Biodegradation of organic pollutants, reduction of metals and energy generation involving Clostridium Sp.

The role of the genus Clostridium is of upmost importance in environmental biotechnology; the great metabolic diversity that the members of this genus exhibit allows them to participate in biodegradation of organic pollutants, biotransformation of metals and generation of energy. In this review, the following topics will be presented: biodegradation of some relevant organic pollutants such as Azo-dyes, explosives, chlorinated compounds, biotransformation of metals such as Chromium, Vanadium, Iron, Uranium and Plutonium, and the generation of energy in Microbial Fuel Cells (MFCs) including the main metabolic features of some members of the Clostridium genus involved in these processes. Therefore, some key points in this review include: The Azo-dyes (synthetic dyes) compounds, which are widely used in industry and can be biodegraded under anaerobic conditions by azoreductases, enzymes expressed by some Clostridium sp. A singular dehydrogenase identified in the case of Clostridium perfringens allows to this microorganism the reduction of nitro aromatic compounds as well. The anaerobic biodegradation of nitrogenated compounds including the explosives 2,4,6- trinitrotoluene (TNT), hexahydro- 1,3,5, trinitro- 1,3,5 triazine (RDX) and octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine (HMX) are linked to Clostridium nitrophenolicum, which is able to survive in mixtures of the mentioned explosives. Among the several recalcitrant organic pollutants that can be biodegraded by microorganisms belonging to the genus Clostridium are also the chlorinated aliphatic and aromatic hydrocarbons, in which case Clostridium bifermentans DPH-1 is known to be capable of dehalorespiration. Successful results on polychlorinated biphenyls (PCBs) dechlorination involve Clostridium pascui. Reduction of metals and radionuclids is another capability of Clostridia. The mechanisms for these reductions are not completely known in some cases, and both fermentation and respiration have been pointed out for the reductive process. Recently, it has been proposed that Clostridium acetobutylicum may reduce U(VI) enzymatically. Some of the most cited metals and radionuclids that several Clostridium sp. can reduce are Fe (III), by Clostridium celerecrescens, V(V), by Clostridium pasteurianum and U(VI) by Clostridium acetobutylicum. A novel strategy for energy generation taking advantage of microorganisms is the microbial fuel cell (MFC). Some of the most studied microorganisms used in microbial fuel cells (MFCs) include Clostridium butyricum and Clostridium acetobutylicum. Most recently, an interesting application of MFCs is to couple the energy generation to anaerobic biodegradation of hydrocarbons, a process that also involves several microorganisms belonging to the genus Clostridium. Certainly, the genus Clostridium offers a great source of bacteria that ensures a physiology complex enough to be taken advantage of in environmental biotechnology; however, there are still places to look at in which Clostridium sp. can be found and therefore, capabilities to be discovered. This may be true when exploring microbial niches in extreme environments such as the Antarctica, where a microorganism closely related to Clostridium subterminale exhibited the capability of producing an extracellular protease, active in a wide range of pH. On the other hand, there is relatively little information of Clostridium sp. in marine environments around the world, and this is particularly true in thermophilic marine environments such as hydrothermal vents, for which only two Clostridium species have been reported: Clostridium caminithermale and Clostridium tepidiprofundi. Along these lines, a microorganism closely related to Clostridium ganghwense has been related to biodegradation of trichloroethylene (TCE) when enrichment cultures of hydrothermal vents sediments were cultivated under sulfate-reducing conditions. Finally, in sight of the valuable biological tool that this genus is, emerging new contaminants such as "green solvents" have also been evaluated on the toxicitythat may exert on Clostridium sp.; this is recent, and may become a great concern in a near future, since for the scientific community dedicated to biodegradation and bioremediation, the fact that some members of Clostridium are ubiquitous is definitely useful.