The effect of crude oil concentration on hydrocarbon degradation and associated bacterial communities under experimental condition

Chemical dispersants are an important tool for mitigating the environmental impact of marine oil pollution. Crude oil concentration used in experiments to study the efficacy of dispersants has been shown to impact oil biodegradation rates, which may explain contradictory conclusions between studies. Using high crude oil concentration could be particularly problematic when extrapolating laboratory results to the field, as dispersed oil in the open sea would rapidly dilute to sub-ppm concentrations. This study aimed to quantify the impact of crude oil concentrations ranging from 1 to 1000 mg L-1 on biodegradation and microbial community composition in dispersed versus non-dispersed crude oil, Oil biodegradation rates increased linearly from 1 to 100 mg L-1 but plateaued at a concentration of 1000 mg L-1. However, with the addition of chemical dispersant, degradation rates were linear across the full range of concentrations, and significantly greater than non-dispersed treatments at 1 mg L-1 (81% increase), 10 mg L-1 (80%), 100 mg L-1 (64%) and 1000 mg L-1 (388%). Based on 16S rRNA gene sequence analysis, Alteromonadaceae and Cellulophaga were more dominant at lower oil concentrations, potentially being inhibited or outcompeted at higher concentrations. Use of dispersant resulted in more rapid changes in community composition. These results highlight the need to consider oil concentration when performing biodegradation experiments and will inform the Net Environment Benefit Analysis (NEBA).