Substrate-dependent competition and cooperation relationships between Geobacter and Dehalococcoides for their organohalide

Both obligate and non-obligate organohalide-respiring bacteria (OHRB) play central roles in the natural cycling and environmental bioremediation of organohalides. The coexistence of obligate and non-obligate OHRB may provide functional redundancy to assure organohalide respiration efficiency but, at the same, complicate isolation and characterization of specific OHRB. In this study, we successfully employed the growth rate/yield tradeoff strategy to selectively enrich and isolate a rare non-obligate perchloroethene (PCE)-respiring Geobacter lovleyi LYY from a Dehalococcoides-predominant microcosm. Further physiological and genomic characterizations, together with co-culture experiments, suggested very unique substrate-dependent competition and cooperation relationships between Geobacter and Dehalococcoides for their organohalide respiration: (1) free competition in medium amended with PCE, acetate and hydrogen; (2) conditional competition in medium amended with PCE and propionate; (3) syntrophic cooperation in medium amended with polychlorinated biphenyls (PCBs) and propionate. Our study provide the first insight into substrate-dependent interactions between obligate and non-obligate OHRB, as well as a new strategy to isolate fastidious microorganisms, for better understanding and mediation of the organohalide cycling in natural environments.