Carbon Isotopic Composition Research Articles

Bacterial glycerol tetraethers as a potential tool to trace marine methane cycling

AbstractBranched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial lipids that can be preserved in sedimentary archives for tens of millions of years and are ubiquitous in diverse environments, including cold seep systems. Their potential implications for detecting methane activity in deep time are, however, hampered by the multiple sources of brGDGTs in cold seeps and the lack of evidence of their stable carbon isotopes. Here, we show that brGDGTs in cold seeps are characterized by depleted stable carbon isotopic compositions of the alkyl moieties (δ13C = −32.9‰ to −82.7‰), indicating a methane metabolizing community origin, which is supported by the association between 16S rRNA genes and brGDGTs. We further identify unique seep‐derived brGDGT signals from the global published dataset by a tree‐based machine‐learning algorithm. This trained model, named light gradient‐boosting machine classification for paleoSEEP (GBM_SEEP), is further applied on a paleorecord across the Paleocene–Eocene Thermal Maximum (PETM), which suggests potential methane emission events during the PETM recovery phase. Collectively, our study links brGDGT production in cold seeps with methane metabolizing communities and provides a potential strategy to capture significant methane emission events using the machine‐learning model, which warrants further investigation.

Influence of methane seep activities and sea-level changes on elemental and isotopic compositions and abundance of carbonates in sediments of the Okinawa Trough since the last glacial period

The elemental and isotopic compositions and abundance of carbonates in marine sediments are used to reconstruct ancient climate and ocean conditions. These carbonates usually include biogenic and authigenic carbonates in origin, which are controlled by sedimentary and diagenetic processes, respectively. However, the specific mechanisms involved in controlling the origin of biogenic and authigenic carbonates are still unclear. To address this knowledge gap, we analyzed core CSHC-4 sediment from the Okinawa Trough to examine the elemental and isotopic compositions of carbonates and their relationship to methane seepage and sea-level changes. Our results indicate that high-Mg calcite is the dominant authigenic carbonate, indicating moderate methane seep intensity. Negative carbon isotope values as low as −4.06‰ V-PDB of the bulk sediments deposited in the glacial period suggest a connection between the formation of authigenic carbonates and methane seepage. Negative oxygen isotope values as low as −2.6‰ V-PDB may be attributed to the input of 18O-depleted fluids influenced by gas hydrate formation. Biogenic carbonates contribute the majority (96.3%) of the total carbonate contents in sediments. Biogenic carbonate contents are lower during glacial periods and higher during interglacial periods. The reduced carbonate content during glacial periods may be due to lower export productivity influenced by the deepening of North Pacific Intermediate Water, as well as terrestrial inputs and anoxic conditions during low sea level periods. Our findings highlight the importance of considering both biogenic and authigenic carbonates when reconstructing paleoclimate and paleoceanography based on their geochemical characteristics.

Digestive system and feeding of deep-sea acorn worm Quatuoralisia malakhovi (Enteropneusta: Torquaratoridae) from the Bering sea

We studied the structure of digestive system of deep-sea torquaratorid Quatuoralisia malakhovi including the food-obtaining apparatus, microanatomy of all regions of digestive tract, and fine structure of various regions of the gut. The mouth of Q. malakhovi is surrounded by two symmetrical triangular lateral collar lips and one midventral collar lip, which may retract into the mouth. The ventral ciliary grooves run along the anterior-ventral edges of lateral lips and lead into the internal canals of the lips. The pharyngeal cavity is divided by left and right parabranchial ridges into the median pharyngeal zone with gill bars and two symmetrical peripheral canals, which fuse behind the site of genesis of new gill bars. The gill pharynx is followed by the hepatic region with metameric hepatic sacculations comprising cells with numerous large phagosomes indicating active intracellular digestion. The intestine is filled with undigested food residues including detritus flakes and remnants of various organisms. The intestine content was examined using SEM and histological studies. We analyzed the ratio of organic and mineral components of the bottom sediment from the habitat of Q. malakhovi, and the stable isotope composition of carbon and nitrogen in the intestine content, in the tissues, and in the sediment from the habitat of the studied acorn worm. Using the obtained data and the photographs and videotapes in situ, we have proposed a hypothesis explaining the feeding mechanism of Q. malakhovi. The width of the ventral ciliary groove of the lateral collar lips (100–200 μm) regulates the maximum size of particles that enter the pharynx of the animal. The minimum size of the sediment particles selected by Q. malakhovi is determined by the width of the gill pores and is 1–3 μm. The selected sediment particles move through the peripheral pharyngeal canals to the hepatic region, where the digestion of the food is carried out. In the posthepatic intestine, weight of the gut contents might serve as an anchor that holds the heavily watered jelly-like torquaratorid body at the bottom during the feeding.

