MOLECULAR SIGNATURES FOR CHANGES IN POPULATIONS OF NITROGEN-FIXING CYANOBACTERIA IN RESPONSE TO FLUCTUATIONS IN OXYGENATION LEVELS DURING THE EARLY APTIAN OCEANIC ANOXIC EVENT

2-Methylhopanoid (2-MH) biomarkers provide a signature for cyanobacterial (cyano) contributions to organic matter (OM) in sedimentary rocks back to 2,700 Ma, although their occurrence is sporadic, unlike that of their hopanoid analogs. 2-MH are prevalent in sediment sequences deposited during oceanic anoxic events (OAEs) and are often associated with low d¹⁵N values indicative of nitrogen (N₂) fixation by cyanos. Representative low d¹⁵N values occur throughout the Early Aptian OAE (OAE1a; 120 Ma) at Shatsky Rise (ODP Site 1207) in the Pacific, whilst the proportions of 2-MH are higher during time intervals characterized by cooler sea surface temperatures and oxygenated waters, determined from the TEX₈₆ proxy and biomarkers, respectively. In modern marine environments higher temperatures favor filamentous non-heterocystous cyanos as the dominant ₂-fixing organisms, and exclude heterocystous species, whereas unicellular cyanos are favored by low pO₂. Thus, the variations observed within OAE1a suggest that N₂-fixing cyano populations changed during these episodes of carbon cycle perturbation.

The coupled response of 2-MH, cyano speciation, and O₂ levels during OAE bears on their relationships in earlier times, especially during evolutionary diversification of cyanos on the early Earth. The proposition that 2-MH originated in heterocystous cyanos that perform N₂ fixation helps explain their occurrence in the Late Archean during a global-scale expansion of oxygenated habitats, and their scarcity in anoxic Paleoproterozoic sequences. The presence of isorenieratane in OAE sequences that contain 2-MH suggests changes in populations of green sulfur bacteria and cyanos as environmental conditions alternately favoured anoxygenic and oxygenic photosynthesis, respectively. Perhaps changes in phytoplankton communities during OAE may re-enact the dynamic interchange of these organisms that first accompanied the transition to an oxygenated world. This interpretation requires biosynthesis of 2-MH by N₂-fixing heterocystous cyanos prior to the initial accumulation of atmospheric O₂, and the fact that these heterocystous cyano groups are phylogenetically more evolved members of the clade also argues against the interpretation that specific groups of cyanos post-date the Great Oxygenation Event.