Novel strategies to improve niche colonization and microbial interactions of organohalide-respiring bacteria for remediation of chloroethenes-contaminated sites
Organohalide-respiring bacteria (OHRB)-mediated reductive dehalogenation is promising in in-situ remediation of chloroethenes-contaminated sites. Nonetheless, bioremediation efficiency is largely determined by successful colonization of fastidious OHRB, which highly depends on their growth niches and interactions with other microorganisms. In this study, based on two ecological principles (i.e., Priority Effects and Coexistence Theory), three strategies were developed to enhance niche colonization of OHRB, which were tested in both laboratory experiments and field applications: (i) pre-inoculation of niche-preparing culture (NPC); (ii) staggered fermentation; and (iii) increased OHRB inoculation ratio. The batch experiments were setup with an NPC culture being mainly composed of fermenting bacteria and methanogens, and a Dehalococcoides-containing tetrachloroethene-to-ethene dechlorinating culture (CE40). Results showed significant higher chlorine removal efficiencies, as well as lower concentrations of volatile fatty acids (VFAs) and methane, in experimental sets with staggered fermentation and niche-preconditioning with NPC for 4 days (50-50) relative to control sets without NPC inoculum (CE40), e.g., 49.90 ± 0.02 and 0.37 ± 0.05 µM/day of chlorine removal rates in the 50-50 and CE40, respectively. Accordingly, comparatively higher abundance of Dehalococcoides as major OHRB, together with lower abundance of fermenting bacteria and methanogens, were observed in the CE40-NPC-4 with staggered fermentation, which indicated the balanced syntrophic-and-competitive interactions between OHRB and other populations for efficient dechlorination. Further experiments with microbial source tracking analyses suggested enhanced colonization of OHRB by increasing inoculation ratio. The final optimized conditions for enhanced colonization of OHRB were successfully employed to field bioremediation of two chloroethenes-contaminated sites, i.e., as high as 96.62% and 99.67% complete dechlorination to ethene were observed within 5 and 3 months at trichloroethene (TCE)- and vinyl chloride (VC)-contaminated sites, respectively. This study provides novel strategies for efficient bioremediation of chloroethenes-contaminated sites, which may be also employed for removal of other emerging organohalide pollutants.