Oxygen respiration by Desulfovibrio species

Throughout the first 90 years after their discovery, sulfate-reducing bacte ria were thought to be strict anaerobes. During the last 15 years, however, it has turned out that they have manifold properties that enable them to c ope with oxygen. Sulfate-reducing bacteria not only survive oxygen exposure for at least days, but many of them even reduce oxygen to water. This proc ess can be a true respiration process when it is coupled to energy conserva tion. Various oxygen-reducing systems are present in Desulfovibrio species. In Desulfovibrio vulgaris and Desulfovibrio desulfuricans, oxygen reductio n was coupled to proton translocation and ATP conservation. In these specie s, the periplasmic fraction, which contains hydrogenase and cytochrome c(3) , was found to catalyze oxygen reduction with high rates. In Desulfovibrio gigas, a cytoplasmic rubredoxin oxidase was identified as an oxygen-reducin g terminal oxidase. Generally, the same substrates as with sulfate are oxid ized with oxygen. As additional electron donors, reduced sulfur compounds c an be oxidized to sulfate. Sulfate-reducing bacteria are thus able to catal yze all reactions of a complete sulfur cycle. Despite a high respiration ra te and energy coupling, aerobic growth of pure cultures is poor or absent. Instead, the respiration capacity appears to have a protective function. Hi gh numbers of sulfate-reducing bacteria are present in the oxic zones and n ear the oxic-anoxic boundaries of sediments and in stratified water bodies, microbial mats and termite guts. Community structure analyses and microbio logical studies have shown that the populations in those zones are especial ly adapted to oxygen. How dissimilatory sulfate reduction can occur in the presence of oxygen is still enigmatic, because in pure culture oxygen block s sulfate reduction. Behavioral responses to oxygen include aggregation, mi gration to anoxic zones, and aerotaxis. The latter leads to band formation in oxygen-containing zones at concentrations of less than or equal to 20% a ir saturation.