Carbon isotopic fractionation during a mesocosm bloom experiment dominated by Emiliania huxleyi: Effects of CO2 concentration and primary production

We investigated the effect of CO₂ and primary production on the carbon isotopic fractionation of alkenones and particulate organic matter (POC) during a natural phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi. In nine semi-closed mesocosms (∼11 m³ each), three different CO₂ partial pressures (pCO₂) in triplicate represented glacial (∼180 ppmv CO₂), present (∼380 ppmv CO₂), and year 2100 (∼710 ppmv CO₂) CO₂ conditions. The largest shift in alkenone isotopic composition (4-5‰) occurred during the exponential growth phase, regardless of the CO₂ concentration in the respective treatment. Despite the difference of ∼500 ppmv, the influence of pCO₂ on isotopic fractionation was marginal (1-2‰). During the stationary phase, E. huxleyi continued to produce alkenones, accumulating cellular concentrations almost four times higher than those of exponentially dividing cells. Our isotope data indicate that, while alkenone production was maintained, the interaction of carbon source and cellular uptake dynamics by E. huxleyi reached a steady state. During stationary phase, we further observed a remarkable increase in the difference between δ¹³C of bulk organic matter and of alkenones spanning 7-12‰. We suggest that this phenomenon is caused mainly by a combination of extracellular release of ¹³C-enriched polysaccharides and subsequent particle aggregation induced by the production of transparent exopolymer particles (TEP).     

N and O isotope effects during nitrate assimilation by unicellular prokaryotic and eukaryotic plankton cultures

In order to provide biological systematics from which to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (¹⁵N/¹⁴N, ¹⁸O/¹⁶O, respectively) in the environment, we previously investigated the isotopic fractionation of nitrate during its assimilation by mono-cultures of eukaryotic algae (Granger et al., 2004). In this study, we extended our analysis to investigate nitrate assimilation by strains of prokaryotic plankton. We measured the N and O isotope effects, ¹⁵ε and ¹⁸ε, during nitrate consumption by cultures of prokaryotic strains and by additional eukaryotic phytoplankton strains (where ε is the ratio of reaction rate constants of the light vs. heavy isotopologues, ^lightk and ^heavyk; ε = ^lightk/^heavyk − 1 × 1000, expressed in per mil). The observed ¹⁵ε ranged from 5‰ to 8‰ among eukaryotes, whereas it did not exceed 5‰ for three cyanobacterial strains, and was as low as 0.4‰ for a heterotrophic α-protoeobacterium. Eukaryotic phytoplankton fractionated the N and O isotopes of nitrate to the same extent (i.e., ¹⁸ε ∼ ¹⁵ε). The ¹⁸ε:¹⁵ε among the cyanobacteria was also ∼1, whereas the heterotrophic α-proteobacterial strain, which showed the lowest ¹⁵ε, between 0.4‰ and 1‰, had a distinct ¹⁸ε:¹⁵ε of ∼2, unlike any plankton strain observed previously. Equivalent N vs. O isotope discrimination is thought to occur during internal nitrate reduction by nitrate reductase, such that the cellular efflux of the fractionated nitrate into the medium drives the typically observed ¹⁸ε:¹⁵ε of ∼1. We hypothesize that the higher in the ¹⁸ε:¹⁵ε of the α-proteobacterium may result from isotope discrimination by nitrate transport, which is evident only at low amplitude of ε. These observations warrant investigating whether heterotrophic bacterial assimilation of nitrate decreases the community isotope effects at the surface ocean.     

Different types of methane monooxygenases produce similar carbon and hydrogen isotope fractionation patterns during methane oxidation

