Pigment production and isotopic fractionations in continuous culture: okenone producing purple sulfur bacteria Part II

Okenone is a carotenoid pigment unique to certain members of Chromatiaceae, the dominant family of purple sulfur bacteria (PSB) found in euxinic photic zones. Diagenetic alteration of okenone produces okenane, the only recognized molecular fossil unique to PSB. The in vivo concentrations of okenone and bacteriochlorophyll a (Bchl a) on a per cell basis were monitored and quantified as a function of light intensity in continuous cultures of the purple sulfur bacterium Marichromatium purpuratum (Mpurp1591). We show that okenone‐producing PSB have constant bacteriochlorophyll to carotenoid ratios in light‐harvesting antenna complexes. The in vivo concentrations of Bchl a, 0.151 ± 0.012 fmol cell^?1, and okenone, 0.103 ± 0.012 fmol cell^?1, were not dependent on average light intensity (10–225 Lux) at both steady and non‐steady states. This observation revealed that in autotrophic continuous cultures of Mpurp1591, there was a constant ratio for okenone to Bchl a of 1:1.5. Okenone was therefore constitutively produced in planktonic cultures of PSB, regardless of light intensity. This confirms the legitimacy of okenone as a signature for autotrophic planktonic PSB and by extrapolation water column euxinia. We measured the δ¹³C, δ¹⁵N, and δ³⁴S bulk biomass values from cells collected daily and determined the isotopic fractionations of Mpurp1591. There was no statistical relationship in the bulk isotope measurements or stable isotope fractionations to light intensity or cell density under steady and non‐steady‐state conditions. The carbon isotope fractionation between okenone and Bchl a with respect to overall bulk biomass (¹³ε_{pigment – biomass}) was 2.2 ± 0.4‰ and ?4.1 ± 0.9‰, respectively. The carbon isotopic fractionation () for the production of pigments in PSB is more variable than previously thought with our reported values for okenone at ?15.5 ± 1.2‰ and ?21.8 ± 1.7‰ for Bchl a.     

Effects of Metabolism and Physiology on the Production of Okenone and Bacteriochlorophyll a in Purple Sulfur Bacteria

Purple sulfur bacteria (PSB) are important photoautotrophs inhabiting chemoclines in euxinic and meromictic lakes. These organisms are the only producers of the carotenoid, okenone, a compound that has been targeted as a biomarker for photic zone euxinia, particularly in ancient sedimentary environments. Although the natural occurrence and geochemistry of this compound has been studied previously, this is the first systematic and comprehensive report on the microbial physiology of okenone production in pure cultures. Four strains/species: Marichromatium purpuratum DSMZ 1591, Marichromatium purpuratum DSMZ 1711, Thiocapsa marina DSMZ 5653, and FGL21 (isolated from Fayetteville Green Lake, New York) were chosen because they produce okenone and Bacteriochlorophyll a (Bchl a). We developed a new, in vivo technique for the quantification of okenone allowing for more rapid and accurate quantification. The ratio of okenone to Bchl a differs among species and strains of PSB, varying from 0.463 ± 0.002 to 0.864 ± 0.002. Photoheterotrophically grown PSB have statistically significant, lowered okenone:Bchl a ratios, decreasing from 0.784 ± 0.009 under autotrophic metabolism to 0.681 ± 0.002, which we interpret to indicate a decreased requirement for okenone when PSB are provided with a complex (> C1) carbon source. The variation in okenone production raises the question on whether okenone expression is constitutive or inducible. The broader implication is that concentrations of okenone in sediments are dependent on metabolism and species composition, and not solely on PSB cell density.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Methanotrophy in a Paleoproterozoic oil field ecosystem,Zaonega Formation,Karelia, Russia

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ¹³C_org from c. ?25‰ in the lower part of the succession to c. ?40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward ¹³C‐depleted organic matter (δ¹³C_org < ?25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH₄ triggered the activity of methanotrophic organisms, resulting in the production of anomalously ¹³C‐depleted biomass. The stratigraphic shift in δ¹³C_org records the change from CO₂‐fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ¹³C_org reflects global forcing and progressive oxidation of the Earth. However, the lithologic indication for local thermogenic CH₄, sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ¹³C_org negative isotopic shift observed in the ZF.     

