Nitrogen Fixation By Reductively Dechlorinating Bacteria

A wide range of man-made and naturally produced chlorinated hydrocarbons have been released into the environment, and most of them were toxic and environmentally persistent pollutants. Their wide distribution caused a significant risk to public health and the environment. Many of these compounds were resistant to aerobic microbial metabolism often because the chlorine substitution blocked oxygenase attack. Fortunately, reductively dechlorinating bacteria were capable of transforming chlorinated hydrocarbons through reductive dechlorination in anaerobic environments such as anoxic soils, groundwaters and sediments, in an energy-generating process termed dechlorinating respiration. Bacterial culture, carbon isotope fractionation and molecular biology studies have showed that halorespiring anaerobes are widely distributed in nature, implying their important environmental roles in bioremediation and in the global chlorine cycle.Biological nitrogen fixation was the conversion of atmospheric nitrogen into ammonia by symbiotic, associative and freeliving bacteria, which played a tremendous role in ecological sustenance and world agriculture. Nitrogen fixation was an important part of global nitrogen cycles as it replenished the overall nitrogen content of biosphere and compensated for the losses that were incurred owing to denitrification. Although nitrogen fixation was not found in eukaryotes, it was widely distributed among the Bacteria and the Archera, revealing considerable biodiversity among dizaotrophic organisms. This significant physiological feature has not been revealed in reductively dechlorinating bacteria. Results from sequencing of three whole genomes of the dechlorinating bacteria Desulfitobacterium hafniense and Dehalococcoides indicated that nitrogen fixation-related genes were found in D. hafniense strain Y51 and Dehalococcoides strain 195, but were missing in Dehalococcoides strain CBDB1. In this research, we provided experimental evidence that reductively dechlorinating bacteria were capable of fixing atmospheric nitrogen.Significant advances in the ecology, physiology, and genetics of reductively dechlorinating bacteria have revealed their important environmental roles in bioremediation and in the global chlorine cycle. In our research, we firstly showed physiological and molecular evidence that reductively dechlorinating bacteria were capable of fixing atmospheric nitrogen. In some of these dechlorinating bacteria, N₂ fixation stimulated reductive dechlorination and nifH homologous expression discrepancy under different growth conditions, which should help predict and regulate the environmental function of dechlorinating bacteria in in situ bioremediation of chlorinated pollutants. These results implied that N₂ fixation in dechlorinating bacteria interacted with other biogeochemical cycles to control the nitrogen status of the anaerobic ecosystem.In order to further study the mechanisms and regulatory networks of N₂ fixation in dechlorinating bacteria that exhibited some important phenotypes under N₂-dependent growth, the supposed nif gene clusters were be cloned by constructing and screening genome BAC libraries, and sequenced. These genes were annotated according to bioinformatics analysis and after that, the functions of them designated in the nif gene clusters would be experimentally identified.