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 H₂-fed biofilms, we used two H₂-based membrane biofilm reactors (MBfRs) with or without restrictions on H₂ availability. DB and SRB compete for H₂ and space in the biofilm, and sulfate (SO₄^2-) reduction should be out-competed when H₂ is limiting inside the biofilm. With H₂ availability restricted, nitrate (NO₃^-) reduction was proportional to the H₂ pressure and was complete at a H₂ pressure of 3 atm; SO₄^2- reduction began at H₂ ≥ 3.4 atm. Without restriction on H₂ availability, NO₃^- was the preferred electron acceptor, and SO ₄^2- was reduced only when the NO₃^- surface loading was ≤0.13 g N/m²-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 H₂ pressures when H₂ availability was limiting, SRB did not decline with higher NO₃^- removal flux when H₂ availability was not limiting, even when SO₄ ^2- 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 SO₄^2-. In all scenarios tested, the SRB were able to initiate strong SO₄^2- reduction only when competition for H₂ inside the biofilm was relieved by nearly complete removal of NO₃^-.