acid aspartate and its non-protein degradation product β-alanine confirmed this observation. Furthermore, estimates of THAA mineralization showed that sedimentary amino acid reactivity decreased with both increasing water depth as well as progressive degradation status of the organic matter that was incorporated into the sediment. Reactivity of organic matter in the sediment was also assessed using the Degradation Index (DI) developed by [Dauwe, B., Middelburg, J.J., 1998. Amino acids and hexosamines as indicators of organic matter degradation state in North Sea sediments. Limnol. Oceanogr.43, pp. 782-798.]. Off Concepción, DI was successfully applied to examine the degradation status of sedimentary organic matter at different water depths. However, unexpected results were obtained at the Antofagasta stations as DI increased with sediment depth, suggesting more degraded organic matter at the surface than deeper in the cores. The contribution of peptidoglycan amino acids to THAA was estimated from the concentrations of d-aspartate, d-glutamic acid, d-serine, and d-alanine. Peptidoglycan amino acids accounted for >18% of THAA in all investigated samples. In surface sediments peptidoglycan amino acids accounted for a progressively larger fraction of THAA at increasing water depths (up to >26%). Further, the contribution of peptidoglycan amino acids to THAA increased with increased sediment depth and age (up to 288-year-old) reaching up to 59%. Independent estimates based on d-amino acid concentrations in selected laboratory strains, bacterial counts and the sedimentary concentrations of d-amino acids indicate that a large fraction of the measured d-amino acids (>47 to >97%) originated from cell wall residues rather than from enumerated cells.     

Biogeochemical processes involving acetate in sub-seafloor sediments from the Bering Sea shelf break

Biogeochemical processes involving acetate in sub-seafloor sediments from piston core PC23B from the Bering Sea shelf break were inferred by examining the stable carbon isotopic relationships between acetate and other relevant carbon compounds: total organic carbon (TOC) in the sediment solid phase, and dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in pore water. Throughout the core, the isotopic composition of acetate (δ¹³C_acetate), from −31‰ to −29‰, was ¹³C-depleted by ca. 7‰ vs. DOC (δ¹³C_DOC) and its depth profile approximately paralleled that of δ¹³C_DOC, suggesting that the principal process producing acetate was fermentation of dissolved organic compounds. However, the ¹³C depletion in δ¹³C_acetate indicates some contribution of acetogenesis to total acetate production, because acetogenesis results in ¹³C depletion of the acetate produced. The relative contribution of acetogenesis via the H₂/CO₂ reaction, calculated by using a two source isotope mixing model, increased with depth in the sulfate reduction zone from 10% to 15% and was constant at 19% in the methanogenic zone. The acetogenic contribution to acetate production in the methanogenic zone underlying the sulfate reduction zone is consistent with reported observations, whereas the occurrence of acetogenesis in the sulfate reduction zone may be related to the contribution of terrestrial organic matter (OM) to the sedimentary OM in that depth interval, because the terrestrial component likely includes precursors that favor organoautotrophic acetogenesis. The high acetate concentration (up to 81 μM) and TOC content (up to 1.4%) at the same depth (<200 cmbsf) suggest that some relationship exists between acetate production rate and TOC content, or that a temperature increase during core storage at room temperature might stimulate acetate-producing microbial activity in the high TOC sediment.     

Generation of aliphatic acid anions and carbon dioxide by hydrous pyrolysis of crude oils

Two crude oils with relatively high (0.60 wt%) and low (0.18 wt%) oxygen contents were heated in the presence of water in gold-plated reactors at 300°C for 2348 h. The high-oxygen oil was also heated at 200°C for 5711 h. The compositions of aqueous organic acid anions of the oils and of the headspace gases were monitored inn order to investigate the distribution of organic acids that can be generated from liquid petroleum.The oil with higher oxygen content generated about five times as much organic anions as the other oil. The dominant organic anions produced were acetate, propionate and butyrate. Small amounts of formate, succinate, methyl succinate and oxalate were also produced. The dominant oxygen-containing product was CO₂, as has been observed in similar studies on the hydrous pyrolysis of kerogen. These results indicate that a significant portion (10–30%) of organic acid anions reported i be generated by thermal alteration of oils in reservoir rocks. The bulk of organic acid anions present in formation waters, however, is most likely generated by thermal alteration of kerogen in source rocks. Kerogen is more abundant than oil in sedimentary basins and the relative yields of organic acid anions reported from the hydrous pyrolysis of kerogen are much higher than the yields obtained for the two oils.     

