Nitrate effects on chromate reduction in a methane-based biofilm

Abstract The effects of nitrate (NO 3 − ) on chromate (Cr(VI)) reduction in a membrane biofilm reactor (MBfR) were studied when CH 4 was the sole electron donor supplied with a non-limiting delivery capacity. A high surface loading of NO 3 − gave significant and irreversible inhibition of Cr(VI) reduction. At a surface loading of 500 mg Cr/m 2 -d, the Cr(VI)-removal percentage was 100% when NO 3 − was absent (Stage 1), but was dramatically lowered to  −2 -d NO 3 − (Stage 2). After ∼50 days operation in Stage 2, the Cr(VI) reduction recovered to only ∼70% in Stage 3, when NO 3 − was removed from the influent; thus, NO 3 − had a significant long-term inhibition effect on Cr(VI) reduction. Weighted PCoA and UniFrac analyses proved that the introduction of NO 3 − had a strong impact on the microbial community in the biofilms, and the changes possibly were linked to the irreversible inhibition of Cr(VI) reduction. For example, Meiothermus , the main genus involved in Cr(VI) reduction at first, declined with introduction of NO 3 − . The denitrifier Chitinophagaceae was enriched after the addition of NO 3 − , while Pelomonas became important when nitrate was removed, suggesting its potential role as a Cr(VI) reducer. Moreover, introducing NO 3 − led to a decrease in the number of genes predicted (by PICRUSt) to be related to chromate reduction, but genes predicted to be related to denitrification, methane oxidation, and fermentation increased.