Benchmarking Water-Limited Yield Potential and Yield Gaps of Shiraz in the Barossa and Eden Valleys

Background and Aims. Vineyard performance is impacted by water availability including the amount and seasonality of rainfall, evapotranspiration, and irrigation volume. We benchmarked water-limited yield potential (Yw), calculated yield gaps as the difference between Yw and actual yield, and explored the underlying environmental and management causes of these gaps. Methods and Results. The yield and its components in two sections of 24 Shiraz vineyards were monitored during three vintages in the Barossa zone (GI). The frequency distribution of yield was L-shaped, with half the vineyards below 5.2 t·ha−1, and an extended tail of the distribution that reached 24.9 t·ha−1. The seasonal ratio of actual crop evapotranspiration and reference evapotranspiration was below 0.48 in 85% of cases, with a maximum of 0.65, highlighting a substantial water deficit in these vineyards. A boundary function relating actual yield and seasonal rainfall was fitted to quantify Yw. Yield gaps increased with an increasing vine water deficit, as quantified by the carbon isotope composition of the fruit. The yield gap was smaller with higher rainfall before budburst, putatively favouring early-season vegetative growth and allocation to reproduction, and with higher rainfall between flowering and veraison, putatively favouring fruit set and berry growth. The gap was larger with higher rainfall and lower radiation between budburst and flowering. The yield gap increased linearly with vine age between 6 and 33 yr at a rate of 0.3 t·ha−1·yr−1. The correlation between yield gap and yield components ranked bunch weight ≈ berries per bunch > bunch number > berry weight; the minimum to close the yield gap was 185,000 bunches ha−1, 105 g bunch−1, 108 berries bunch−1, and 1.1 g berry−1. Conclusions. Water deficit and vine age were major causes of yield gaps. Irrigation during winter and spring provides an opportunity to improve productivity. The cost of dealing with older, less productive vines needs to be weighed against the rate of increase in yield gap with vine age. Significance of the Study. A boundary function to estimate water-limited yield potential returned viticulturally meaningful yield gaps and highlighted potential targets to improve vineyard productivity.

Preservation of organic C and N isotope signatures from water column to sediments in the anoxic and ferruginous Pavin lake

Organic carbon (Corg) and bulk nitrogen (Nbulk) isotope compositions (respectively δ13Corg and δ15Nbulk) of sedimentary rocks deposited in the Archean and Proterozoic eons are commonly used as proxies for the biomass δ13Corg and δ15Nbulk values in paleo-environment and paleo-ecosystem studies. Many sedimentary rocks from these eons were deposited under ferruginous conditions, but it remains unknown whether C and N isotope signatures are preserved during diagenetic degradation of organic matter under this chemical state. Lake Pavin, Massif Central, France, is a permanently stratified freshwater lake with anoxic and ferruginous deep waters and represents a unique natural site for evaluating the impact of organic matter decomposition predominantly through iron redox cycling and methanogenesis. Here we present C and N isotope compositions of sediment collected from traps placed at different depths in the water column and from cores recovered in the anoxic zone of the lake. The δ13Corg and δ15Ntot values in the living biomass of Lake Pavin are estimated from the shallowest sediment traps. The biomass δ13Corg values show a seasonally-controlled bimodal distribution, around −23.3 ± 1.7 ‰ (2σ) in September and November (organic matter produced in summer and autumn) and −28.0 ± 2.6 ‰ (2σ) in April and June (organic matter produced in winter and spring). These seasonal variations are attributed to variable δ13C values in dissolved inorganic carbon, inherited from fluctuation in the relative rates of photosynthesis versus respiration. The δ15Ntot values are near −1 ‰ for most seasons, indicating that the dominant N source to photosynthetic biomass is atmospheric N2.Along the water column profile, organic matter mineralization released about 20% of C and 30% of N, with only moderate effects on δ13Corg and δ15Ntot (decreased by 1.3 and 0.4 ‰, respectively). Although the water column data show significant seasonal variations, global annual fluxes, estimated from regular trap collection over 15 months, are remarkably consistent with the top sediment composition. In the sedimentary column, microbial degradation through ferric iron reduction and methanogenesis induces a loss of 40–50% Corg and 50–60% Ntot, with negligible modification of δ13Corg and δ15Ntot. We conclude that although the C/N ratio increases significantly as organic matter is degraded in the water column and in the first ∼15 cm of the sedimentary column, δ13Corg and δ15Ntot are well preserved, supporting the idea that C and N isotope signatures can be reliably interpreted as tracers of primary biomass in sedimentary rocks deposited in anoxic and ferruginous environments.

