Interactions between Nitrate-Reducing and Sulfate-Reducing Bacteria Coexisting in a Hydrogen-Fed Biofilm

To explore the relationships between denitrifying bacteria (DB) and sulfate-reducing bacteria (SRB) in H-2-fed biofilms, we used two H-2-based membrane biofilm reactors (MBfRs) with or without restrictions on H-2 availability. DB and SRB compete for H-2 and space in the biofilm, and sulfate (SO42-) reduction should be out-competed when H-2 is limiting inside the biofilm. With H-2 availability restricted, nitrate (NO3-) reduction was proportional to the H-2 pressure and was complete at a H-2 pressure of 3 atm; SO42- reduction began at H-2 >= 3.4 atm. Without restriction on H-2 availability, NO3- was the preferred electron acceptor, and SO42- was reduced only when the NO3- surface loading was <= 0.13 g N/m(2)-day. We assayed DB and SRB by quantitative polymerase chain reaction targeting the nitrite reductases and dissimilatory sulfite reductase, respectively. Whereas DB and SRB increased with higher H-2 pressures when H-2 availability was limiting, SRB did not decline with higher NO3- removal flux when H-2 availability was not limiting, even when SO42- reduction was absent. The SRB trend reflects that the SRB's metabolic diversity allowed them to remain in the biofilm whether or not they were reducing SO42-. In all scenarios tested, the SRB were able to initiate strong SO42- reduction only when competition for H-2 inside the biofilm was relieved by nearly complete removal of NO3-.