Early Evidence for photosynthesis
IPGP - Campus Jussieu
Roger Buick du Earth & Space Sciences and Astrobiology, Universi
Résumé: Though anoxygenic photosynthesis oxidizing H2S to sulfate (instead of H2O to O2 in oxygenic photosynthesis) is hypothesized from biochemical data to have evolved first, the geological evidence for it only goes back to hydrocarbon biomarkers derived from photo-pigments of green and purple anoxygenic photosynthetic bacteria in 1.65 Ga rocks. Paradoxically, evidence for oxygenic photosynthesis is much more ancient. There are 3 hypotheses regarding the timing of its evolution: 1) extremely ancient such that the atmosphere has always been oxygenated; 2) immediately before the atmosphere became permanently and highly oxygenated during the "Great Oxidation Event" ~2.35 Ga ago; and 3) hundreds of millions of years before atmospheric oxygenation but taking aeons for molecular oxygen to accumulate in the atmosphere. There is abundant evidence for low or no environmental oxygen in early Archean rocks, invalidating the first hypothesis, and consistent but controversial evidence from hydrocarbon biomarkers, redox-sensitive metals and minerals, and lacustrine stromatolites for Archean oxygen production, apparently invalidating the second hypothesis. In particular, biomarkers from contamination-proof fluid inclusions from before the Great Oxidation Event support the third hypothesis, implying that it indeed took aeons to oxidize the reservoirs and fluxes of reduced volcanic gases, hydrothermal fluids and volcanic crust. Thus, oxygenic photosynthetic cyanobacteria apparently evolved well before their excreted oxygen permanently polluted the atmosphere.