Photosynthesis 3.5 thousand million years ago.

The recent discovery of stromatolites and microfossils in 3.5-Ga-old sedimentary rock formations is evidence for the existence of phototrophic prokaryotes at that time. Values of δ(13)C for sedimentary organic carbon strongly suggest autotrophic CO2 fixation, and the existence of large deposits of sedimentary sulfate is consistent with a photosynthesis dependent on reduced sulfur compounds for reducing power. The ancient photoautotrophs are though to have contained only one kind of reaction center with either chlorophyll a or bacteriochlorophyll a as primary electron donor and with one or more iron-sulfur centers as secondary electron acceptors. Light-harvesting pigments might have been chlorophyll a, bacteriochlorophyll a, or possibly bacteriochlorophyll c.A new proposal is made to explain how these organisms could have survived an intense UV flux at the earth's surface in the absence of an ozone layer. Photochemically produced ferric iron was abundant in sediments, and the UV-absorption of this ferric iron would have been sufficient to shield those organisms living below the watersediment interface.