Isotopic evidence of environmental changes during the Devonian–Carboniferous transition in South China and its implications for the biotic crisis

The Devonian–Carboniferous (D–C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo-ocean environment of different depositional facies, we studied a shallow-water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well-preserved succession spanning the D–C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ¹⁵N excursion (~−3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium-bearing waters. Negative excursions in the δ¹³C_carb and δ¹³C_org values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered ¹³C-depleted anoxic water masses. Decreased δ³⁴S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water-column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ¹³C_org values associated with maximal △¹³C. The integrated δ¹⁵N-δ¹³C-δ³⁴S data suggest that significant ocean-redox variation was recorded in South China during the D–C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.     

Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ¹⁵N signatures. Here, we studied how biomass allocation and δ¹⁵N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ¹⁵N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ¹⁵N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ¹⁵N and fungal biomass on root tips were positively correlated in ammonium treatments (r ²?=?0.52) but not in nitrate treatments (r ²?=?0.00). Fungal biomass on root tips was enriched in ¹⁵N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ¹⁵N. We conclude that: (1) transfer of ¹⁵N-depleted N from mycorrhizal fungi to plants produces low plant δ¹⁵N signatures and high root tip and fungal δ¹⁵N signatures; (2) limited nitrate reduction in fungi restricted transfer of ¹⁵N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ¹⁵N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without ¹⁵N fractionation. These factors probably control plant δ¹⁵N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.     

The mechanisms of iron isotope fractionation produced during dissimilatory Fe(III) reduction by Shewanella putrefaciens and Geobacter sulfurreducens

Microbial dissimilatory iron reduction (DIR) is widespread in anaerobic sediments and is a key producer of aqueous Fe(II) in suboxic sediments that contain reactive ferric oxides. Previous studies have shown that DIR produces some of the largest natural fractionations of stable Fe isotopes, although the mechanism of this isotopic fractionation is not yet well understood. Here we compare Fe isotope fractionations produced by similar cultures of Geobacter sulfurreducens strain PCA and Shewanella putrefaciens strain CN32 during reduction of hematite and goethite. Both species produce aqueous Fe(II) that is depleted in the heavy Fe isotopes, as expressed by a decrease in ⁵⁶Fe/⁵⁴Fe ratios or δ⁵⁶Fe values. The low δ⁵⁶Fe values for aqueous Fe(II) produced by DIR reflect isotopic exchange among three Fe inventories: aqueous Fe(II) (Fe(II)_aq), sorbed Fe(II) (Fe(II)_sorb), and a reactive Fe(III) component on the ferric oxide surface (Fe(III)_reac). The fractionation in ⁵⁶Fe/⁵⁴Fe ratios between Fe(II)_aq and Fe(III)_reac was –2.95‰, and this remained constant over the timescales of the experiments (280 d). The Fe(II)_aq – Fe(III)_reac fractionation was independent of the ferric Fe substrate (hematite or goethite) and bacterial species, indicating a common mechanism for Fe isotope fractionation during DIR. Moreover, the Fe(II)_aq – Fe(III)_reac fractionation in ⁵⁶Fe/⁵⁴Fe ratios during DIR is identical within error of the equilibrium Fe(II)_aq – ferric oxide fractionation in abiological systems at room temperatures. This suggests that the role of bacteria in producing Fe isotope fractionations during DIR lies in catalyzing coupled atom and electron exchange between Fe(II)_aq and Fe(III)_reac so that equilibrium Fe isotope partitioning occurs. Although Fe isotope fractionation between Fe(II)_aq and Fe(III)_reac remained constant, the absolute δ⁵⁶Fe values for Fe(II)_aq varied as a function of the relative proportions of Fe(II)_aq, Fe(II)_sorb, and Fe(III)_reac during reduction. The temporal variations in these proportions were unique to hematite or goethite but independent of bacterial species. In the case of hematite reduction, the small measured Fe(II)_aq – Fe(II)_sorb fractionation of −0.30‰ in ⁵⁶Fe/⁵⁴Fe ratios, combined with the small proportion of Fe(II)_sorb, produced insignificant (<0.05‰) isotopic effects due to sorption of Fe(II). Sorption of Fe(II) produced small, but significant effects during reduction of goethite, reflecting the higher proportion of Fe(II)_sorb and larger measured Fe(II)_aq – Fe(II)_sorb fractionation of –0.87‰ in ⁵⁶Fe/⁵⁴Fe ratios for goethite. The isotopic effects of sorption on the δ⁵⁶Fe values for Fe(II)_aq were largest during the initial stages of reduction when Fe(II)_sorb was the major ferrous Fe species during goethite reduction, on the order of 0.3 to 0.4‰. With continued reduction, however, the isotopic effects of sorption decreased to <0.2‰. These results provide insight into the mechanisms that produce Fe isotope fractionation during DIR, and form the basis for interpretation of Fe isotope variations in modern and ancient natural systems where DIR may have driven Fe cycling.     

