posted on 2021-04-19, 04:35authored byBaogang Zhang, Jun Liu, Yizhi Sheng, Jiaxin Shi, Hailiang Dong
Hexavalent chromium [Cr(VI)] is one
of the common heavy-metal contaminants
in groundwater, and the availability of electron donors is considered
to be a key parameter for Cr(VI) biotransformation. During the autotrophic
remediation process, however, much remains to be illuminated about
how complex syntrophic microbial communities couple Cr(VI) reduction
with other elemental cycles. Two series of Cr(VI)-reducing groundwater
bioreactors were independently amended by elemental sulfur and iron
and inoculated with the same inoculum. After 160 days of incubation,
both bioreactors showed similar archaea-dominating microbiota compositions,
whereas a higher Cr(VI)-reducing rate and more methane production
were detected in the Fe0-driven one. Metabolic reconstruction
of 23 retrieved genomes revealed complex symbiotic relationships driving
distinct elemental cycles coupled with Cr(VI) reduction in bioreactors.
In both bioreactors, these Cr(VI) reducers were assumed to live in
syntrophy with oxidizers of sulfur, iron, hydrogen, and volatile fatty
acids and methane produced by carbon fixers and multitrophic methanogens,
respectively. The significant difference in methane production was
mainly due to the fact that the yielded sulfate greatly retarded acetoclastic
methanogenesis in the S-bioreactor. These findings provide insights
into mutualistic symbioses of carbon, sulfur, iron, and chromium metabolisms
in groundwater systems and have implications for bioremediation of
Cr(VI)-contaminated groundwater.