Microbial ecology in selenate-reducing biofilm communities: Rare biosphere and their interactions with abundant...

Selenate (SeO₄^2- ) reduction in hydrogen (H₂ )-fed membrane biofilm reactors (H₂ -MBfRs) was studied in combinations with other common electron acceptors. We employed H₂ -MBfRs with two distinctly different conditions: R1, with ample electron-donor availability and acceptors SeO₄^2- and sulfate (SO₄^2- ), and R2, with electron-donor limitation and the presence of electron acceptors SeO₄^2- , nitrate (NO₃^- ), and SO₄^2- . Even though H₂ was available to reduce all input SeO₄^2- and SO₄^2- in R1, SeO₄^2- reduction was preferred over SO₄^2- reduction. In R2, co-reduction of NO₃^- and SeO₄^2- occurred, and SO₄^2- reduction was mostly suppressed. Biofilms in all MBfRs had high microbial diversity that was influenced by the "rare biosphere" (RB), phylotypes with relative abundance less than 1%. While all MBfR biofilms had abundant members, such as Dechloromonas and Methyloversatilis, the bacterial communities were significantly different between R1 and R2. For R1, abundant genera were Methyloversatilis, Melioribacter, and Propionivibrio; for R2, abundant genera were Dechloromonas, Hydrogenophaga, Cystobacter, Methyloversatilis, and Thauera. Although changes in electron-acceptor or -donor loading altered the phylogenetic structure of the microbial communities, the biofilm communities were resilient in terms of SeO₄^2- and NO₃^- reductions, because interacting members of the RB had the capacity of respiring these electron acceptors.