Effects of charring on the carbon isotopic composition of grass (Poaceae) epidermis

Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C₄ grasses or sedges preserve a carbon isotopic value within the range typical for C₄ plants (−17 to −10‰), and charred epidermal fragments from C₃ plants have carbon isotopic values between −30 and −26‰. The process of charring results in a slightly enriched carbon isotopic signature (−11.9‰ mean charred value as compared to −12.8‰ mean unaltered grass tissue value). δ¹³C measurements of replicate samples from the same plant vary within 1–2‰, yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. δ¹³C measurements on >180-μm charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C₄ grass input (δ¹³C=−16 to −19‰). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C₃ vs. C₄ grassland assemblages on the landscape. Furthermore, such downcore δ¹³C profiles can be used to highlight key zones of C₃ vs. C₄ grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition.     

Spatial variability in photosynthetic and heterotrophic activity drives localized δ13Corg fluctuations and carbonate precipitation in hypersaline microbial mats

Modern laminated photosynthetic microbial mats are ideal environments to study how microbial activity creates and modifies carbon and sulfur isotopic signatures prior to lithification. Laminated microbial mats from a hypersaline lagoon (Guerrero Negro, Baja California, Mexico) maintained in a flume in a greenhouse at NASA Ames Research Center were sampled for δ¹³C of organic material and carbonate to assess the impact of carbon fixation (e.g., photosynthesis) and decomposition (e.g., bacterial respiration) on δ¹³C signatures. In the photic zone, the δ¹³C_org signature records a complex relationship between the activities of cyanobacteria under variable conditions of CO₂ limitation with a significant contribution from green sulfur bacteria using the reductive TCA cycle for carbon fixation. Carbonate is present in some layers of the mat, associated with high concentrations of bacteriochlorophyll e (characteristic of green sulfur bacteria) and exhibits δ¹³C signatures similar to DIC in the overlying water column (?2.0‰), with small but variable decreases consistent with localized heterotrophic activity from sulfate‐reducing bacteria (SRB). Model results indicate respiration rates in the upper 12 mm of the mat alter in situ pH and concentrations to create both phototrophic CO₂ limitation and carbonate supersaturation, leading to local precipitation of carbonate minerals. The measured activity of SRB with depth suggests they variably contribute to decomposition in the mat dependent on organic substrate concentrations. Millimeter‐scale variability in the δ¹³C_org signature beneath the photic zone in the mat is a result of shifting dominance between cyanobacteria and green sulfur bacteria with the aggregate signature overprinted by heterotrophic reworking by SRB and methanogens. These observations highlight the impact of sedimentary microbial processes on δ¹³C_org signatures; these processes need to be considered when attempting to relate observed isotopic signatures in ancient sedimentary strata to conditions in the overlying water column at the time of deposition and associated inferences about carbon cycling.     

