Organohalide-respiring bacteria as members of microbial communities: Catabolic food webs and biochemical interactions

Abstract

Organohalide-respiring bacteria (OHRB) have been isolated from a wide range of anoxic environments worldwide and can easily be enriched in the laboratory. Obligate OHRB generally thrive best in mixed communities as part of anaerobic food webs that typically involve interspecies hydrogen (H2) transfer from fermenters to OHRB, and often OHRB compete for H2 with hydrogenotrophic methanogens. In laboratory enrichments, the community composition of the non-OHRB fraction of the communities is dependent on which electron donor is used for enrichment as well as other factors (e.g., the concentrations of organohalide substrate). In addition to catabolic food webs, other biochemical interactions in these communities include provision of key cofactors (e.g., corrinoids), relief of toxicity due to reactive oxygen species, as well as the organohalides themselves. Multiple OHRB often coexist stably in enrichment cultures and environmental communities. This diversity in OHRB populations creates complex interactions among different OHRB-with the partially dehalogenated end product of one population serving as substrate for other populations. Recent broad surveys of bacterial and archaeal community structure at sites undergoing in situ bioremediation are confirming that fermenters, methanogens, and OHRB are all stimulated by enhanced bioremediation efforts but that aerobes including methanotrophs and organohalide-oxidizing aerobes are also stimulated-especially in downgradient plume regions. The chapter will also discuss roles of OHRB populations in pristine environments including soils and sediments where they dehalogenate naturally produced halogenated organic matter and may compete with sulfate reducers and iron reducers when appropriate electron acceptors are available.