BIOGEOCHEMICAL EVIDENCE FOR ENVIRONMENTAL CONSTRAINTS ON ANCIENT CYANOBACTERIAL ASSEMBLAGES

2-Methylhopanoids (2-MH) provide a signature for cyanobacterial contributions to organic matter in sediments as old as the Paleoproterozoic. However, the precise biological and environmental controls that regulate production of 2-MH during hopanoid biosynthesis in cyanobacteria (CB) remain unknown. Values for the 2-MH index (2-MHI; ratio of 2-MH to hopanoids) are often high during intervals of oceanic dysoxia known as Oceanic Anoxic Events (OAE), and variations in 2-MHI throughout the sedimentary record have been presumed to reflect differing proportions or assemblages of CB within microbial communities. Sediments from Shatsky Rise (ODP Site 1207) corresponding to the Early Aptian OAE (OAE1a; 120 Myr) deposited in the tropical Pacific exhibit low d¹⁵N values indicative of N₂ fixation by CB. Yet values of 2-MHI are higher during episodes of cooler sea surface temperatures and more oxygenated waters, as determined from the TEX₈₆ proxy and biomarkers, respectively. In modern marine environments higher temperatures favor filamentous non-heterocystous CB as the dominant N₂-fixing organisms, and exclude heterocystous species, whereas unicellular CB are favored by low pO₂. Thus, 2-MHI variations during OAE1a may reflect a change in N₂-fixing cyanobacterial populations from heterocystous species during cooler, more oxygenated intervals to unicellular and non-heterocystous species during warmer, dysoxic intervals. A link between 2-MH, CB speciation, and O₂ levels during OAE may also reflect the evolutionary diversification of CB on the early Earth. An origin for 2-MH in heterocystous CB that perform N₂ fixation would explain their occurrence in the Late Archean during a global-scale expansion of oxygenated habitats. It would also require biosynthesis of 2-MH by N₂-fixing heterocystous CB prior to the initial accumulation of atmospheric O₂, consistent with their status as phylogenetically more evolved members of the clade, which argues against the interpretation that specific groups of CB post-date the Great Oxygenation Event. Moreover, intermittent oceanic anoxia over geological time may have served to sustain those CB whose evolutionary inheritance best equips them for low O₂ environments.