Identifying emission sources of CH4 in East Asia based on in-situ observations of atmospheric δ13C-CH4 and C2H6

Methane (CH4) is the second most important greenhouse gas influenced by human activity. The increase in atmospheric CH4 concentrations contributed ~23 % to the anthropogenic radiative forcing (Saunois et al., 2020). The current anthropogenic CH4 emissions trajectory implies that large emissions reductions are needed to meet the target of the Paris Agreement (Nisbet et al., 2019). For effective regulation of CH4, it is important to identify spatiotemporal emission sources, in particular those from East Asia – one of the largest CH4 emitters. In this study, we present in-situ observations of atmospheric CH4 concentrations (i.e., dry air mole fractions in part per billion (ppb)) and carbon isotopic compositions of CH4 made during 2017–2020 at the Gosan station (GSN, 33.3°N, 126.2°E, 72 m a.s.l) which is representative of regional background conditions in East Asia. The annual growth rate of the observed CH4 baseline concentrations was 11 ± 1 ppb yr−1. The enhanced pollution concentrations of CH4 showed seasonally distinctive correlations with the corresponding δ13C-CH4. The CH4 source isotopic signature for winter derived based on both the Keeling and Miller-Tans approaches was −40.7 ± 3.4 ‰, suggesting dominant thermogenic sources (e.g., coal and/or gas combustion), whereas the source signature for summer was estimated as −54.1 ± 1.2 ‰, which seemed to represent both microbial sources (e.g., rice paddies) and fossil fuel sources of CH4 emissions. Based on the δ13C-CH4 source signatures, we were able to infer that the proportional contribution of microbial sources to CH4 summer emissions was ranges from 45 to 79 %. The finding indicates that microbial sources account for a substantial portion of CH4 summer emissions, consistent with estimates of 74–80 % derived from the observed correlation between CH4 and C2H6, which serves as a complementary tracer for fossil fuel sources.

Woody species with higher hydraulic efficiency or lower photosynthetic capacity discriminate more against 13C at the global scale

Leaf carbon isotope composition (δ13C) provides an integrative record on the carbon and water balance of plants over long periods. Photosynthetic ability and hydraulic traits which are highly associated with stomatal behavior could affect leaf δ13C. Association between photosynthetic ability and leaf δ13C has been examined, however, how hydraulic traits influence leaf δ13C has not been fully understood. To fill this gap, we investigated the variations in leaf δ13C among 2591 woody species (547 shrub and 2044 tree species), and analyzed the link of leaf δ13C with leaf photosynthetic and xylem hydraulic traits. Our result showed that leaf δ13C was positively correlated to leaf photosynthetic ability and capacity. For hydraulic traits, leaf δ13C was negatively related to hydraulic conductivity (Ks), xylem pressure inducing 50 % loss of hydraulic conductivity (P50) and vessel diameter (Vdia). Associations of leaf δ13C with xylem hydraulic traits indicate woody species with stronger hydraulic safety discriminated less against 13C, while woody species with higher hydraulic efficiency had more negative leaf δ13C. Shrub species, which showed a lower Vdia and P50, had a significant less negative leaf δ13C than tree species. Furthermore, woody species inhabiting in dry regions discriminated less against 13C than those growing in humid regions. Moreover, leaf δ13C displayed a low phylogenetic signal based on Blomberg's K statistic. Overall, woody species with a higher leaf photosynthetic ability or stronger hydraulic safety system discriminated less against 13C and adopt the provident water use strategy.