We determined the stable carbon and hydrogen isotope fractionation factors for methane oxidation under oxic conditions using strains with known degradation pathways. The aerobic oxidation of methane can be initiated by two different forms of enzymes known as methane monooxygenases (MMO). The expression of these enzymes is type-specific and dependent upon the adjusted copper concentration in the medium (or environment). In this study, the expression of either the soluble MMO or the particulate MMO was supported by adjusting the copper concentrations in the growth medium. Taxonomically different aerobic methanotrophic strains, mainly belonging to the alpha- and gamma- classes of Proteobacteria, produced methane isotope enrichment factors (ε_bulk) ranging from −14.8 to −27.9‰ for carbon, and from −110.0 to −231.5‰ for hydrogen. The ratio of hydrogen versus carbon discrimination (Λ = (α_H⁻¹ − 1)/(α_C⁻¹ − 1) ≈ Δ(δ²H)/Δ(δ¹³C)) were similar for all tested cultures, and are also identical to values calculated from previously published enrichment factors for aerobic and anaerobic methane degradation. In contrast, Λ-values for the abiotic oxidation of methane with OH radicals (this process is considered as the main removal process for methane from the atmosphere) were significantly higher than the values derived from biotic oxidation. Due to the low variability of microbial methane isotope fractionation patterns, we propose that combined carbon and hydrogen isotope fractionation analyses can be used to monitor and assess the occurrence of microbial methane oxidation in marine or terrestrial environments. However, it is not possible to distinguish distinct aerobic or anaerobic methane-oxidation pathways by this approach.     

Indoor and outdoor urban atmospheric CO2: Stable carbon isotope constraints on mixing and mass balance

From July to November 2009, concentrations of CO₂ in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ¹³C values varied from −8.9‰ to −19.4‰. The CO₂ from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ¹³C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO₂ exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ¹³C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO₂ gas. Collectively, the δ¹³C values of the indoor CO₂ samples were linearly correlated with the reciprocal of CO₂ concentration, exhibiting an intercept of −21.8‰, with r² = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO₂ data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ¹³C value for human-exhaled CO₂ demonstrates simple mixing between two inputs: (1) outdoor CO₂ introduced to the interior spaces by ventilation systems, and (2) CO₂ exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO₂ input from the local ventilation system and CO₂ in the ambient air of the room) could be a reasonable estimate of the average δ¹³C value of the CO₂ exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO₂ that can be used to determine the average proportions of C₃ and C₄-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C₄-derived C appears to have constituted ∼40% of the average diet.     

Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

Limitations on the climatic and ecological signals provided by the δC values of phytoliths from a C4 North American prairie grass

Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ¹³C values of phytoliths from a C₄ grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r² < 0.3) between climate variables and the δ¹³C values of C. longifolia tissues (average δ¹³C_tissue = −13.1 ± 0.6 ‰; n = 70) across the North American prairies. However, plant tissue δ¹³C values are lower for grasses collected in populated areas where the δ¹³C value of atmospheric CO₂ is expected to be lower because of fossil fuel burning. Phytolith δ¹³C values are more variable (δ¹³C = −27.3 to −23.0‰; average = −25.1 ± 1.3‰; n = 34) and more sensitive to changes in aridity than whole tissue δ¹³C values. The strongest correlations are obtained between the δ¹³C values of stem or sheath phytoliths and humidity (r² = 0.3), latitude (r² = 0.4) and amount of precipitation (r² = 0.5). However, use of these relationships is limited by the wide spread in δ¹³C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ¹³C values of phytoliths and expected variations in the δ¹³C values of atmospheric CO₂. The C. longifolia phytoliths are depleted of ¹³C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C₃ plants. This observation may further limit the usefulness of soil-phytolith assemblage δ¹³C values for identifying shifts in grassland C₃:C₄ ratios.     

Note on the isotopic geochemistry of fossil-lacustrine tufas in carbonate plateau—A study from Dungul region (SW Egypt)

Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)_molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ¹³C_mean = −2.49 ± 0.99‰; δ¹⁸O_mean = −9.43 ± 1.40‰). The δ¹³C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ¹⁸O values are typical of fresh water carbonates. Covariation between δ¹³C and δ¹⁸O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ¹³C_mean = −2.06 ± 0.84‰; δ¹⁸O_mean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)_molar, δ¹³C and δ¹⁸O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.     

The stable carbon isotope biogeochemistry of acetate and other dissolved carbon species in deep subseafloor sediments at the northern Cascadia Margin

Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190 m of sediment. At Site U1329, δ¹³C values of acetate span a wide range from −46.0‰ to −11.0‰ vs. VPDB and change systematically with sediment depth. In contrast, δ¹³C values of both the bulk dissolved organic carbon (DOC) (−21.6 ± 1.3‰ vs. VPDB) and the low-molecular-weight compound lactate (−20.9 ± 1.8‰ vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1‰ depleted and up to 9.1‰ enriched in ¹³C. Broadly, ¹³C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while ¹³C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO₂-reduction can coexist with methanogenic CO₂-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.     

