Sulfur cycle is the most important cycle conjugated to Corg. Assimilation of sulfate into Sorg is quantitatively of minor importance in spite of the fact that it is the main source of dimethylsulfide - a volatile compound contributing to the source of S in the atmosphere. Its photochemical oxidation leads to the formation of aerosol in the stratosphere and is most important for the climate. In the destructive pathway coupled to the Corg-cycle, sulfate is reduced to H2S by sulfate-reducing bacteria (SRBs), which by now are taxo-nomically numerous but functionally uniform. There are the following trophic groups: H2-utilizers, SRBs with incomplete oxidation of organic acids (Desulfovibrio-type) producing acetate, and complete oxidizers (Desulfobacter-type), which use various unfermentable products of fermentations. Hydrogenotrophic SRBs are H2-scavengers, which allow them to serve as intermediary oxidants to H2-producing syntrophic bacteria and thus oxidize a variety of organic compounds. Most interesting is their interaction with methanogens in anaerobic methane oxidation in marine sediments. Methane is oxidized by reversed methanogenesis with the formation of isotopically light carbonates and evolution of H2S by SRBs. H2S, if not bound by iron into pyrite, escapes to the surface of the mud where it is oxidized by pelophilic sulfur bacteria, which can either use intracellular S0 for oxidation into sulfate, if O2 or NO3 - is available, or use it as an oxidant in sulfur reduction. Magnificent benthic mats of trichomic sulfur bacteria are found on the shelf close to Chile and West Africa. Here, the so-called thiobios is formed by sulfur bacteria of Thioploca-type. Large filamentous bacteria cross the ox-red boundary and receive H2S from the anaerobic layer and the current near the surface of the mud brings oxidant as nitrate or O2, which is used for chemo-synthesis. H2S escaping in the water mass in the bodies of water with limited circulation makes a chemocline with the reductive zone below the oxic zone; Black Sea is a conventional example. The same occurs in stratified lakes. It is supposed that Mid-Proterozoic stratified ocean had the same structure. If H2S zone comes to the photic zone, anoxic sulfur phototrophs develop. There are a variety of anoxygenic phototrophic bacteria, which belong to phylo-genetically distant phyla. Purple layers of phototrophs make a remarkable landscape when they come up to the surface on the beach or in the soda lakes. In oxygenated photic zone, H2S is oxidized into sulfate by various thionic bacteria. It is noteworthy that the appearance of sulfates in the palaeocean correlates with the oxygenation of the atmosphere around 2.4 Ga, and before 1 Ga its composition became close to the present one. It might be speculated that sulfates of the sea are biogenic in their origin. What was the initial source of mineral S? If the source was massive sulfides, then for their mobilization oxidative step was needed by aerobic acidothiobacteria used now in biohydrometallurgy in the general reaction, FeS + O2 ! Fe3+ + SO42- . The reaction strongly depends on the availability of O2. Oxidation of sulfides leads to the formation of extreme acid conditions. It is most spectacular on volcanic thermal fields with sulfur exhalations, so-called solfataras. When A. Humboldt visited Vesuvius before its eruption, he noted that hot vapors were neutral in spite of possible SO2 production in the heat, while cold walls of the crater were strongly acidic for Lakmus paper. Now it is known that oxidation of sulfur occurs mainly by acidophilic thionic bacteria and only in outlets of fumaroles, extremely thermophilic archaea are active. In the deeper parts of thermal fields, S0 is used as an oxidant by anaerobic archaea with H2S production. Short cycle S0 $ H2S works also in microbial mats where white sulfur is deposited from H2S by microaerobic sulfur bacteria and reduced when oxidant is not available. A large variety of microorganisms are involved in the cycle. Sulfur cycle closes destruction of organic matter in anoxic zone with sulfate regeneration either by anaerobic phototrophs or by aerobic sulfur oxidizers. Its function strongly depends on the transport processes across chemocline. The outcome from the cycle depends on availability of iron, which forms insoluble sulfides first as hydrotroillit and then pyrite.
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