Studies on the microbial halogen cycle : Reactions of fungal peroxidases and bacterial reductive dehalogenases
Halogenated metabolites are produced e. g. by lignin degrading fungi and may be available to organohalide respiring anaerobes in natural environments. However, it is unclear if there is any connection between aerobic halogenation and anaerobic dehalogenation. The aims of the present study were to answer this question and to shed some light on microbial halogenation and dehalogenation involving halide ions other than the well studied Cl- (e. g. Br-, F-). Using Sulfurospirillum multivorans and Desulfitobacterium hafniense PCE-S and their purified tetrachloroethene reductive dehalogenases as a model system, dehalogenation of brominated alkenes was investigated and compared with that of their chlorinated analogues. It was found that debromination proceeded at a lower stereospecificity than dechlorination; the reaction mechanism of dechlorination and debromination of the halogenated alkenes appeared to be very similar and that the carbon backbone had a more significant effect on the reaction mechanism than the type of halogen. Peroxidases of the halogenating fungus Bjerkandera adusta Ud1 were studied for halide ion-involved reactions. The versatile peroxidases purified from this fungus were found to exhibit brominating activity. For the first time, a variety of substrates were found to be fluoride-dependently converted by manganese peroxidase in the absence of Mn(II). A hypothesis was brought up that a long-range electron transfer pathway might have been activated by the conformational changes occurred upon fluoride binding. The putative interaction of halogenating fungi and dehalogenating anaerobic bacteria was studied. Halogenated metabolites were produced by B. adusta strain Ud1 and dehalogenated by the anaerobe D. hafniense DCB-2. Furthermore, a dehalogenating Desulfitobacterium strain was isolated from forest soil, where ligninolytic enzyme activities were detected, pointing to an interaction between halogenating lignin-degrading fungi and dehalogenating anaerobe.