The paradoxical role of sulfur bacteria on the thermodynamic maintenance of aquatic ecosystems

The sulfur bacteria are probably one of the most complex and heterogeneous groups of microorganisms that exist with a controversial ecological and health importance. They can be autotrophic or heterotrophic and participate in complex processes that allow the feedback among the detrital and tropic circuits. They can be aerobic, anaerobic, or facultative; however, being one of the oldest microorganisms, they are generally anaerobic. On one hand, they are essential to maintain the homeostasis of the aquatic systems avoiding eutrophication; nevertheless, when a load of organic matter, whether autochthonous or allochthonous, is excessive, the predominance of anoxygenic environments is favored, and the products of their metabolism, such as H₂S, make an environment hostile to aerobic life in aquatic ecosystems and considerably limit the multiple uses of water. However, through quite complex relationships with other groups in the microbial circuit and with physicochemical properties of the environment and the water column, a balance can be achieved in aquatic ecosystems. In this chapter, some aspects of sulfur bacteria and the water column are dealt with in a general way, with no other intention than to provide a broad vision of these processes.