Initial oil concentration affects hydrocarbon biodegradation rates and bacterial community composition in seawater

Highlights: • n-Alkane biodegradation rate decreased with increasing initial oil concentration. • PAHs biodegradation rate increased with increasing initial oil concentration. • Residual alkanes were ~50%, while residual PAHs increased with oil concentration. • Increased Rhodobacterales, Altererythrobacter, and Neptuniibacter with oil concentration. • High oil concentrations (400 ppm and 800 ppm) inhibited Alcanivorax. During oil spills in the field or for laboratory incubation studies, different oil concentrations are often encountered or applied, yet how initial oil concentration affects biodegradation rates of hydrocarbons and the development of oil degraders remains unclear. We incubated seawater for 50 d with different oil concentrations (0, 50, 100, 200, 400 and 800 ppm). n-Alkanes and polycyclic aromatic hydrocarbons (PAHs), and the bacterial community were analyzed periodically. Results show that the biodegradation rates of alkanes, derived from first order kinetics, decreased with increasing oil concentration, but percent residual was ~50% regardless of the initial concentration. In contrast, the biodegradation rates of PAHs increased with concentration, and the percent residual increased with oil concentration. Increasing oil concentration resulted in increased abundances of Rhodobacterales, Altererythrobacter, and Neptuniibacter. However, Alcanivorax abundance was barely detected in 400 and 800 ppm. Overall, oil concentration critically affected the degradation of hydrocarbons and the bacterial community.