δ13C Compositions of Bacteriohopanetrol Isomers Reveal Bacterial Processes Involved in the Carbon Cycle

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.