Short-chain carboxylates in fluid inclusions in minerals

The carboxylate (formate, acetate, propionate and oxalate) and common inorganic anions (F⁻, Cl⁻ and SO²⁻₄) compositions for aqueous fluid inclusion leachates from 17 mineral samples collected from various deposits have been determined using ion chromatography in conjunction with microthermometric measurements on the fluid inclusions of their host minerals. The minerals, quartz, fluorite, barite, beryl and a few `ore' minerals (wolframite, pyrite and galena), came from hydrothermal vein-type deposits in felsic igneous rocks or Archean metamorphic rocks. The results indicate that short-chain carboxylates are common components in hydrothermal fluids and can be present in considerable amounts. Formic acid (as formate) is the dominant species over other carboxylic acids. The present study raises new questions about the origin and geochemical significance of carboxylates in hydrothermal ore-forming processes.     

Accumulation of short-chain fatty acids in an aquitard linked to anaerobic biodegradation of petroleum hydrocarbons

Investigation of a 17 m vertical profile of a silt and clay aquitard at a natural gas well site in Alberta, Canada revealed a contaminant plume of gas condensate, along with high concentrations of acetate, propionate and butyrate. The pattern of the distribution of these short-chain fatty acids in groundwater and sediment samples suggested that they have been produced by microorganisms in a process associated with degradation of the condensate hydrocarbons. It is suggested that, in certain zones, under water-saturated and/or anaerobic conditions, these acids were actively consumed by SO₄-reducing bacteria. Analyses of DNA extracts by denaturing gel gradient electrophoresis (DGGE) indicated that, compared to sediment samples collected from outside the condensate plume, contaminated samples tended to have fewer, but more strongly developed bands of DNA, which typically had closest affinities to known anaerobes, including species of Fe-reducing Geobacter, and SO₄-reducing Desulfosporosinus.     

The effect of organic matter and oxygen on the degradation of bacterial membrane lipids in marine sediments

The biodegradation of purified radiolabelled membrane lipids from a methanogenic bacterium and a pseudomonad were investigated in mangrove, beach and high marsh marine sediments under aerobic and anaerobic conditions. The effect of organic matter on the amount and rate of degradation was also examined by supplementing beach sediments with humic acids. In aerobic sediments, CO₂ was the major product of lipid degradation while under anaerobic conditions both CO₂ and CH₄ were major end products and the overall rates were reduced (up to 40%) relative to aerobic conditions. Total bacterial numbers increased during all incubations with the largest increases occurring in anaerobic sediments supplemented with humic acids. No lipid degradation occurred in aerobic or anaerobic sediments treated with formaldehyde or autoclaving. In low organic beach sediments, the ester-linked phospholipid of the pseudomonad was degraded much more rapidly than the diphytanyl glycerol diether of the methanogen with 69% of the phospholipid degraded in 96 hours versus only 4% of the methanogen lipid. Lipid degradation in both aerobic and anaerobic sediments was highly correlated to organic matter content with increasing amounts of organic matter inhibiting degradation. Long incubations (75 days) of the diphytanyl glycerol ether resulted in 51% degraded to CO₂ in low (0.5%) organic mangrove sediments while only 9% was mineralized in high (10.8%) organic marsh sediments. Physicochemical sorption of membrane lipids to the organic matrix is proposed as a mechanism which protects membrane lipids from microbial attack and degradation.     

Stable carbon isotopes of HCO3− in oil-field waters—implications for the origin of CO2

The δ¹³C values of dissolved HCO₃^? in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO₂ of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ¹³C values of total HCO₃^? indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.The range of δ¹³C values for the HCO₃^? of these waters is ?20–28 per mil relative to PDB. This wide range of δ¹³C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ¹³C values of HCO₃^? in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ¹³C values (< ?10 per mil) are obtained in waters where concentrations of SO₄^2? are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO₄^2? + CH₃COO^? → 2HCO₃^? + HS^?). The high δ¹³C values probably result from the degradation of these anions by methanogenic bacteria (CH₃COO^? + H₂OaiHCO₃^? + CH₄).Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO₂ and CH₄ is probably the major source of CO₂ for production zones with subsurface temperatures greater than 80°C. The δ¹³C values of HCO₃^? for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO₂ and CH₄. At these high temperatures, δ¹³C values of HCO₃^? decrease with increasing temperatures and decreasing concentrations of these acid anions.     

