Formate: A promising electron donor to enhance trichloroethene-to-ethene dechlorination in Dehalococcoides-augmented groundwater ecosystems with minimal bacterial growth

Abstract

Various substrates have been used to stimulate habitat microbes in chloroethene-contaminated groundwater, however, the specific efficiency and minimum growth of microbes have rarely been studied. This study investigated the effects of seven substrates on trichloroethene (TCE) dechlorination by augmentation of groundwater with Dehalococcoides mccartyi NIT01 and its contribution to the microbial community. Three out of eight test groups completed dechlorination of 1 mM TCE-to-ethene in varying durations; groundwater supplemented with formate (FOR) required 78 days, whereas the microcosms with lactate (LAC) and citrate (CIT) required approximately twice as long (143 days). The calculated efficiency of how much produced H2 was used in dechlorination indicated a higher efficiency in FOR (36%) compared with LAC (1.9%) or CIT (2.9%). FOR showed lower microbial growth (3.4 × 105 copies/mL) than LAC (1.5 × 106) or CIT (4.4 × 106), and maintained a higher Shannon diversity index (5.65) than LAC (4.97) and CIT (4.30). The rapid and higher H2 transfer efficiency with lower bacterial growth by using formate was attributed to the slightly positive Gibbs free energy identified in H2 production requiring a H2-utilizer, lower carbon in the molecule, and adaptation to metabolic potential of the original groundwater microbiome. Formate is, therefore, a promising electron donor for rapid Dehalococcoides-augmented remediation with minimum bacterial growth. Sequential transferring of the FOR culture successfully maintained TCE-to-ethene dechlorination activity and enriched the members of genera Dehalococcoides (33%), Methanosphaerula (23%), Rectinema (13%), and Desulfitobacterium (5.6%). This suggests that formate is transferred to H2 and acetate, and provided to Dehalococcoides.