Microbial mats early

The Moon was created from the mantle-derived ejecta. Physically, over the aeons this may have played a useful sheltering role in protecting the planet from some meteorite impacts. Arguably more important, the presence of the Moon leads to the tides. These create the intertidal and near-subtidal habitat with rapidly varying geochemical settings, from wet submarine to dry subaerial, in which sediment is repeatedly flushed with fluid. Such cyclically varying habitats may have been vital in the early evolution of microbial biofilms and eventually microbial mats. 

Possibly the earliest habitats were thin biofilms of bacteria and archaea that existed by processing redox contrast between hydrothermal products and the external environment (sea water and atmosphere). The productivity of these early microbial mats would have been severely limited by the inorganic sources of redox power from below. Volcanic and hydro-thermal processes would have ensured a small but steady supply of H2, H2S, CH4 and possibly HCN from below. Nitrate (from dissolved NO2), and sulphate are crucial. Some sulphate would have come from dissolved magmatically exhaled SO3. Some would have been supplied by disproportionation of sulphite in seawater, the sulphite having come from magmatic SO2. Some sulphate would have been made photo-chemically in air. 

There are different types of microbial mats, based on colorless sulfur bacteria, purple sulfur bacteria, iron bacteria and cyanobacteria. The most studied mats are those represented by filamentous cyanobacteria. They are widely distributed in protected intertidal sand flats where periodic desiccation discourages colonization hy marine invertebrates. The characteristic, sharply defined, colored hands of such mats (green in the top, red or purple a few millimeters down and heneath this hlack) were recorded as early as the middle of the ninetieth century. Such shallow waters or intertidal mats have now been recorded in many places of the World. These mats are all ephemeral or seasonal, and only in some specific conditions of tropical and subtropical lagoons can they form permanently. 

Goldhaber MB, Kaplan IR (1980) Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the Gulf of California. Marine Chem 9 95-143 Habicht KS, Canfield DE (1996) Sulphur isotope fractionation in modem microbial mats and the evolution of the sulphur cycle. Nature 382 342-343... 

Westall F, de Ronde CEJ, Southam G, Grassineau N, Golas M, Cockell C, Lam-mer H Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt. South Africa for the UV environmental conditions on the early Earth. PhilosTrans RSoc Lond B Biol Sci 2006, 36l(l474) 1857— 1875. 

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