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Abstract

The 2-methylhopanes (2-MeHops) are molecular fossils of 2-methylbacteriohopanepolyols (2-MeBHPs) and among the oldest biomarkers on Earth. However, these biomarkers’ specific source is currently unexplained, including whether they reflect an expansion of marine cyanobacteria. Here we study the occurrence of 2-MeBHPs and the genes involved in their synthesis in modern bacteria and explore the occurrence of 2-MeHops in the geological record. We find that the gene responsible for 2-MeBHP synthesis (hpnP) is widespread in cyano- and ⍺-proteobacteria, but absent or very limited in other classes/phyla of bacteria. This result is consistent with the dominance of 2-MeBHP in cyano- and ⍺-proteobacterial cultures. The review of their geological occurrence indicates that 2-MeHops are found from the Paleoproterozoic onwards, although some Precambrian samples might be biased by drilling contamination. During the Phanerozoic, high 2-MeHops relative abundances (index > 15 %) are associated with climatic and biogeochemical perturbations such as the Permo/Triassic boundary and the Oceanic Anoxic Events. We analysed the modern habitat of all hpnP-containing bacteria and find that the only one species coming from an undisputed open marine habitat is an ⍺-proteobacterium acting upon the marine nitrogen cycle. Although organisms can change their habitat in response to environmental stress and evolutionary pressure, we speculate that the high sedimentary 2-MeHops occurrence observed during the Phanerozoic reflect ⍺- proteobacteria expansion and marine N-cycle perturbations in response to climatic and environmental change.
Original languageEnglish
Pages (from-to)41-59
Number of pages19
JournalGeobiology
Volume20
Issue number1
Early online date22 Jul 2021
DOIs
Publication statusPublished - Jan 2022

Bibliographical note

Funding Information:
BDAN was funded through a Royal Society Tata University Research Fellowship, GB was funded by a postgraduate scholarship from the University of Bristol, and PS‐B was funded through a Royal Society University Research Fellowship, while FMM was supported by a NERC standard grant (NE/N011112/1). We thank Simon Brassell, Roger Summons, Jochen Brocks, Shelby Lyons, and Kate French for their help with compiling the 2‐MeHops’ data from the geological record. Felix Elling is acknowledged for discussing the modern 2‐MeBHP producers, and we thank Jenan Kharbush for providing the biomass. We thank Ian Bull for help with the HT‐GC‐MS system. Bioinformatic analyses were carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol— http://www.bris.ac.uk/acrc/ . The data that support the findings of this study are available in the supplementary material of this article. Synechoccus

Publisher Copyright:
© 2021 The Authors. Geobiology published by John Wiley & Sons Ltd

Keywords

  • 2-methylhopanes
  • bacteria
  • biomarker
  • cyanobacteria
  • hopanoids

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