Interactions between perchlorate and nitrate reductions in the biofilm of a hydrogen-based membrane biofilm reactor

We studied the microbial functional and structural interactions between nitrate (NO ₃ ^-) and perchlorate (ClO ₄ ^-) reductions in the hydrogen (H ₂)-based membrane biofilm reactor (MBfR). When H ₂ was not limiting, ClO ₄ ^- and NO ₃ ^- reductions were complete, and the MBfR's biofilm was composed mainly of bacteria from the ε- and β-proteobacteria classes, with autotrophic genera Sulfuricurvum, Hydrogenophaga, and Dechloromonas dominating the biofilm. Based on functional-gene and pyrosequencing assays, Dechloromonas played the most important role in ClO ₄ ^- reduction, while Sulfuricurvum and Hydrogenophaga were responsible for NO ₃ ^- reduction. When H ₂ delivery was insufficient to completely reduce both electron acceptors, NO ₃ ^- reduction out-competed ClO ₄ ^- reduction for electrons from H ₂, and mixotrophs become important in the MBfR biofilm. β-Proteobacteria became the dominant class, and Azonexus replaced Sulfuricurvum as a main genus. The changes suggest that facultative, NO ₃ ^--reducing bacteria had advantages over strict autotrophs when H ₂ was limiting, because organic microbial products became important electron donors when H ₂ was severely limiting.