Grain carbon isotopes indicate the ability of wheat plants to maintain enhanced intrinsic water-use efficiency even after short-term exposure to high temperatures and drought

Minimizing the impact of heat and drought on crop yields requires varieties with effective protective mechanisms. We tested the hypothesis that even a short-term high temperature amplifies the negative effects of reduced water availability on leaf gas-exchange, but can induce long-lasting improvement in plant water-use efficiency after the stress period. Accordingly, three common varieties of winter wheat (Triticum aestivum) were grown under field conditions. During the stem extension, the plants were exposed to distinct temperatures (daily maximum 26 vs. 38 °C), water availabilities (75% of field water capacity vs. permanent wilting point), and their combination for 14 days.All treatments reduced light-saturated rates of CO2 assimilation and transpiration, particularly when heat and drought were combined. Drought enhanced water-use efficiency (WUE) in all varieties (31.4–36.4%), but not at high temperatures (decrease by 17–52%). Intrinsic WUE (iWUE), determined from the stable carbon isotope composition of grains, was enhanced by 7.9–37% in all treatments and varieties; however, not all changes were significant. The combination of heat and drought tended to increase total protein content in grains but reduced spike productivity. Noticeably, the strongest decline in spike productivity was observed in Elan – the variety displaying the smallest enhancement of iWUE, while it was negligible in Pannonia which shows the most pronounced improvement of iWUE. We conclude that even several hot and dry days can improve iWUE for the rest of the vegetation season. This improvement, however, does not necessarily lead to increased crop productivity possibly due to physiological trade-offs.

Combined use of 87Sr/86Sr and carbon isotopes with multielemental analysis for the geographical authentication of Tunisian and European olive oils.

The geographical authentication in the agrifood industry has become a major issue to guarantee the quality of food products. Olive oil (OO) is particularly a complex matrix and establishing a reliable approach for linking OO samples to their origin is an analytical challenge. In this study, the isotopic composition of carbon, strontium and the concentrations of seventeen elements were determined in OOs from Tunisia, Southern France and the South Basque country. The preliminary results overlapped and showed that, taken individually, the isotopic and elemental approaches were not discriminant. A linear discriminant analysis applied to δ13C, 87Sr/86Sr and to the concentrations of 4 selected trace elements (Fe, Mn, V and Cr) allowed to classify, with high resolution, olive oils into 3 groups according to their provenance. The combination of the plant growing environment, the geological background, the mineral composition of the soil and the production process lead to a novel approach to deal with fraudulent practices in OO sector.

Agrivoltaics mitigate drought effects in winter wheat.

Climate change is expected to decrease water availability in many agricultural production areas around the globe. At the same time renewable energy concepts such as agrivoltaics (AV) are necessary to manage the energy transition. Several studies showed that evapotranspiration can be reduced in AV systems, resulting in increased water availability for crops. However, effects on crop performance and productivity remain unclear to date. Carbon-13 isotopic composition (δ13 C and discrimination against carbon-13) can be used as a proxy for the effects of water availability on plant performance, integrating crop responses over the entire growing season. The aim of this study was to assess these effects via carbon isotopic composition in grains, as well as grain yield of winter wheat in an AV system in southwest Germany. Crops were cultivated over four seasons from 2016-2020 in the AV system and on an unshaded adjacent reference (REF) site. Across all seasons, average grain yield did not significantly differ between AV and REF (4.7 vs 5.2 t ha-1 ), with higher interannual yield stability in the AV system. However, δ13 C as well as carbon-13 isotope discrimination differed significantly across the seasons by 1‰ (AV: -29.0‰ vs REF: -28.0‰ and AV: 21.6‰ vs REF: 20.6‰) between the AV system and the REF site. These drought mitigation effects as indicated by the results of this study will become crucial for the resilience of agricultural production in the near future when drought events will become significantly more frequent and severe.

From forest to pastures and silvopastoral systems: Soil carbon and nitrogen stocks changes in northeast Amazônia