Carbon: freshwater plants

δ¹³C values for freshwater aquatic plant matter varies from ?11 to ?50‰ and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect δ¹³C of aquatic plant matter. These include: (1) The δ¹³C signature of the source carbon has been observed to range from +1‰ for HCO₃^? derived from limestone to ?30‰ for CO₂ derived from respiration. (2) Some plants assimilate HCO₃^?, which is –7 to –11‰ less negative than CO₂. (3) C₃, C₄, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a δ¹³C value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use δ¹³C values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO₂ diffusion and carbon source.     

Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals

Despite theories of large-scale movement and assimilation of carbon in estuaries, recent evidence suggests that in some estuaries much more limited exchange occurs. We measured the fine-scale movement and assimilation of carbon by resident macroinvertebrates between adjacent saltmarsh and mangrove habitats in an Australian estuary using δ¹³C analysis of animals at different distances into adjacent patches of habitat. δ¹³C values of crabs (Parasesarma erythrodactyla –15.7 ± 0.1‰, Australoplax tridentata –14.7 ± 0.1‰) and slugs (Onchidina australis –16.2 ± 0.3‰) in saltmarsh closely matched that of the salt couch grass Sporobolus virginicus (–15.5 ± 0.1‰). In mangroves, δ¹³C values of crabs (P. erythrodactyla –22.0 ± 0.2‰, A. tridentata –19.2 ± 0.3‰) and slugs (–19.7 ± 0.3‰) were enriched relative to those of mangroves (–27.9 ± 0.2‰) but were more similar to those of microphytobenthos (–23.7 ± 0.3‰). The δ¹³C values of animals across the saltmarsh-mangrove interface fitted a sigmoidal curve, with a transition zone of rapidly changing values at the saltmarsh-mangrove boundary. The width of this transition indicated that the movement and assimilation of carbon is limited to between 5 and 7 m. The δ¹³C values of crabs and slugs, especially those in saltmarsh habitat, clearly indicate that the movement and assimilation of carbon between adjacent saltmarsh and mangrove habitat is restricted to just a few metres, although some contribution from unmeasured sources elsewhere in the estuary is possible. Such evidence demonstrating the extent of carbon movement and assimilation by animals in estuarine habitats is useful in determining the spatial arrangement of habitats needed in marine protected areas to capture food web processes.     

Abundance and activity of microorganisms at the water-sediment interface and their effect on the carbon isotopic composition of suspended organic matter and sediments of the Kara Sea

At ten stations of the meridian profile in the eastern Kara Sea from the Yenisei estuary through the shallow shelf and further through the St. Anna trough, total microbial numbers (TMN) determined by direct counting, total activity of the microbial community determined by dark CO₂ assimilation (DCA), and the carbon isotopic composition of organic matter in suspension and upper sediment horizons (δ¹³C, ‰) were investigated. Three horizons were studied in detail: (1) the near-bottom water layer (20–30 cm above the sediment); (2) the uppermost, strongly hydrated sediment horizon, further termed fluffy layer (5–10 mm); and (3) the upper sediment horizon (1–5 cm). Due to a decrease in the amount of isotopically light carbon of terrigenous origin with increasing distance from the Yenisei estuary, the TMN and DCA values decreased, and the δ¹³C changed gradually from ?29.7 to ?23.9‰. At most stations, a noticeable decrease in TMN and DCA values with depth was observed in the water column, while the carbon isotopic composition of suspended organic matter did not change significantly. Considerable changes of all parameters were detected in the interface zone: TMN and DCA increased in the sediments compared to their values in near-bottom water, while the 13C content increased significantly, with δ¹³C of organic matter in the sediments being at some stations 3.5–4.0‰ higher than in the near-bottom water. Due to insufficient illumination in the near-bottom zone, newly formed isotopically heavy organic matter (δ¹³C ～ ?20‰) could not be formed by photosynthesis; active growth of chemoautotrophic microorganisms in this zone is suggested, which may use reduced sulfur, nitrogen, and carbon compounds diffusing from anaerobic sediments. High DCA values for the interface zone samples confirm this hypothesis. Moreover, neutrophilic sulfur-oxidizing bacteria were retrieved from the samples of this zone.     

