Bacterial metabolism of selenium-for survival or profit

Abstract

Selenium (Se) is transformed by phylogenetically diverse bacteria following several basic strategies which include: (1) satisfying a trace element requirement for bacterial synthetic machinery (assimilatory metabolism), (2) cellular energy production coupled to oxidation/reduction reactions (dissimilatory metabolism), and (3) detoxification processes. Some bacteria can use Se for respiration under limiting anaerobic conditions, generating energy to sustain growth. Under aerobic conditions, Se behaves as a toxicant and bacteria have evolved different strategies to counteract it. An important detoxification mechanism involves the formation of Se nanoparticles with a diminished toxic potential, but the cells have to properly manage these products in order to maintain their integrity. The bacterial metabolism of Se can be regarded as a survival mechanism when Se compounds prove to be highly toxic. Secondly, selenium is used to obtain energy in a nutrient-depleted environment, therefore allowing to specialized bacterial species to prevail over competitors that cannot exploit this resource. To achieve the Se metabolic activities, numerous unique enzymes are employed. While some enzymes have been isolated and are markedly specific for Se, many of the Se enzymes remain to be isolated. The formation of Se nanoparticles inside bacteria and the transportation mechanisms to the extracellular environment are still under debate. Se nanoparticles do not appear to play a nutritional (energy storage) or ecological function for bacteria, being by-products of bacterial metabolism. However, from a biotechnological standpoint, these conversions could be used to (1) clean up industrial effluents rich in Se and (2) to produce biomaterials with industrial applications (biofactory).