The Cerrado-Amazon ecotone has been under intense pressure over the years from agricultural and urban expansion, both of which are land uses that directly affect soil quality. The objective of this work was to evaluate the responses of soil carbon and nitrogen stocks, soil organic matter (SOM) quality, carbon isotopic composition (13C) from C3 and C4 plants and carbon sequestration and emission to silvopastoral systems, natural vegetation (NV), fallow pasture (FP), and intensive management pasture (IMP). Silvopastoral systems had different shading levels: 25 % (SP25), 50 % (SP50) and 75 % (SP75). The grass cultivated in all treatments was Megathyrsus maximus cv. Mombaça. The experimental design consisted of four replicates, and collection sites were distributed in strips throughout the study areas at different soil layers (0.00–0.05; 0.05–0.15; 0.15–0.30; 0.30–0.60 and 0.60–1.0 m deep). The conversion of natural vegetation in FP areas and silvopastoral systems (SP25 and SP75) led to increases in total C and N stocks (up to 1.0 m) when compared to other land use systems (SP50, IMP, and NV), which did not occur with total labile-C and C-POM. FP, SP25 and SP75 significantly increased labile C stocks in MAOM found in the 0.30–0.60 m layer. A greater enrichment of 13C in MAOM was observed with increasing depth for silvopastoral systems (SP25, SP50 and SP75) and NV. C-MAOM stocks derived from C3 plants were higher in soils under SP25 and SP75 and from C4 plants under FP. C-POM stocks were higher in all silvopastoral systems under study. IMP and FP affected δ13C values in MAOM and POM, especially in the 0.00–0.05 m layer. C sequestration increased under FP and SP25, with greater contributions from C4 and C3 plants, respectively. Of the silvopastoral systems, SP25 had the highest C stock in soil and contributed to the sequestration of 1.67 Mg C ha−1 yr−1.

Clumped isotope evidence for microbial alteration of thermogenic methane in terrestrial mud volcanoes

Abstract Methane in oil reservoirs originates mostly from thermogenic sources, yet secondary microbial methane production from petroleum biodegradation is known to be pervasive. The conventional approach for identifying this secondary microbial methane commonly relies on geochemical characteristics of other gas molecules such as the carbon isotopic composition of carbon dioxide and propane. This information is sometimes obscured by isotopic variations in source material and may not be available in certain geological reservoirs. To better constrain the presence of secondary microbial methane, we studied the clumped isotopologue compositions of methane in terrestrial Azerbaijanian mud volcanoes, which support the occurrence of secondary microbial gas. Here, a deficit in Δ12CH2D2 of thermogenic methane occurs due to different δD of hydrogen sources that contribute to the formation of methane molecules (i.e., combinatorial effect). The Δ12CH2D2 is expected to move toward equilibrium as thermal maturity increases. More importantly, both Δ13CH3D and Δ12CH2D2 values of methane approach low-temperature thermodynamic equilibrium in most gases, suggesting that the original thermogenic methane has been altered by newly formed microbial methane in addition to isotope exchange among methane molecules catalyzed by the methyl-coenzyme M reductase enzyme. We conclude that methane clumped isotopes provide a unique proxy for identifying secondary microbial methane and understanding the exact evolution stages for natural gases.

Subcretion of altered oceanic crust beneath the SW São Francisco Craton, Brazil – A stable isotope study on diamonds and their inclusions

The presence of diamonds of lherzolitic, eclogitic and websteritic paragenesis in proximal alluvial deposits on the southwestern edge of the São Francisco Craton documents the incorporation of subducted oceanic crust and associated metasomatism through slab dehydration fluids affecting the local SCLM. To better constrain the subduction-association of diamond substrates and metasomatic events, we conducted a combined study of the δ13C-δ15N-[N] characteristics of 81 diamonds and the δ18O values of four of their eclogitic garnet inclusions. Diamond carbon isotope compositions range from −25.5 to +0.5‰, with 13C-depleted diamonds (≤ −7‰) being exclusively of eclogitic/websteritic paragenesis while the 13C-enriched (≥ −2‰) tail of the distribution is related to diamonds with lherzolitic inclusions. Nitrogen isotope values range from −14.2 to +25.5‰, with about half of the values being positive. A general absence of coherent trends in δ13C-δ15N-[N] across growth zones implies that diamond formation did not occur under fluid-limited conditions. Instead, the observed heterogeneity in carbon and nitrogen isotope compositions reflects contributions of distinct source reservoirs hosted in both altered oceanic crust and Earth's mantle. Nitrogen contents peak around a δ15N value of −3.5‰, indicating that more N-rich fluids, presumably representing a primitive endmember composition, have a mantle-like δ15N signature. While positive and negative δ15N values occur equally near the δ13C mantle value (−5 ± 2‰), 13C-depleted diamonds have nitrogen isotope compositions skewed towards positive values.13C depletion and 15N enrichment is a signature of biogenic carbonates/organic matter and low-T clays in uppermost, basaltic sections of oceanic crust that experienced low-temperature seawater alteration prior to subduction. Correspondingly, the oxygen isotope compositions of eclogitic garnet inclusions fall in a restricted range between +5.5‰ to +7.0‰. For three of the four samples, the stable isotope signatures of inclusions and host diamonds display perfect agreement, with the intensity of seawater alteration signatures, in the form of garnet inclusion 18O enrichment and host diamond 13C depletion and 15N enrichment, increasing together. For the fourth sample, the δ18O signature of the garnet inclusion (+5.5‰) and δ13C-δ15N signatures of the diamond host (−25 and + 19‰, respectively) are decoupled. While the mantle-like δ18O signature indicates a diamond substrate derived from deeper levels in oceanic crust (e.g., deep sheeted dikes), the diamond-forming fluids must have originated from sources that originally resided near the sea water interface. A viable mode of mixing such disparate isotopic signatures is the interaction of diamond-forming fluids derived from shallow oceanic crust altered at low-temperatures with eclogitized substrates originally formed in deeper levels of oceanic crust. This process likely occurred during diamond formation in a tectonic subduction mélange, which juxtaposes deeper and shallower levels of oceanic crust. In the lithospheric mantle above the subcreted oceanic slab, the elevated carbon isotope and highly variable nitrogen isotope compositions of diamonds formed in lherzolitic substrates likely relate to devolatilization and/or melting of principally oceanic sediments containing marine carbonates and clays and subsequent mixing with mantle-derived volatile components. In combination, our diamond and inclusion stable isotope data provide insight into multiple processes that promote diamond formation both inside subducted slabs accreted to the São Francisco cratonic keel and in adjacent subcontinental lithospheric mantle.

