## Abstract

H<latex>$_2$</latex>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<latex>$_2$</latex>. The H<latex>$_2$</latex>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<latex>$_2$</latex>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<latex>$_2$</latex> and H<latex>$_2$</latex>S regulate the bacterial involvement in the sulphide oxidation. The H<latex>$_2$</latex>S specialists, Beggiatoa and Thiovulum, are optimally adapted to compete with the autocatalytic oxidation of H<latex>$_2$</latex>S by O<latex>$_2$</latex>. Microelectrode measurements show retention times of O<latex>$_2$</latex>-H<latex>$_2$</latex>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<latex>$_2$</latex>-H<latex>$_2$</latex>S interface. Short cycles between H<latex>$_2$</latex>S and intermediate oxidation products, S<latex>$^0$</latex> or S<latex>$_2$</latex>O<latex>$^{2-}_3$</latex>, are created. The bacteria of the sulfuretum are highly adapted to the diurnal rhythm of light, O<latex>$_2$</latex> and H<latex>$_2$</latex>S.