a cyanobacterial biomarker was challenged by a number of microbiological discoveries. Geobacter sulfurreducens was demonstrated to synthesize diverse hopanols, although not 2-methyl-hopanols, when grown under strictly anaerobic conditions. Furthermore, the anoxygenic phototroph Rhodopseudomonas palustris was found to produce 2-methyl-BHPs only under anoxic conditions. This latter discovery also lead to the identification of the gene encoding the key methyltransferase HpnP. hpnP was subsequently identified in an acidobacterium and numerous alphaproteobacteria, and phylogenetic analysis of the gene concluded that it originated in the alphaproteobacteria and was acquired by the cyanobacteria and acidobacteriota via horizontal gene transfer. Among cyanobacteria, hopanoid production is generally limited to terrestrial cyanobacteria. Among marine cyanobacteria, culture experiments in conducted by Helen Talbot and colleagues concluded that only two marine species–Trichodesmium and Crocosphaera–produced bacteriohopanepolyols. A later gene-based search for hpnP in available cyanobacterial genomes and Metagenome Assembled Genomes (MAGs) drew similar conclusions, identifying the gene in ~30% of terrestrial and freshwater species, and only one of the 739 marine cyanobacterial genomes and MAGs. Additionally, Nostoc punctiforme produces the greatest amount of 2-methylhopanoids when differentiated into akinetes. These cold- and desiccation-resistant cell structures are dormant and therefore not photosynthetically active, further challenging the association between 2-methylhopanes and oxygenic photosynthesis. Other interpretations Research demonstrating that the nitrite-oxidizing bacteria (NOB) Nitrobacter vulgaris increases its production of 2-methylhopanoids 33-fold when supplemented with cobalamin has furthered a non-cyanobacterial explanation for the observed abundance of 2-methylhopanes associated with Cretaceous OAEs. Felix Elling and colleagues propose that overturning circulation brought ammonia- and cobalt-rich deep waters to the surface, promoting aerobic nitrite oxidation and cobalamin synthesis, respectively. This model also addresses the conspicuous lack of 2-methylhopanes associated with Mediterranean sapropel events and in modern Black Sea sediments. Because both environments feature much less upwelling, 2-methylhopanoid-producing NOB such as N. vulgaris are outcompeted by NOB with higher nitrite affinity and anammox bacteria. An environmental survey by Jessica Ricci and coauthors using metagenomes and clone libraries found significant correlation between plant-associated microbial communities and hpnP presence, based on which they propose that 2-methylhopanoids are a biomarker for sessile microbial communities high in osmolarity and low in oxygen and fixed nitrogen. 3-methylhopanoids 3-methylhopanoids have historically been associated with aerobic methanotrophy based on culture experiments and co-occurrence with aerobic methanotrophs in the environment. As such, the presence of 3-methylhopanes, together with 13C depletion, are considered markers of ancient aerobic methanotrophy. However, acetic acid bacteria have been known for decades to also