Biodegradation of naphthalene by enriched marine denitrifying bacteria

Numerous studies have been investigated on the PAHs biodegradation in aerobic and anaerobic environments; however, the biodegradation of PAHs under anoxic conditions, especially denitrifying conditions, has drawn less attention. In this study, four series of batch experiments were conducted to investigate the effect of temperature, pH, naphthalene concentration and nitrate concentration on the naphthalene degradation under denitrification condition. Our results showed that the degradation of naphthalene was most favorable at pH 7 and 25 °C. Results also indicated that 30 mg/l naphthalene inhibited the biodegradation and the removal efficiency was only 20.2%. Significant degradation (91.7% and 96.3%) of naphthalene occurred when nitrate concentrations were 1.0 and 5.0 mM. Moreover, the maximum degradation rates were 0.13 and 0.18 mg-NAP/(l h) depending on the concentration of nitrate. Based on 16S rDNA analysis, the denitrifying enriched culture was mainly composed of γ-Proteobacteria (19 clones out of a total of 23 clones) and Actinobacteria (4 clones). Using a primer set specific for naphthalene degrading functional gene nahAc, two operational taxonomy units were obtained in the clone library of nahAc. Both of them were closely related to nahAc genes of known species of Pseudomonas. Quantitative polymerase chain reaction (qPCR) was employed to quantify the change of naphthalene-degrading population during the degradation of naphthalene using nahAc gene as the biomarker. The maximum degradation rate and removal efficiency were strongly correlated with nahAc gene copy number, with R2 of 0.69 and 0.79, respectively.