Microorganisms and the Sulfur Cycle

Abstract

Abstract The biogeochemical sulfur cycle consists of an assimilatory and a dissimilatory side. On the assimilatory side inorganic sulfur in reduced or oxidized form is taken up into microbes, plants and animals (via plants) and transformed there into amino acids, proteins, and coenzymes that function there until they are excreted or the organisms die and are decomposed. The turnover rates during these processes are low as compared with those in dissimilatory processes, during which inorganic sulfur compounds serve as electron donors or acceptors in microbial metabolism. The different metabolic types involved are dissimilatory sulfate reduction (anaerobic; sulfate as electron acceptor), dissimilatory sulfur reduction (anaerobic; elemental sulfur as electron acceptor), phototrophic sulfur oxidation (anaerobic; reduced sulfur compounds as electron donors for photosynthesis), chemotrophic sulfur oxidation (anaerobic and aerobic; reduced sulfur compounds as electron donors for respiration). Communities of sulfur-oxidizing and -reducing bacteria form natural sulfur cycles, so-called sulfureta. Microbial mass transformations of inorganic sulfur compounds occured already very early in the geological record and are traceable by their kinetic isotope effects.