Correlation of patterns of denitrification instability in replicated bioreactor communities with shifts in the relative abundance and the denitrification patterns of specific populations

To assess the effects of community structure on the stability of denitrification, six chemostat cultures derived from the same denitrifying community were subjected to step increases in feed nitrate concentration and monitored for evidence that denitrification was either not occurring (indicated by the presence of nitrate) or was incomplete (indicated by the presence of nitrite or nitrous oxide). Functional stability was defined and quantified from the pattern of effluent concentration trends of nitrate and denitrification intermediates. Microbial community structure and dynamics were analyzed by terminal restriction fragment length polymorphism analysis of the 16S rRNA gene. Functional stability varied: one chemostat community lost the ability to reduce all of the influent nitrate; others continued to reduce all of the influent nitrate, but accumulated varying amounts of nitrous oxide. The microbial community structure in two of the chemostats diverged from the others, and variation of functional response among chemostats corresponded with the divergence of community structure. The Acidovorax-like terminal restriction fragment (T-RF) dominated the chemostat that accumulated nitrate, and an Acidovorax-like isolate reduced nitrate directly to dinitrogen gas in batch nitrate reduction assays. In the nitrous oxide-accumulating chemostats, the relative abundance of the Pseudomonas-like T-RF was strongly and significantly correlated with the magnitude of nitrous oxide accumulation, and a Pseudomonas-like isolate accumulated nitrous oxide in batch assays.