Search

Summary Here, we explore the use of bacterial and archaeal lipid carbon and hydrogen isotopic compositions to explore carbon cycling processes in modern settings and in response to ancient warming events. Investigations of Holocene peat and Eocene lignites reveal both stability over long time-scales but rapid, transient responses to climate perturbations.

Summary Elementar UK Ltd present present two pioneering new techniques for the analysis of larger and/or more polar organic molecules of biogeochemical interest which are traditionally considered unamenable for gas chromatography stable isotope ratio mass spectrometry (GC-IRMS) analysis, which have been developed by and in collaboratoration with Elementar GC-IRMS users from a variety of laboratories. This includes a rapid one-step derivatization procedure for the isotope analysis of the non-exchangeable hydrogen in mono and disaccharides and subsequent conversion to H2 by chromium reduction ( Abrahim et al, 2020 ), and the development of a high-temperature GC-IRMS (HTGC-IRMS) methodology employed for the analysis of a suite of compounds of interest ( Lengger et al., 2021 ).

Summary Bacteria play key roles in the carbon cycle. In many sediments and peatlands, methanotrophic bacteria consume a portion of released methane, reducing the emissions of this potent greenhouse gas. In marine oxygen minimum zones (OMZ) and other anoxic settings, anaerobic ammonium oxidizing (anammox) bacteria remove bioavailable nitrogen while performing chemoautotrophic carbon fixation. Methanotrophic and anammox bacteria synthesize a wide number of complex bacteriohopanepolyols (BHPs), comprising notably several stereoisomers of bacteriohopanetetrols (BHTs), which are used as biomarker lipids. We used a gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) method to measure the δ13C of BHTs of cultured bacteria. These δ13C values were combined with bulk isotopic measurements of the bacterial biomass and δ13C analyses of the bacterial growth substrates to establish carbon isotopic fractionation from substrate to biomass to BHT lipid. We demonstrated that bacteria using different metabolic pathways produced distinct fractionation factors between substrate and BHTs, which potentially allows for distinguishing BHTs produced by ‘Ca. Brocadia’ and methanotrophs from other freshwater producers (e.g. in peatlands). Measurement of BHT-specific fractionation factors allowed us to better constrain the contribution of anammox bacteria to fixed carbon in OMZ. This work expands the application of BHT isomers to isotopically identify carbon cycle processes.

Rationale: The hydrogen isotopic composition of lipids (δ 2Hlipid) is widely used in foodscience and as a proxy for past hydrological conditions. Determining the δ 2H values of large,well-preserved triacylglycerides and other uniquely microbial lipids, such as glycerol dialkylglycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been possible due to their size which prohibits analysis by traditional gas chromatographypyrolysis isotope ratio mass spectrometry (GC-P-IRMS).Methods: We determined the δ 2H values of large, polar molecules and applied hightemperature gas chromatography (GC) methods on a modified GC-P-IRMS system. Themethods were validated using authentic standards of large, functionalised molecules(triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2Hvalues determined by elemental analyser pyrolysis isotope ratio mass spectrometry (EA-PIRMS); and subsequently applied to the analysis of GDGTs in a sample from a methaneseep and a Welsh peat.Results: δ 2H values of TAGs agreed within error between different between GC-P-IRMS andEA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipidGDGTs with up to three cyclisations could be analysed: δ 2H values were not significantlydifferent between methods with standard deviations of 5 to 6 ‰. When environmentalsamples were analysed, δ 38 2H values of isoGDGTs were 50 ‰ more negative than those ofterrestrial brGDGTs.Conclusions: Our results indicate that the high temperature GC-P-IRMS (HTGC-P-IRMS)method developed here is appropriate to determine the δ 2H values of TAGs, GDGT lipidswith up to two cyclisations, and potentially other high molecular weight compounds. Themethodology will widen the current analytical window for biomarker and alimentary lightstable isotope analyses. Moreover, our initial measurements suggest that bacterial andarchaeal GDGT δ 2H values can record environmental and ecological conditions

In response to rising CO2 concentrations and increasing global sea surface temperatures, oxygen minimum zones (OMZ), or “dead zones”, are expected to expand. OMZs are fueled by high primary productivity, resulting in enhanced biological oxygen demand at depth, subsequent oxygen depletion, and attenuation of remineralization. This results in the deposition of organic carbon‐rich sediments. Carbon drawdown is estimated by biogeochemical models; however, a major process is ignored: carbon fixation in the mid‐ and lower water column. Here, we show that chemoautotrophic carbon fixation is important in the Arabian Sea OMZ; and manifests in a 13C‐depleted signature of sedimentary organic carbon. We determined the δ13C values of Corg deposited in close spatial proximity but over a steep bottom‐water oxygen gradient, and the δ13C composition of biomarkers of chemoautotrophic bacteria capable of anaerobic ammonia oxidation (anammox). Isotope mixing models show that detritus from anammox bacteria or other chemoautotrophs likely forms a substantial part of the organic matter deposited within the Arabian Sea OMZ (~17%), implying that the contribution of chemoautotrophs to settling organic matter is exported to the sediment. This has implications for the evaluation of past, and future, OMZs: biogeochemical models that operate on the assumption that all sinking organic matter is photosynthetically derived, without new addition of carbon, could significantly underestimate the extent of remineralization. Oxygen demand in oxygen minimum zones could thus be higher than projections suggest, leading to a more intense expansion of OMZs than expected.