Biogeochemical probing of microbial communities in a basalt‐hosted hot spring at Kverkfjöll volcano,Iceland

We investigated bacterial and archaeal communities along an ice‐fed surficial hot spring at Kverkfjöll volcano—a partially ice‐covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and <1 mg/L (±0.1%)) changing to lower temperatures and higher dissolved oxygen downstream (34.7°C and 5.9 mg/L), with sulfate the dominant anion (225 mg/L at the source). Sediments are comprised of detrital basalt, low‐temperature alteration phases and pyrite, with <0.4 wt. % total organic carbon (TOC). 16S rRNA gene profiles reveal that organisms affiliated with Hydrogenobaculum (54%–87% bacterial population) and Thermoproteales (35%–63% archaeal population) dominate the micro‐oxic hot spring source, while sulfur‐oxidizing archaea (Sulfolobales, 57%–82%), and putative sulfur‐oxidizing and heterotrophic bacterial groups dominate oxic downstream environments. The δ¹³C_org (‰ V‐PDB) values for sediment TOC and microbial biomass range from ?9.4‰ at the spring's source decreasing to ?12.6‰ downstream. A reverse effect isotope fractionation of ~3‰ between sediment sulfide (δ³⁴S ~0‰) and dissolved water sulfate (δ³⁴S +3.2‰), and δ¹⁸O values of ~ ?5.3‰ suggest pyrite forms abiogenically from volcanic sulfide, followed by abiogenic and microbial oxidation. These environments represent an unexplored surficial geothermal environment analogous to transient volcanogenic habitats during putative “snowball Earth” scenarios and volcano–ice geothermal environments on Mars.     

Isotopic and chemical investigations of Fe (III) reduction by Shewanella putrefaciens strain 200R

Experiments were conducted using the Fe⁺³‐reducing bacterium Shewanella putrefaciens strain 200R to determine the stable carbon isotope fractionation during dissimilatory Fe (III) reduction and associated lactate oxidation at circum‐neutral pH. Previous studies used equilibrium fractionation factors (~14.3‰) between bacterial biomass and synthesized fatty acids to identify the predominant carbon fixation pathways for some of the most frequently isolated microbes including Shewanella under anaerobic conditions. We investigated the carbon isotope disproportionation among organic carbon substrate (lactate), biomass and respired carbon dioxide at the lag to stationary phase of the growth curve. Ferric citrate and sodium lactate were used as electron acceptor and donor, respectively. Sodium bicarbonate or potassium phosphate was used as buffering agent. Iron (II), iron (III), dissolved inorganic carbon (DIC) and carbon isotope ratios were measured for both bicarbonate‐ and phosphate‐buffered systems. Carbon isotope ratio measurements were made on the respired CO₂ (as DIC) and microbial biomass for both buffering conditions. The fraction of lactate consumed was estimated using DIC as a proxy and was verified by direct measurement using HPLC. Our result showed that bicarbonate‐buffered system has an enhancing effect in the reduction process compared to the phosphate system. Both systems resulted in carbon isotope fractionations between the lactate substrate and DIC that could be modelled as a Rayleigh process. The biomass produced under both buffer conditions was depleted on average by ~2‰ relative to the substrate and enriched by ~5‰ relative to the DIC. This translates to an overall isotopic fractionation of 10–12‰ between the biomass and respired CO₂ in both buffering systems.     

Isotope discrimination by form IC RubisCO from Ralstonia eutropha and Rhodobacter sphaeroides,metabolically versatile members of ‘Proteobacteria’ from aquatic and soil habitats

