Microbial reactions in estuaries are governed by chemical equilibria. Bacteria may catalyze reactions, which they use as a source of energy, but their over-all activity follows the direction of energy transfer. Bacteria are active in the water and in the sediments of estuaries. They reduce sulfate to sulfide, oxidize sulfide to sulfur and sulfur to sulfate, reduce ferric to ferrous iron, cause the precipitation of gypsum, release phosphate through the action of microbial hydrogen sulfide, produce methane and hydrogen, synthesize simple hydrocarbons, simplify the structure of complex hydrocarbons and fatty acids, decompose proteins and amino acids to produce ammonia and nitrogen, and precipitate calcium carbonate. In addition to bacteria a large number of microorganisms are active in estuarine waters and sediments, but the details of the geomicrobiological processes that take place are largely undetermined.