Petrological, Geochemical and Chronological Characteristics of Dolomites in the Permian Maokou Formation and Constraints to the Reservoir Genesis, Central Sichuan Basin, China

The Middle Permian Maokou Formation in the Sichuan Basin has huge resources and is an important target for natural gas exploration. In recent years, significant exploration breakthroughs have been made in the dolomite field of member Mao-2 in central Sichuan, and the gas production of several wells has exceeded 1 × 106 m3/d, indicating promising prospects for exploration. However, the origin of the dolomite reservoir in member Mao-2 remains ambiguous, which restricts the accurate prediction of favorable reservoirs. This study focuses on drilling in the Hechuan area as its research object, by using a detailed description of the cores from member Mao-2 of seven wells; samples were selected for tests of the degree of dolomite cation ordering, stable carbon and oxygen isotopic compositions, strontium isotopic composition, rare earth elements, LA-ICP-MS element mapping and U-Pb dating. It is clarified that: (1) The crystalline dolomite of member Mao-2 in the Hechuan area is the main reservoir rock, and the heterogeneous vugs and fractures are the main reservoir space. The dolomite in member Mao-2 has been characterized by a low degree of cation ordering value (avg. 0.59), with values of δ13C (avg. 3.87‰), δ18O (avg. −7.15‰) and 87Sr/86Sr (avg. 0.707474) having similar geochemical characteristics to Middle Permian seawater; the REEs normalized distribution patterns have similar characteristics to limestone; and the U-Pb age (261.0~262.0 Ma) corresponds to the age in the Capitanian stage of the Permian Guadalupian Series. (2) Petrological studies show that member Mao-2 has vertical karstification zonation characteristics; syngenetic karstification controls the formation of a large-scale fracture-cave system in the phreatic zone; the dolomitization of sediment in the fracture-cave system occurred during the penecontemporaneous period with locally restricted seawater. (3) The main controlling factors of the reservoir were syngenetic karstification, early dolomitization and hydrothermal dissolution related to Emei taphrogenesis. The research results are of great significance for dolomite reservoir prediction; the highlands of paleogeomorphology with syndepositional faults are favorable areas for dolomite reservoirs.

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Studying the isotopic composition of microbial methane with a genetically-tractable methanogen