Fractionation of carbon isotopes in biosynthesis of fatty acids by a piezophilic bacterium Moritella japonica strain DSK1

We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C_14-19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed Δδ_FA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in ¹³C with pressure. Average carbon isotope fractionation (Δδ_FA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more ¹³C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a ¹³C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.     

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.     

Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH₄ and CO₂ in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in ¹³C by ∼10‰ in values of δ¹³C of CH₄, and by ∼0.55‰ in values of δ¹³C of CO₂, relative to the values of the high-temperature source fluid (δ¹³C of CH₄ =−20.1 ± 1.2‰, δ¹³C of CO₂ =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in ¹³C of both CH₄ and CO₂ at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and ¹³C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH₄ and CO₂ concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable ¹³C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.     

Stable isotope fractionation during bacterial sulfate reduction is controlled by reoxidation of intermediates

Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of ¹⁸O between residual sulfate and ambient water. Batch experiments were conducted with ¹⁸O-enriched (δ¹⁸O_water = +700‰) and depleted water (δ¹⁸O_water = −40‰), respectively, and the stable ¹⁸O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (ε_S > −13.2‰), δ¹⁸O values in the remaining sulfate increased by only 50‰ during growth when ¹⁸O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (ε_S < −22.7‰) the residual sulfate showed an increase of the sulfate δ¹⁸O close to the values of the enriched water of +700‰. In the experiments with δ¹⁸O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (ε_S < −38.7‰) showed slightly decreasing δ¹⁸O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall ¹⁸O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of ¹⁸O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.     

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‰).     

Insights into C and H storage in the altered oceanic crust: Results from ODP/IODP Hole 1256D

Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with δD values from −64‰ to −25‰. All samples are enriched in water relative to fresh basalts. The δD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO₂, extracted by step-heating technique, ranges between 564 and 2823 ppm with δ¹³C values from −14.9‰ to −26.6‰. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with δ¹³C = −4.5‰ and (2) an organic compound with δ¹³C = −26.6‰. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO₂ yield after H₃PO₄ attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO₂ degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO₂ is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when “fresh” oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO₂, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ∼ −4.7‰, similar to the δ¹³C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 × 10¹² molC/yr.     

Cl,N, C isotopic analysis of common agro-chemicals for identifying non-point source agricultural contaminants

The isotopic compositions of commercially available herbicides were analyzed to determine their respective ¹⁵N, ¹³C and ³⁷Cl signatures for the purposes of developing a discrete tool for tracing and identifying non-point source contaminants in agricultural watersheds. Findings demonstrate that of the agrochemicals evaluated, chlorine stable isotopes signatures range between δ³⁷Cl = −4.55‰ and +3.40‰, whereas most naturally occurring chlorine stable isotopes signatures, including those of road salt, sewage sludge and fertilizers, vary in a narrow range about the Standard Mean Ocean Chloride (SMOC) between −2.00‰ and +1.00‰. Nitrogen stable isotope values varied widely from δ¹⁵N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ¹³C = −37.13‰ and −21.35‰ for the entire suite of agro-chemicals analyzed. When nitrogen, carbon and chlorine stable isotope analyses were compared in a cross-correlation analysis, statistically independent isotopic signatures exist suggesting a new potential tracer tool for identifying herbicides in the environment.     

Determination of carbon fractionation factor between aqueous carbonate and CO2(g) in two-direction isotope equilibration

The experiments were conducted in the open CO₂ system to find out the equilibrium fractionation between the carbonate ion and CO₂(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO₂−N₂ gases with different δ¹³C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ¹³C = − 4.2‰. The Δ_{CO3T²⁻−CO2(g)} equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of Δ_{CO3T²⁻−CO2(g)} and Δ_{HCO3T⁻−CO2(g)} fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.     

