Nitrate Shaped the Selenate-Reducing Microbial Community in a Hydrogen-Based Biofilm Reactor

To study the effect of nitrate (NO3-) on selenate (SeO42-) reduction, we tested a H-2-based biofilm with a range of influent NO3- loadings. When SeO42- was the only electron acceptor (stage 1), 40% of the influent SeO42- was reduced to insoluble elemental selenium (Se). SeO42- reduction was dramatically inhibited when NO3- was added at a surface loading larger than 1.14 g of N m(-2) day(-1), when H-2 delivery became limiting and only 80% of the input NO3- was reduced (stage 2). In stage 3, when NO3- was again removed from the influent, SeO42- reduction was re-established and increased to 60% conversion to Se-0. SeO42- reduction remained stable at 60% in stages 4 and 5, when the NO3- surface loading was re-introduced at <= 50.53 g of N m(-2) day(-1), allowing for complete NO3- reduction. The selenate-reducing microbial community was significantly reshaped by the high NO3- surface loading in stage 2, and it remained stable through stages 3-5. In particular, the abundance of alpha-Proteobacteria decreased from 30% in stage 1 to less than 10% of total bacteria in stage 2. beta-Proteobacteria, which represented about 55% of total bacteria in the biofilm in stage 1, increased to more than 90% of phylotypes in stage 2. Hydrogenophaga, an autotrophic denitrifier, was positively correlated with NO3- flux. Thus, introducing a NO3- loading high enough to cause H-2 limitation and suppress SeO42- reduction had a long-lasting effect on the microbial community structure, which was confirmed by principal coordinate analysis, although SeO42- reduction remained intact.