Anoxygenic photosynthesis, evolution, and the origin of continents and cyanobacteria


Wednesday, April 23 2008 - 12:00 pm, PDT
Norm Sleep

Pre-photosynthetic niches were meager with productivity << 10-4 of modern photosynthesis. Serpentinisation, arc volcanism, and ridge-axis volcanism reliably provided H2. Methanogens and acetogens reacted CO2 with H2 to obtain energy and make organic matter. These skills preadapted a bacterium for anoxygenic photosynthesis, probably starting with H2 in lieu of an oxygen acceptor. Use of ferrous iron and sulphide followed as abundant oxygen acceptors, allowing productivity to approach modern levels. The “photo-bacterium” proliferated rooting much of the bacterial tree. Land photosynthetic microbes faced a dearth of oxygen acceptors and nutrients. A consortium of photosynthetic and soil bacteria aided weathering and access to ferrous iron. Weathering led to formation of shales and, and ultimately to granitic rocks. Already oxidized and Fe-poor sedimentary rocks and low-Fe granites provided scant oxygen acceptors. So did fresh water in their drainages. Cyanobacteria evolved di-oxygen production that relieved them of these vicissitudes. They did not immediately dominate the planet. Eventually anoxygenic and oxygenic photosynthesis oxidized much of the Earth’s crust and supplied sulphate to the ocean. Anoxygenic photosynthesis remained important until there was enough O2 in downwelling seawater to quantitatively oxidize massive sulphides at ridge axes. 

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