Microorganisms Involved in the Nitrogen and Sulfur Cycles

Abstract

The changes of the outer surface of the earth, which have occurred within the last three or four billion years, are by no means only due to plutonic activities, tectonic shifts, weathering and dissolution of minerals and other geochemical processes; rather prokaryotic microorganisms were actively involved. Due to the latter chemical activities the highly reduced atmosphere and lithosphere became oxidized as exemplified by the transformation of ammonia, methane, water, and hydrogen sulfide to dinitrogen, carbon dioxide, oxygen, and sulfate. The precipitation of limestone, deposition of iron oxides, the separation of various minerals and later the deposition of carbon and reduced carbon compounds are at least partially due to microbial activities. The basic metabolism of living organisms developed on the level of prokaryotic life. Prokaryotes occupied ecological niches long before higher forms of life developed and were always present during the evolution of plants and animals. It is, therefore, not surprising that various transformations in the cycle of matter are the domain and the monopoly of prokaryotes still to-day. In fact, the cycles of nitrogen and sulfur in the biosphere mainly deal with the activities of bacteria. Transformations under anaerobic conditions are exclusively brought about by bacteria, and the utilization of inorganic nitrogen and sulfur for biosynthesis is confined to microorganisms and plants.