Biodegradation Potential of Halo(alkali)philic Prokaryotes

Oil and gas exploitation and downstream processing may generate large volumes of contaminated aqueous wastewaters. Without treatment, the discharge of these streams into the environment would lead to considerable environmental problems. A combination of physicochemical and biological treatment processes are applied to treat such polluted streams (e. g., in conventional water purification plants). However, the application of microorganisms in these streams is rather limited streams to salt levels well below those of seawater. The potential of microorganisms to degrade various organic and inorganic pollutants at higher salinities was surveyed in the open literature. The advantage of biological treatment processes that operate at elevated salinities is that no dilution water is needed, which is particularly relevant for arid regions. The available literature is scarce, especially concerning truly halophilic bacteria that thrive at salt concentrations significantly higher than in seawater. The obvious trend of most of the publications indicates a significant inhibition of the biodegradation activity at salt concentrations above 10 wt.%. However, a specialized group of extremophilic microorganisms, called halo(alkali)philes, could replace freshwater or marine organisms in the biodegradation processes at salinities above 1.5 M and pH values above 9. They are present in natural hypersaline habitats and can be activated either directly in situ by addition of micronutrients and aeration, or introduced into engineered water treatment systems. In a number of studies, the possibility of microbial HC degradation up to 5 M NaCl has been mentioned. Relatively little is known about the removal of toxic inorganic compounds, such as H2S, CO, and CN-, at halo(alkaline) conditions. These compounds are clearly associated with the processing of oil, coal, and gas. An exception is the application of sulfur-cycling bacteria, as these organisms have been intensively studied during the last decade. Sulfur-oxidizing and sulfidogenic bacteria can function efficiently at extremely saline and alkaline conditions. The general conclusion from this survey is that many biodegradation routs are, in principal, possible at high salt/pH but more research is needed to make a real progress in the development of engineered processes that can be applied to protect the environment.