# Hexavalent Chromium Inhibits Nitrate-Dependent Anaerobic Methane Oxidation While Enriching Denitrifiers: Insights into Microbial Interactions for Simultaneous Methane, Nitrate, and Chromate Removal

## TL;DR

Hexavalent chromium harms methane and nitrate removal in wastewater by inhibiting key microbes, but it gets reduced by other bacteria.

## Contribution

Shows Cr(VI) inhibits N-DAMO microbes but is reduced by denitrifiers, offering insights for wastewater treatment.

## Key findings

- Cr(VI) significantly inhibits methane oxidation by N-DAMO organisms.
- Cr(VI) reduction occurs independently of methane oxidation, likely via denitrifiers or abiotic reactions.
- Microbial consortia show resilience but Cr(VI) limits N-DAMO-based treatment effectiveness.

## Abstract

Chromate [Cr­(VI)] is a toxic heavy metal frequently detected
in
wastewater, often alongside nitrate (NO3
–). Nitrate-dependent anaerobic methane oxidation (N-DAMO) is a promising
process for the simultaneous removal of methane (CH4) and
NO3
– in wastewater treatment plants.
Because Cr­(VI) can serve as an alternative electron acceptor, its
presence may alter the N-DAMO performance. Here, we investigated the
impact of Cr­(VI) on an enrichment culture containing Candidatus
Methanoperedens and Candidatus Methylomirabilis, using NO3
– as the electron acceptor
and 13C–CH4 as the electron donor. Cultures
were exposed to varying Cr­(VI) concentrations, and microbial activity
was assessed using GC-MS, 16S rRNA gene sequencing, and qPCR. Cr­(VI)
was reduced within the cultures, but this reduction was not linked
to CH4 oxidation. Instead, CH4 oxidation was
significantly inhibited, with declines in the relative abundances
of both N-DAMO organisms. Cr­(VI) reduction was likely mediated by
denitrifiers through nitrate reductase activity or abiotically via
the reaction with nitrite (NO2
–). These
findings reveal functional resilience of microbial consortia in contaminated
environments but highlight Cr­(VI) toxicity as a constraint for N-DAMO-based
wastewater treatment.