Antioxidative defense systems of anaerobic sulfate-reducing microorganisms

Sulfate-reducing bacteria (SRB) are traditionally considered as strict anaerobic microorganisms. But there are a lot of data up to date about survival and metabolic activity of SRB in biotopes periodically exposed to oxygen. The main factor of O2 toxicity is the action of products of oxygen incomplete reduction, so called the reactive oxygen species (superoxide anion radical, hydrogen peroxide and hydroxyl radical). The main targets of them are cell macromolecules. Also molecular oxygen is able to directly inactivate key enzymes of sulfate reduction metabolism. The SRB have thus developed complicated and tightly regulated systems of physiological and enzymatic mechanisms of oxidative stress response which provide their relative aerotolerance. They can form aggregates in the presence of O2, establish symbiotic relationships with aerobic microorganisms and possess aerotaxis. In addition to classical enzymes (superoxide dismutase, catalase, peroxidases) which are usual for aerobes, the cells of many SRB possess also unique alternative enzymes with superoxide reductase (desulfoferrodoxin, neelaredoxin) and peroxidase (rubrerythrin, nigerythrin) activities.