The occurrence of 2-methylhopanoids in modern bacteria and the geological record

The 2--methylhopanes (2--MeHops) are molecular fossils of 2--methylbacteriohopanepolyols (2--MeBHPs) and among the oldest biomarkers on Earth. However, these biomarkers' specific sources are 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 analyzed 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.

Automated summary

The 2-methylhopanes are molecular fossils of 2-methylbacteriohopanepolyols and are among the oldest biomarkers on Earth. Studies show that the gene responsible for their synthesis is widespread in cyanobacteria and proteobacteria, suggesting their dominant presence in these bacterial cultures. Geological records indicate the occurrence of 2-methylhopanes from the Paleoproterozoic onwards, with high abundances associated with climatic and biogeochemical perturbations during the Phanerozoic.