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.