Small-scale oxygen distribution determines the vinyl chloride biodegradation pathway in surficial sediments of riverbed hyporheic zones.

Surficial riverbed sediments are often characterized by sharp redox gradients between the aerobic benthic sediment and underlying anoxic sediment, potentially representing an ideal niche for aerobic and anaerobic vinyl chloride (VC) degraders. To test this, the fate of VC in aerobic and anaerobic microcosms containing surficial sediment of a riverbed hyporheic zone receiving VC-contaminated groundwater was explored. Quantitative PCR showed that Dehalococcoides 16S rRNA gene and VC reductive dehalogenase-encoding genes (vcrA, bvcA) were highly enriched in anaerobic microcosms, with stoichiometric conversion of VC to ethene. In aerobic microcosms, etnC and etnE involved in aerobic ethene/VC oxidation were enriched with concomitant low or no accumulation of ethene. However, Dehalococcoides 16S rRNA gene, vcrA and bvcA copy numbers were also enriched in oxygen-exposed microcosms containing sediment with high organic carbon and small grain size, whereas they were reduced in oxygen-exposed sediment with low organic carbon and larger grain size in line with extensive oxygen penetration into the sediment. These results suggest the coexistence and coactivity of anaerobic and aerobic VC degraders in the same small volume of surficial sediment and that oxygen distribution, as determined by sediment grain size and organic matter content, affects the local VC-degrading bacterial community and VC biodegradation pathway.