The unique characteristic of the phototrophic purple and green sulfur bacteria as well as of members of the purple nonsulfur bacteria is their ability to oxidize H2S to sulfate under anaerobic conditions in the course of their anoxigenic photosynthesis. Carbon dioxide - assimilated into cell material - is the electron acceptor of this oxidation process. Thus, a reoxidation of the H2S formed under anaerobic conditions by sulfate reducing bacteria is achieved without the consumption of molecular oxygen. Five different types of complete or incomplete oxidation of H2S and elemental sulfur to sulfate carried out by different species and genera of the phototrophic bacteria are described. The simplest biological systems catalyzing a complete microbial sulfur cycle had been verified experimentally with anaerobic illuminated mixed pure cultures of Desulfovibrio and Chromatium or Chlorobium. These systems are closed with respect to the sulfur cycle; they depend, however, on the supply of both reducing power for the sulfate reducers and light energy for the phototrophs. The special conditions for the mass development of the phototrophic sulfur bacteria under conditions of competition with aerobic colourless sulfur bacteria are characterized. The significance and catalytic role of the sulfur cycle for the degradation of organic material under anaerobic conditions is demonstrated. The phototrophic bacteria are shown to be beneficial for the higher forms of life in lakes.