The geochemistry of dissolved inorganic carbon in a surficial groundwater aquifer in North Inlet, South Carolina, and the carbon fluxes to the coastal ocean

We report measurements of pH, total dissolved inorganic carbon (DIC), total or titration alkalinity (TAlk), Ca²⁺, Mg²⁺, sulfate, and sulfide data at the seawater-freshwater interface in a shallow groundwater aquifer in North Inlet, South Carolina. These measurements and a diagenetic modeling analysis indicate that the groundwaters at North Inlet are mixtures of seawater and freshwater end-members and are seriously modified by carbon dioxide inputs from organic carbon degradation via SO₄²⁻ reduction across the entire salinity range and fermentation and CaCO₃ dissolution in the low-salinity region. DIC and TAlk are several times higher than the theoretical dilution line, whereas Ca²⁺ is slightly higher and SO₄²⁻ is somewhat lower than the dilution line. Partial pressure of CO₂ in the groundwater is extremely high (0.05 to 0.12 atm). These deviations are consistent with theoretical predictions from known diagenetic reactions. Estimated groundwater DIC fluxes to the South Atlantic Bight from either the surficial aquifer (via salt marshes) or the Upper Floridan Aquifer (direct input) are significant when compared to riverine flux in this area.     

Temporal change of C-isotope signatures and methanogenic pathways in rice field soil incubated anoxically at different temperatures

Production of CH₄ and CO₂ was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37°C. The δ¹³C of the accumulated CO₂, CH₄ and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH₄ production. The calculations were further constrained by the determination of ¹⁴CH₄ production from ¹⁴CO₂ at steady state. At 50°C, CH₄ was exclusively produced from CO₂, indicating a fractionation factor of α_CO2/CH4 = 1.073. Between 10 and 37°C, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO₂-derived fraction of CH₄ production in the beginning was followed by a phase in which contribution of CO₂-derived CH₄ decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of ¹³C during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation.     

Volatile fatty acid cycling in organic-rich marine sediments

Volatile fatty acid (VFA) apparent turnover rates were determined by measuring whole sediment VFA concentrations and the corresponding reaction rate constants. The following ranges of VFA concentrations were measured in Cape Lookout Bight, N.C. sediments (μmole·l_s^?1): acetate 54–660, propionate 1–24, butyrate <0.5–22, iso-butyrate <0.5–6. Apparent turnover rates measured over a one-year period ranged from 18–600 μmole·l_s^?1·h^?1 for acetate and 0.7–7 μmole·l_s^?1·h^?1 for the carboxyl carbon of propionate. Methane production was observed only with acetate and only in sulfatedepleted sediments; total acetate turnover attained approximately the same maximum value in both sulfate-reducing and sulfate-depleted sediments.Apparent turnover rates for acetate and propionate appeared to be controlled by similar factors: in sulfate-reducing (surface) sediments the turnover rates were stimulated by autumn storm-mediated deposition/resuspension events; in deeper sulfate-depleted sediments the turnover rates followed changes in the ambient temperature. Changes in VFA poolsizes were proportionally much larger than changes in corresponding rate constants. The ratio of CO₂ to CH₄ produced from acetate vs. depth suggested that non-methanogenic bacteria accounted for 60% of the acetate turnover in sulfate-depleted sediments.VFA concentrations were much lower in N.C. continental slope mud than in Cape Lookout sediments; acetate was the only VFA detectable throughout the top 40 cm of the slope sediments. The estimated production rate of CO₂ from acetate decreased rapidly with depth. The surface rate was approximately 20 times less than that measured at similar temperatures in sulfate-reducing Cape Lookout sediments.     

Impact of suboxia on sinking particulate organic carbon: Enhanced carbon flux and preferential degradation of amino acids via denitrification

Fluxes of particulate organic carbon (POC) through the oxygen deficient waters in the eastern tropical North Pacific were found to be relatively less attenuated with depth than elsewhere in the eastern North Pacific. The attenuation coefficient (b) for the flux was found to be 0.40 versus the composite value of 0.86 determined by Martin et al. (1987). To examine this further, sinking POC was collected using sediment traps and allowed to degrade in oxic and suboxic experiments. Using a kinetic model, it was found that degradation proceeded at similar rates (roughly 0.8 day⁻¹) under oxic and suboxic conditions, but a greater fraction of bulk POC was resistant to degradation in the suboxic experiments (61% vs. 23%). Amino acids accounted for 37% of POC collected at 75m, but following degradation the value dropped to 17% and 16% in the oxic and suboxic experiments respectively. POC collected from 500m was 10% amino acids. The non-AA component of POC collected at 75m was not degraded under suboxic conditions, while under oxic conditions it was. These results suggest that microbes degrading OC under suboxic conditions via denitrification preferentially utilize nitrogen-rich amino acids. This preferential degradation of amino acids suggests that 9% more nitrogen may be lost via water column denitrification than is accounted for when a more “Redfieldian” stoichiometry for POC is assumed.     

