Concurrent haloalkanoate degradation and chlorate reduction by Pseudomonas chloritidismutans AW-1^T

Haloalkanoates are environmental pollutants that can be degraded aerobically by microorganisms producing hydrolytic dehalogenases. However, there is a lack of information about the anaerobic degradation of haloalkanoates. Genome analysis of Pseudomonas chloritidismutans AW-1^T, a facultative anaerobic chloratereducing bacterium, showed the 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), respectively. Hence, we studied the 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 to 37% and 26 to 86% identities with biochemically/structurally characterized L-DEX and the D- and DL-2-haloacid dehalogenase enzymes, respectively. Physiological experiments confirmed that strain AW-1^T can grow on chloroacetate, bromoacetate, and both L- and D-α-halogenated propionates with chlorate as an electron acceptor. Interestingly, growth and haloalkanoate degradation were generally faster with chlorate as an electron acceptor than with oxygen as an electron acceptor. 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 conditions showed up to 3.5-fold higher dehalogenase activity than the CFE obtained from AW-1^T cells grown on DL-2-chloropropionate under aerobic conditions. Reverse transcriptionquantitative PCR showed that the L-DEX gene was expressed constitutively independently of the electron donor (haloalkanoates or acetate) or acceptor (chlorate or oxygen), whereas the 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 of the puzzle of the microbial halogen cycle.