Sulfate reduction and sulfide oxidation in aerobic mixed population biofilms

Abstract

The microzonation of O2 respiration, H2S oxidation and S042- reduction in aerobic biofilms grown on rotating disk reactors of a sewage treatment plant was studied by measuring concentration profiles with microelectrodes for O2, S2- and pH. The vertical distribution of sulfate-reducing bacteria (SRB) in the hiolilms was also determined by the conventional culture-dependent MPN method and fluorescently labeled 16S rRNA-targeted oligonucleotide probes for SRB. The SRB probe stained cells were distributed throughout the biofilm with a distinct higher fluorescence intensity to the middle part of the biofilm. This result corresponded well with O2 and S2- concentration gradients measured by microelectrodes, showing sulfate reducing activity was largely restricted to a narrow anaerobic zone located in the middle of the biofilm. Measurements of accumulation of reduced sulfur compounds (FeS, FeS2 and SO) in the biofilm indicated that the H2S produced by SRB became oxidized by O2 and other oxidants, probably ferric/ferrous hydrates, and precipitated as FeS and SO just above the sulfate reduction zone.