Relationship between the concentration of denitrifiers and Pseudomonas spp. in soils: Implications for BTX bioremediation

Aquifer microcosms were used to investigate the effect of stimulating denitrification on microbial population shifts and BTX degradation potential. Selective pressure for facultative denitrifiers was applied to a treatment set by feeding acetate and nitrate, and cycling electron acceptor conditions twice between aerobic and denitrifying stages. A second (control) set degraded the same amount of acetate under aerobic conditions. The resulting concentrations of total heterotrophs were not significantly different between the two sets. Nevertheless, the concentrations of denitrifiers, Pseudomonas spp., and BTX degraders were significantly higher in the cycled microcosms than in the aerobic controls. The predominant isolates from the cycled microcosms were fluorescent Pseudomonas species that are known to degrade BTX. Following the complete removal of acetate, cycled microcosms also showed higher aerobic BTX degradation activity. These results suggest that nitrate addition to oxygen-limited aquifers might enhance BTX bioremediation not only by supplementing the electron acceptor pool as is widely accepted, but also by fostering favorable changes in the composition of the microbial consortium. Specifically, denitrifying conditions could have the ancillary benefit of fortuitously selecting for Pseudomonas spp. that can degrade BTX. This syllogism is supported by a survey of international soils (from France, Denmark, Brazil and Iowa, USA), which showed a correlation between the concentration of denitrifers and Pseudomonas spp.