Life in the Absence of Oxygen: Alternative Electron Acceptors for Anaerobic Microorganisms in a Petroleum Environment

Anaerobic microorganisms derive energy by transferring electrons from an external source or donor to an external electron sink or terminal acceptor and often have the capacity to reduce 2 or more terminal electron acceptors. The well-known type of microbial respiration, in which oxygen serves as an electron acceptor for the oxidation of organic carbon and/or hydrogen, has been studied elsewhere in detail. Anaerobic microorganisms are widely distributed in oil-producing vents, hydrothermal vents, volcanic hot springs, non-volcanic geothermally heated subsurface aquifers, and soil. In this study, anaerobic, thermophilic, and fermenting microorganisms in a petroleum sample from the Ad›yaman region of Turkey were examined for their ability to use different electron acceptors. The temperature range for growth of the enrichment culture (TP1) was between 40 and 65 °C and the optimum pH ranged from 4.5 to 8.0. TP1 had the ability to use a wide variety of mono-, di-, and polysaccharides to form acetate, lactate, ethanol, H2, and CO2. No sulfate-reducing or methanogenic microorganisms were found. As an electron acceptor, TP1 reduces thiosulfate, elemental sulfur, sulfite, Fe(III), anthraquinone-2,6-disulfonate (AQDS), arsenake, and MnO2, but not sulfate, nitrate, (per)chlorate, or selenate. Herein, we show that the enrichment culture from the petroleum environment was able to reduce multiple electron acceptors. The utilization of these electron acceptors by TP1 also indicated their presence in this area. The results presented suggest that TP1 may occupy a niche as an environmental opportunist by taking advantage of diverse electron acceptors.