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.

中文翻译：

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现代细菌中2-甲基藿香的存在与地质记录

2-甲基藿烷 (2-MeHops) 是 2-甲基细菌藿烷多元醇 (2-MeBHP) 的分子化石，是地球上最古老的生物标志物之一。然而，这些生物标志物的具体来源目前无法解释，包括它们是否反映了海洋蓝藻的扩张。在这里，我们研究了现代细菌中 2-MeBHPs 的出现及其合成相关基因，并探索了地质记录中 2-MeHops 的出现。我们发现负责 2-MeBHP 合成的基因 ( hpnP) 在氰基和⍺-变形菌中广泛存在，但在其他细菌类别/门中不存在或非常有限。该结果与 2-MeBHP 在氰基和 ⍺- 变形菌培养物中的优势一致。对其地质产状的回顾表明，从古元古代开始就发现了 2-MeHops，尽管一些前寒武纪样品可能因钻井污染而产生偏差。在显生宙期间，高 2-MeHops 的相对丰度（指数 >15%）与气候和生物地球化学扰动有关，例如二叠纪/三叠纪边界和海洋缺氧事件。我们分析了所有hpnP的现代栖息地- 含有细菌，并发现唯一来自无可争议的开放海洋栖息地的物种是作用于海洋氮循环的⍺-变形杆菌。尽管生物体可以响应环境压力和进化压力而改变其栖息地，但我们推测在显生宙期间观察到的高沉积 2-MeHops 的出现反映了⍺-变形菌的扩张和海洋 N 循环扰动以响应气候和环境变化。