RubisCO, the CO₂ fixing enzyme of the Calvin–Benson–Bassham (CBB) cycle, is responsible for the majority of carbon fixation on Earth. RubisCO fixes ¹²CO₂ faster than ¹³CO₂ resulting in ¹³C-depleted biomass, enabling the use of δ¹³C values to trace CBB activity in contemporary and ancient environments. Enzymatic fractionation is expressed as an ε value, and is routinely used in modelling, for example, the global carbon cycle and climate change, and for interpreting trophic interactions. Although values for spinach RubisCO (ε = ~29‰) have routinely been used in such efforts, there are five different forms of RubisCO utilized by diverse photolithoautotrophs and chemolithoautotrophs and ε values, now known for four forms (IA, B, D and II), vary substantially with ε = 11‰ to 27‰. Given the importance of ε values in δ¹³C evaluation, we measured enzymatic fractionation of the fifth form, form IC RubisCO, which is found widely in aquatic and terrestrial environments. Values were determined for two model organisms, the ‘Proteobacteria’ Ralstonia eutropha (ε = 19.0‰) and Rhodobacter sphaeroides (ε = 22.4‰). It is apparent from these measurements that all RubisCO forms measured to date discriminate less than commonly assumed based on spinach, and that enzyme ε values must be considered when interpreting and modelling variability of δ¹³C values in nature.     

Isotopic evidence of dietary variations and weaning practices in the Gaya cemetery at Yeanri,Gimhae, South Korea

Stable carbon and nitrogen isotope analyses were conducted to investigate dietary variation in human skeletons (n = 109) from the Gaya cemetery at Yeanri located near Gimhae City, South Korea. The cemetery contained three distinct grave types dating to 4th–7th century AD. The main purposes of this research were to reconstruct palaeodiet in the Gaya population and to explore correlations between stable isotope compositions and burial types, inferred age, and sex of these individuals. The isotopic data indicate that the people at Yeanri consumed a predominantly C₃‐based terrestrial diet supplemented with freshwater and/or marine resources. The comparison of isotopic results reveals significant differences in δ¹³C values among three adult burial types (wood‐cist coffin: ?18.5 ± 0.5‰, stone‐cist coffin: ?18.1 ± 0.6‰, mausoleum: ?17.8 ± 0.9‰). Males in wood‐cist and stone‐cist coffins have relatively more elevated mean δ¹³C and δ¹⁵N values than females. The isotopic ratios from the two adult age groups (21–40 years and 40–60 years) indicate that there was no significant dietary change in individuals with age. The isotope data from the infants and children suggest the weaning was a gradual process that was completed between 3 and 4 years of age in the Gaya population. This evidence indicates that the dietary variations within the cemetery reflect social status, sex, and childhood consumption patterns. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Assessing nature and dynamics of POM in transitional environment (the Curonian Lagoon,SE Baltic Sea) using a stable isotope approach

The nature of the particulate organic matter (POM) as well as its temporal and spatial distribution and dynamics in the Curonian Lagoon (south-eastern part of the Baltic Sea) were investigated. The organic matter was characterized by the organic carbon and nitrogen content, δ¹³C and δ¹⁵N signatures as well as POC/Chl-a and C/N ratios. Additionally, data on hydrological, chemical and biological parameters were used for better understanding the POM distribution and dynamics. The sampling was performed at 13 stations in the Curonian Lagoon and its outflow in the Baltic Sea during the 2012–2013 period. Samples were also collected at the Nemunas River mouth in order to test the riverine impact. Obtained results showed that isotopic values of carbon and nitrogen ranged from −36.1‰ to −25.2‰ and from −0.9‰ to 15.5‰, respectively. The isotopic composition, together with the low C/N molar (∼7) and POC/Chl-a ratios (<100) of the POM, suggested the dominance of living phytoplankton in POM throughout the year with the higher input of detrital material (C/N >10, POC/Chl-a ratios >100) in late autumn − winter.The results of multivariate analysis evidenced a spatial distinction of POM distribution in the northern-transitional and central confined areas and allowed us to distinguish the main driving factors. The seasonal variation of the δ¹³C and δ¹⁵N values in POM (towards higher δ¹³C and lower δ¹⁵N values in the summer − early autumn period and lower δ¹³C and higher δ¹⁵N values in the late autumn − spring period) was determined by combination of factors such as availability of inorganic carbon and nitrogen, the riverine discharge, seasonal phytoplankton succession and by the short-term saline water intrusion to the northern-transitional part of the lagoon.     