Nearly all biogenic methane is produced by a group of microorganisms called methanogenic archaea (or methanogens). Methanogens can use a variety of substrates, such as H2 + CO2, acetate, and methylated compounds, for methanogenesis. Previous studies have shown that the stable carbon and hydrogen isotopic compositions of methane produced by methanogens can vary drastically depending on the substrate composition and concentration in the environment. For instance, the concentration of H2 in the environment has a substantial impact on the isotopic composition of methane derived from hydrogenotrophic methanogenesis (reduction of CO2 to methane using H2 as the electron donor) (Valentine et al. 2004, Penning et al. 2005). While there is substantial empirical data on isotopic signatures of methane from different substrates and under different conditions, the physiological and molecular features that control these values are not as well understood. To address this, we are using the metabolically diverse and genetically tractable methanogen, Methanosarcina acetivorans as a model system to uncover key cellular processes that control the stable bulk isotopic composition of methane (i.e., 13C/12C and D/H ratios), and the distributions of the “clumped” 13CH3D and 12CH2D2 isotopologues. The methanogen M. acetivorans grows on a wide variety of compounds such as acetate, methanol, methylamines, and methylsulfides. We found that the methylotrophic pathways (for methanol and trimethylamine) and the aceticlastic pathway have large and similar primary hydrogen isotopic effects (α of ~0.45). These data are in contrast to previous findings and imply a minor isotopic exchange between CH4 and H2O (Valentine et al. 2004, Gruen et al. 2018). Focusing first on the methylotrophic pathway, we generated mutants of two key enzymes in the methylotrophic pathway: a) methyl coenzyme M reductase (Mcr) that catalyzes the last step in methanogenesis and b) methyltransferases that catalyze the first step in methylotrophic methanogenesis from methanol (Mta). A mutant with reduced Mcr expression had no observable change in the hydrogen isotopic effect relative to the wild-type, validating the initial observation of minimal H2O-CH4 hydrogen isotopic exchange. One of the Mta mutants, which only expressed a specific methyltransferase isoform, had a smaller carbon isotopic effect relative to the other isoforms (α of ~1.074 vs. ~1.080). Since the isoforms are thought to be identical in structure, the different isotopic effects could result from differential expression of each isoform, or from different kinetic properties. By combining our genetic approaches with traditional and high-resolution isotopic analytical methods, we aim to develop a quantitative understanding of the mechanisms that control the isotopic compositions of biological methane. Our preliminary results show that M. acetivorans would be an ideal candidate for such research, which could help in understanding methanogens’ physiology in natural environments in past, present, and future Earth.

Lithium isotope systematics of methane-seep carbonates as an archive of fluid origins and flow rates

Fluid sources and flow rates at methane seeps control gas hydrate formation, and are potentially archived in authigenic carbonates. Lithium (Li) in seeping fluids exhibits a broad range of isotopic fractionation during its ascent to the seafloor, providing information on temperatures and flow rates of the fluids. To test the applicability of Li isotopes as a fluid tracer in authigenic carbonates, this study investigates the element and Li isotope composition of methane-seep carbonates collected from the Black Sea and the Japan Sea. The Li/Ca ratios and δ7Li values of the aragonite-rich and calcite-rich samples (10–60 μmol mol−1 and 3.5–15.6‰; 21–142 μmol mol−1 and 8.3–24.4‰, respectively) are distinct from those in carbonates precipitated from seawater, and indicate an input of deep-seated fluids enriched in 6Li. After correcting for a minor admixture of clay-bound Li, the Li/Ca ratios and δ7Li values of the parent fluids were estimated to be 2400–11 000 μmol mol−1 and 17.2–34.5‰, respectively. Variations in the estimated values cannot be explained by fluid mixing alone, but rather reflect the isotopic fractionation caused by interaction of the deep fluids with secondary minerals during fluid ascent. The δ7Lifluid values as low as 17.2‰ for the Japan Sea carbonates indicate a deep fluid source with estimated temperatures of >26 °C derived from >250 m below the seafloor. Reaction-transport modeling shows that the deep fluid was transported to the zone of carbonate precipitation with advection rates of ≥10−2 cm yr−1. Higher advection rates were indicated for the Japan Sea fluids than for Black Sea fluids, probably due to a larger contribution of dissolved methane to hydrate formation. Spatiotemporal variations in fluid sources and advection rates were indicated by distinct (Li/Ca)fluid and δ7Lifluid values between the carbonates and pore fluids. The Li isotope systematics of seep carbonates are a potent indicator of the origin and flow rate of deep-seated fluids and their spatiotemporal variability, which is explored herein in detail for the first time.

Water deficit and potassium affect carbon isotope composition in cassava bulk leaf material and extracted carbohydrates.

