Ecology of the bacteria of the sulphur cycle with special reference to anoxic—oxic interface environments

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Abstract
H$_2$S is produced as a main end-product of anaerobic mineralization in anoxic, sulphate-rich environments by a diverse population of sulphate-reducing bacteria. The sulphate reducers can carry out an almost complete oxidation of detrital organic matter to CO$_2$. The H$_2$S consequently becomes an important electron carrier from the anoxic to the oxic world. Thiobacilli and other colourless sulphur bacteria have the potential to oxidize the H$_2$S at the oxic-anoxic interface in sediments or stratified waters, but their role is still poorly understood. A comparison of sulphide oxidation processes in the chemoclines of the Black Sea, the Solar Lake and in a Beggiatoa mat indicated that depth scales and retention times of coexisting O$_2$ and H$_2$S regulate the bacterial involvement in the sulphide oxidation. The H$_2$S specialists, Beggiatoa and Thiovulum, are optimally adapted to compete with the autocatalytic oxidation of H$_2$S by O$_2$. Microelectrode measurements show retention times of O$_2$-H$_2$S in the bacterial mats or veils of less than 1 s. In photic chemoclines of stratified waters or sulfureta, the phototrophic sulphur bacteria or cyanobacteria interact with the sulphide oxidation at the O$_2$-H$_2$S interface. Short cycles between H$_2$S and intermediate oxidation products, S$^0$ or S$_2$O$^{2-}_3$, are created. The bacteria of the sulfuretum are highly adapted to the diurnal rhythm of light, O$_2$ and H$_2$S.