Environmental context for the terminal Ediacaran biomineralization of animals

In terminal Ediacaran strata of South China, the onset of calcareous biomineralization is preserved in the paleontological transition from Conotubus to Cloudina in repetitious limestone facies of the Dengying Formation. Both fossils have similar size, funnel‐in‐funnel construction, and epibenthic lifestyle, but Cloudina is biomineralized, whereas Conotubus is not. To provide environmental context for this evolutionary milestone, we conducted a high‐resolution elemental and stable isotope study of the richly fossiliferous Gaojiashan Member. Coincident with the first appearance of Cloudina is a significant positive carbonate carbon isotope excursion (up to +6‰) and an increase in the abundance and ³⁴S composition of pyrite. In contrast, δ³⁴S values of carbonate‐associated sulfate remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations in the lower half of the member. The fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation involved in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We speculate that the coupled paleontological and biogeochemical anomalies may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, the spread of an oxygen minimum zone, and the development of euxinic conditions in subtidal and basinal environments. Enhanced production and burial of organic matter is thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic pathway for Conotubus and other soft‐bodied Ediacara biotas. Our studies suggest that the Ediacaran confluence of ecological pressures from predation and environmental pressures from an increase in seawater alkalinity set the stage for an unprecedented geobiological response: the evolutionary novelty of animal biomineralization.     

Stable carbon and nitrogen isotopes in multiple tissues of wild and captive harbor seals (Phoca vitulina) off the California coast

Stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) of serum, red blood cells (RBC), muscle, and blubber were measured in captive and wild northeast Pacific harbor seals (Phoca vitulina richardii) at three coastal California sites (San Francisco Bay, Tomales Bay, and Channel Islands). Trophic discrimination factors (Δ_{Tissue‐Diet}) were calculated for captive seals and then applied in wild counterparts in each habitat to estimate trophic position and feeding behavior. Trophic discrimination factors for δ¹⁵N of serum (+3.8‰), lipid‐extracted muscle (+1.6‰), and lipid‐blubber (+6.5‰) are proposed to determine trophic position. An offset between RBC and serum of +0.3‰ for δ¹³C and ?0.6‰ for δ¹⁵N was observed, which is consistent with previous research. Specifically, weaner seals (<1 yr) had large offsets, suggesting strong trophic position shifts during this life stage. Isotopic values indicated an average trophic position of 3.6 at both San Francisco Bay and Tomales Bay and 4.2 at Channel Islands. Isotopic means were strongly dependent on age class and also suggested that mean diet composition varies considerably between all locations. Together, these data indicate that isotopic composition of blood fractions can be an effective approach to estimate trophic position and dietary behavior in wild pinnipeds.     

Stromatolite records of environmental change in perennially ice-covered Lake Joyce,McMurdo Dry Valleys,Antarctica

Calcite-rich columnar stromatolites grew in perennially ice-covered Lake Joyce in the McMurdo Dry Valleys, Antarctica, during a period of environmental change associated with rising lake level. Stromatolite calcite contains carbon and oxygen isotope records of changes to microbial activity in response to variable light environments and water chemistry through time. The stromatolites grew synchronously with correlative calcite zones. The innermost (oldest) calcite zone has a wide range of δ¹³C_calcite values consistent with variable photosynthetic effects on local DIC ¹³C/¹²C. Subsequent calcite zones preserve a progressive enrichment in δ¹³C_calcite values of approximately + 2.6‰ through time, with δ¹³C_calcite values becoming less variable. This enrichment likely records the removal of ¹²C by photosynthesis from the DIC reservoir over decades, with photosynthetic effects decreasing as light levels became lower and more consistent through time. Mean δ¹⁸O_calcite values of the innermost calcified zone were at least 1‰ lower than those of the other calcified zones (t test p-level < 0.001). The significant difference in δ¹⁸O_calcite values between the innermost and other calcified zones could be a product of mixing source waters with different isotopic values associated with the initiation of lake stratification associated with rising lake level. Overall, Lake Joyce stromatolites record significant lateral variability in relative photosynthetic rate and long-lived lake water stratification with microbial modification of the DIC pool. Such processes provide criteria for interpreting microbial activity within polar paleolake deposits and may shed light on variability in lake environments associated with changing climate in the McMurdo Dry Valleys.