Bacterial physiology highlighted by the δ13C fractionation of bacteriohopanetetrol isomers

Rachel Schwartz-Narbonne, Philippe Schaeffer, Sabine K. Lengger, Jerome Blewett, D. Martin Jones, Estelle Motsch, Andrew Crombie, Mike S.-M. Jetten, Deirdre Mikkelsen, Philippe Normand, Guylaine H. l. Nuijten, Richard D. Pancost, Darci Rush

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Abstract

Lipid biomarkers, such as the various bacteriohopanetetrol (BHT) isomers studied here, are useful tools in tracing bacterially mediated nitrogen and carbon cycle processes affecting greenhouse gas emissions, including the anaerobic oxidation of ammonia. Three BHT isomers occur commonly in the environment. By gas chromatography, BHT-34S elutes first; it is produced by numerous bacteria. The two later eluting isomers are more constrained in their origin. The marine anammox bacteria ‘Ca. Scalindua’ is the only known producer of a BHT isomer of unknown stereochemistry (BHT-x), making BHT-x a diagnostic biomarker in anoxic marine settings. The BHT-34R isomer is produced by three freshwater aerobic heterotrophic producers (Frankia spp., Acetobacter pasteurianus, and Komagataeibacter xylinus), a freshwater serine-cycle (Type II) methanotroph (Methylocella palustris), and the freshwater anammox ‘Ca. Brocadia’, which makes the detection of freshwater anammox using BHT-34R more complicated. We investigated whether the source of BHT-34R in freshwater environments could be ascertained via its δ 13C value. We used conventional on-column gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) (as opposed to high temperature GC-C-IRMS) to determine the δ 13C composition of acetylated BHT isomers in cultured bacteria and bacterial enrichments. We combined these with bulk biomass and substrate δ 13C compositions to establish carbon isotopic fractionation factors. The two anammox genera had large fractionation factors from dissolved inorganic carbon (DIC) to biomass (Δ 13C biomass – DIC = –43.8 to –26.4 ‰) and to BHTs (Δ 13C BHT – DIC = –53.8 to –38.2 ‰), which clearly distinguished them from the freshwater aerobic heterotrophic producers (Δ 13C biomass – substrate = –2.3 to –0.1 ‰; Δ 13C BHT – substrate = –12.8 to 5.2 ‰). Methylocella assimilated mainly carbon from DIC, rather than from methane, into its biomass and BHT, and previous work suggested this assimilation comes with relatively small fractionation. Thus, in peatlands, the BHT δ 13C values of Methylocella would not reflect the low δ 13C values of biogenic methane. Consequently, the presence of BHT-34R with low δ 13C values relates to ‘Ca. Brocadia’ and presents a novel tool to trace anammox in freshwater environments.

Original languageEnglish
Article number104617
JournalOrganic Geochemistry
Volume181
Early online date12 May 2023
DOIs
Publication statusPublished - Jul 2023

Keywords

  • Anammox
  • Bacteriohopanepolyols
  • Bacteriohopanetetrols
  • Carbon isotopes
  • Carbon-13
  • Hopanoids
  • Lipid biomarkers
  • Methanotroph

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