Sources and biological fractionation of Silicon isotopes in the Eastern Equatorial Pacific

Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of ε estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77 ± 0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of ε was −1.08 ± 0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of ε was significantly more positive, −0.61 ± 0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of ε can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant δ³⁰Si of +1.32 ± 0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9-1.0‰) and it is slightly less positive than new measures of the δ³⁰Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)₄ > 180  μM, δ³⁰Si = +1.46 ± 0.12‰ (SD, n = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ³⁰Si(OH)₄ with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ³⁰Si(OH)₄ within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea.     

Paleoenvironmental implications of taxonomic variation among δN values of chloropigments

Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ¹⁵N values of chloropigments of photosynthetic organisms to determine the corresponding δ¹⁵N values of biomass - and by extension, surface waters - the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N₂, NO₃⁻, and NH₄⁺ as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ¹⁵N_biomass − δ¹⁵N_{chloropigment}) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as ε_por, a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of ε_por reflect chloropigments that are ¹⁵N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of ε_por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., ε_por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of ε_por for eukaryotic algae (range = 4.7-8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of ε_por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of ε_por suggests that values of ε_por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.     

Quantifying seasonal precipitation using high-resolution carbon isotope analyses in evergreen wood

High-resolution natural abundance stable carbon isotope analyses across annual growth rings in evergreen trees reveal a cyclic increase and decrease in the measured carbon isotopic composition (δ¹³C), but the causes of this pattern are poorly understood. We compiled new and published high-resolution δ¹³C data from across annual growth rings of 33 modern evergreen trees from 10 genera and 15 globally distributed sites to quantify the parameters that affect the observed δ¹³C pattern. Across a broad range of latitude, temperature, and precipitation regimes, we found that the average, measured seasonal change in δ¹³C (Δδ¹³C_meas, ‰) within tree rings of evergreen species reflects changes in the carbon isotopic composition of atmospheric carbon dioxide (Δδ¹³C_CO2) and changes in seasonal precipitation (ΔP) according to the following equation: Δδ¹³C_meas = Δδ¹³C_CO2 - 0.82(ΔP) + 0.73; R² = 0.96. Seasonal changes in temperature, pCO₂, and light levels were not found to significantly affect Δδ¹³C_meas. We propose that this relationship can be used to quantify seasonal patterns in paleoprecipitation from intra-ring profiles of δ¹³C measured from non-permineralized, fossil wood.     

Stable carbon isotope discrimination in rice field soil during acetate turnover by syntrophic acetate oxidation or acetoclastic methanogenesis

Rice fields are an important source for the greenhouse gas methane. In Italian rice field soil CH₄ is produced either by hydrogenotrophic and acetoclastic methanogenesis, or by hydrogenotrophic methanogenesis and syntrophic acetate oxidation when temperatures are below and above about 40-45 °C, respectively. In order to see whether these acetate consumption pathways differently discriminate the stable carbon isotopes of acetate, we measured the δ¹³C of total acetate and acetate-methyl as well as the δ¹³C of CO₂ and CH₄ in rice field soil that had been pre-incubated at 45 °C and then shifted to different temperatures between 25 and 50 °C. Acetate transiently accumulated to about 6 mM, which is about one-third of the amount of CH₄ produced, irrespective of the incubation temperature and the CH₄ production pathway involved. However, the patterns of δ¹³C of the CH₄ and CO₂ produced were different at low (25, 30, 35 °C) versus high (40, 45, 50 °C) temperatures. These patterns were consistent with CH₄ being exclusively formed by hydrogenotrophic methanogenesis at high temperatures, and by a combination of acetoclastic and hydrogenotrophic methanogenesis at low temperatures. The patterns of δ¹³C of total acetate and acetate-methyl were also different at high versus low temperatures, indicating the involvement of different pathways of production and consumption of acetate at the two temperature regimes. Isotope fractionation during consumption of the methyl group of acetate was more pronounced at low (α = 1.010-1.025) than at high (α = 1.0-1.01) temperatures indicating that acetoclastic methanogenesis exhibits a stronger isotope effect than syntrophic acetate oxidation. Small amounts of propionate also transiently accumulated and were analyzed for δ¹³C. The δ¹³C values slightly increased (by about 10‰) during production and consumption of propionate, but were not affected by incubation temperature. Collectively, our results showed distinct isotope discrimination for different paths of acetate (and propionate) production and consumption, albeit differences were only small, and discrimination between methanogenic and syntrophic acetate consumption in nature may be difficult to detect.