Cell toxic damages during polycyclic aromatic hydrocarbons biodegradation by Pseudomonas aeruginosa G24

Many microorganisms are capable of degrading polycyclic aromatic hydrocarbons (PAHs), but the extent of toxicity of PAHs on these degraders is not yet fully understood. In this study, the physicochemical responses of the PAH-degrading strain Pseudomonas aeruginosa (P. aeruginosa) G24 under PAHs stress were investigated. After 21 days of incubation, the total removal rates of phenanthrene (Phe) and pyrene (Pyr) by P. aeruginosa G24 were 43.8 % and 36.8 %, respectively. Among them, about 11 % of PAHs were absorbed by pellets in the molecular form, resulting in net biodegradation rates of 32.9 % and 25.2 % for Phe and Pyr, respectively. Further investigations revealed that the surface zeta potential of the P. aeruginosa G24 increased from 16.78 to 19.31 mV to 30.32-33.59 mV, the percentage of permeable cells increased from 24.5 %-32.5 % to 52.4 %-56.2 %, and the EPS contents decreased from 2.34 to 3.37 mg L-1 to 0.63-1.80 mg L-1 in the "Phe-MSM + P. aeruginosa G24" and "Pyr-MSM + P. aeruginosa G24" systems. The damages of P. aeruginosa G24 during PAH degradation included increased zeta potential and permeability on the bacterial surface, increased cell apoptosis, reduced EPS contents, as well as injured cell membrane structures. The findings of the study suggest that the damages or apoptosis of P. aeruginosa G24 may inhibit the continuous degradation of residual PAHs during the late stage of incubation.