Microbial transformation of organic and inorganic halogen compounds

Abstract

Haloalkanoates are environmental pollutants that can be degraded aerobically by microorganisms producing hydrolytic dehalogenases. However, there is lack of information about anaerobic degradation of haloalkanoates. Genome analysis of Pseudomonas chloritidismutans AW-1 T , a facultative anaerobic chlorate-reducing bacterium, showed presence of two putative haloacid dehalogenase genes, the L-DEX gene and dehI, encoding an L-2-haloacid dehalogenase (L-DEX) and a halocarboxylic acid dehydrogenase (DehI). Hence, we studied concurrent degradation of haloalkanoates and chlorate as a yet unexplored trait of strain AW-1 T. The deduced amino acid sequences of L-DEX and DehI revealed 33−37% and 26−86% similarities with biochemically/structurally characterized L-DEX and D-, DL-2haloacid dehalogenase enzymes, respectively. Physiological experiments confirmed that strain AW-1 T can grow on chloroacetate, bromoacetate and both Land D-α-halogenated propionates with chlorate as an electron acceptor. Interestingly, growth and haloalkanoates degradation were generally faster with chlorate as an electron acceptor than with oxygen. In line with this, analyses of L-DEX and DehI dehalogenase activities using cell free extract (CFE) of strain AW-1 T grown on DL-2-chloropropionate under chlorate-reducing condition showed up to 3.5-fold higher dehalogenase activity than the CFE obtained from cells grown on DL-2chloropropionate under aerobic condition. Reverse transcription quantitative PCR showed that the L-DEX gene was expressed constitutively independent of the electron donor (haloalkanoates or acetate) or acceptor (chlorate or oxygen), whereas expression of dehI was induced by haloalkanoates. Concurrent degradation of organic and inorganic halogenated compounds by strain AW-1 T represents a unique metabolic capacity in a single bacterium, providing a new piece in the puzzle of the microbial halogen cycle.