Cassava (Manihot esculenta Crantz) is an important root crop, which despite its drought tolerance suffers considerable yield losses under water deficit. One strategy to increase crop yields under water deficit is improving the crop's transpiration efficiency, which could be achieved by variety selection and potassium application. We assessed carbon isotope composition in bulk leaf material and extracted carbohydrates (soluble sugar, starch, and cellulose) of selected leaves one month after inducing water deficit to estimate transpiration efficiency and storage root biomass under varying conditions in a greenhouse experiment. A local and improved variety were grown in sand, supplied with nutrient solution with two potassium levels (1.44 vs. 0.04 mM K+) and were subjected to water deficit five months after planting. Potassium application and selection of the improved variety both increased transpiration efficiency of the roots with 58% and 85% respectively. Only in the improved variety were 13C ratios affected by potassium application (up to - 1.8‰ in δ13C of soluble sugar) and water deficit (up to + 0.6‰ in δ13C of starch and soluble sugar). These data revealed a shift in substrate away from transitory starch for cellulose synthesis in young leaves of the improved variety under potassium deficit. Bulk δ13C of leaves that had fully developed prior to water deficit were the best proxies for storage root biomass (r = - 0.62, r = - 0.70) and transpiration efficiency (r = - 0.68, r = - 0.58) for the local and improved variety respectively, making laborious extractions redundant. Results obtained from the youngest fully developed leaf, commonly used as a diagnostic leaf, were complicated by remobilized assimilates in the improved variety, making them less suitable for carbon isotope analysis. This study highlights the potential of carbon isotope composition to assess transpiration efficiency and yield, depending on the chosen sampling strategy as well as to unravel carbon allocation processes.

Isotopic characteristics of Crimean collection wines

Collection wines are a special category of winemaking products, which acquire their unique characteristics during aging. One of the first wine collections in Russia was created in 1833–1835 in the Magarach state establishment, which is now the AllRussian National Research Institute of Viticulture and Winemaking “Magarach”. The large part of the Magarach collection consists of strong wines. These wines were produced by introducing alcohol of different nature (grape, grain, molasses), which was conditioned by the requirements of the legislation that was in force at the moment of production. It was interesting to find out the nature of alcohol that was used for wine alcoholization in the preceding decades. To this end, the method of isotope mass spectrometry was used. In addition, it was necessary to establish whether the values of isotopic characteristics of carbon, oxygen and hydrogen of ethanol of grape origin have changed over the last decades. The objects of the research were 15 samples of Crimean collection wines, one of which was dry wine of the 1982 harvest and 14 strong wines were from harvests of the last century with the oldest wine made from grape of the 1937 harvest. The composition of the stable carbon isotopes of ethanol in the samples was determined using the analytical complex Delta V Advantage. It has been found that the dry wine of 1982 harvest was produced without adding alcohol. Although the age of the drink is more than 40 years, the values of the isotopic characteristics of the elements of ethanol are typical for the modern products made exclusively from grape. Thus, the isotopic characteristics of carbon, oxygen and hydrogen of sugars of grape berry did not change significantly over the studied period. When analyzing strong wines, it has been established that the values of the isotopic characteristics of carbon were in a range of –27.23‰ to –24.13‰ (except one sample with a value of –15.49‰); the corresponding values for oxygen were from 7.21‰ to 17.09‰ and for hydrogen from –270‰ to –216‰. Therefore, for most studied samples, the obtained values of isotopic characteristics of the elements of ethanol were typical precisely for the mixture of grape and grain alcohols.

Trophic characteristics of zooplankton and zoobenthos in certain water masses in the western Chukchi Sea, by δ<sup>15</sup>N and δ<sup>13</sup>С data

To compare trophic characteristics for pelagic and bottom communities of zooplankton and zoobenthos in the western Chukchi Sea, isotopic composition of carbon and nitrogen (δ13С and δ15N) is determined for three different types of water masses. The main area of the southwestern part of the sea (to the south of 71оN) is occupied by the surface and subsurface water masses invaded from the Bering Sea with relatively high δ13C and low δ15N values. The cold Arctic waters prevail in the northwestern area. The western periphery of the sea is influenced by the water masses from the East Siberian Sea transported by the alongshore Siberian Current, distinguished by significantly lowered δ13С values and somewhat (by 1–3 ‰) increased δ15N values. A stable difference in the carbon isotopic composition between zooplankton and benthic invertebrates is revealed in the southwestern Chukchi Sea that means that the benthic organisms are based mainly on the bottom food chain, with a minimal input of organic matter from the water column. In the area influenced by the water masses from the East Siberian Sea, benthic invertebrates have the lowest values of δ13С comparable to those of zooplankton that is a sign of organic sediments from the water column utilization, partly terrigenous in origin. The role of primary producers for plankton and benthos in the northwestern Chukchi Sea has not been fully elucidated and requires additional studies. However, the main trophic base for the benthic community is assumed there to be organic matter removed from the water column, originated mainly from ice algae.