Identification and valuation of paramagnetic radicals in natural dolomites as an indicator of geological events

 Geological sedimentary dolomite samples from the Superior Proterozoic are studied using electron paramagnetic resonance (EPR) spectroscopy. The complex spectra in the g=2.0 region is composed of Mn²⁺ lines and signals due to crystallization and radiation-induced defects. Measurements in microwave frequencies of 9.5 GHz (X-band) and 35 GHz (Q-band), and thermal and/or radiation treatments allowed identification of seven paramagnetic radicals in the g=2.0 region: (1) isotropic organic radical; (2) axial SO₂ ⁻; (3) axial PO₂ ⁰ or PO₂ ²⁻; (4) isotropic CO₂ ⁻; (5) axial CO₂ ⁻; (6) axial CO₃ ³⁻; (7) isotropic unknown line. The use of these paramagnetic centres as indicators of geological events is discussed. Received: 18 March 2002 / Accepted: 3 October 2002     

Interactions between basalts and oil source rocks in rift basins： CO2 generation

Basalts interbedded with oil source rocks are discovered frequently in rift basins of eastern China, where CO2 is found in reservoirs around or within basalts, for example in the Binnan reservoir of the Dongying Depression. In the reservoirs, CO2 with heavy carbon isotopic composition (δ13C>-10‰ PDB) is in most cases accounts for 40% of the total gas reserve, and is believed to have resulted from degassing of basaltic magma from the mantle. In their investigations of the Binnan reservoir, the authors suggested that the CO2 would result from interactions between the source rocks and basalts. As the source rocks around basalts are rich in carbonate minerals, volcanic minerals, transition metals and organic matter, during their burial history some of the transition metals were catalyzed on the thermal degradation of organic matter into hydrocarbons and on the decomposition of carbonate minerals into CO2, which was reproduced in thermal simulations of the source rocks with the transition metals (Ni and Co). This kind of CO2 accounts for 55%-85% of the total gas reserve generated in the process of thermal simulation, and its δ13C values range from -11‰- -7.2‰ PDB, which are very similar to those of CO2 found in the Binnan reservoir. The co-generation of CO2 and hydrocarbon gases makes it possible their accumulation together in one trap. In other words, if the CO2 resulted directly from degassing of basaltic magma or was derived from the mantle, it could not be accumulated with hydrocarbon gases because it came into the basin much earlier than hydrocarbon generation and much earlier than trap formation. Therefore, the source rocks around basalts generated hydrocarbons and CO2 simultaneously through catalysis of Co and Ni transition metals, which is useful for the explanation of co-accumulation of hydrocarbon gases and CO2 in rift basins in eastern China.     

Volatile organic acids and microbial processes in the Yegua formation,east-central Texas

Geochemical and microbiological evidence indicates that viable microorganisms produce and consume volatile organic acids (VOA) in the Yegua formation. Acetic and propionic acid concentrations in mudstones range from 200 to 1270 and 20 to 38 nmol·gdw⁻¹ respectively, whereas concentrations in sands are 50–200 and less than 20 nmol·gdw⁻¹. VOA concentrations in sediments and in laboratory incubations suggest net production of VOAs by microorganisms in mudstones, and net consumption of VOAs by SO₄ reducing bacteria (SRB) in sands. Notably, SRB activity is mostly confined to aquifer sands.Vertical diffusion and advection were modeled to estimate acetic acid transport from aquitard to aquifer. Assuming that SRB completely respire the acetic acid transported into the aquifer (3.2 μmol·l⁻¹·m·a⁻¹), the CO₂ production rate in the aquifer sands is 5.3 μmol·l⁻¹·a⁻¹. This slow mineralization rate of in situ organic matter is within the range for deep aquifers, and probably accounts for the long-term survival of microorganisms in oligotrophic environments. Finally, the microbial communities in Yegua sediments appear to exhibit a loose commensalism, with microorganisms in aquitards providing VOAs for respiratory processes (i.e., SO₄ reduction) in aquifers.