Evolutionary ecology during the rise of dioxygen in the Earth's atmosphere

Pre-photosynthetic niches were meagre with a productivity of much less than 10−4 of modern photosynthesis. Serpentinization, 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 pre-adapted 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 ‘photobacterium’ 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. Biologically enhanced weathering led to the formation of shales and, ultimately, to granitic rocks. Already oxidized iron-poor sedimentary rocks and low-iron granites provided scant oxygen acceptors, as did freshwater in their drainages. Cyanobacteria evolved dioxygen 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 mid-ocean ridge axes.