The effects of high-precision scale dispersant concentrations on microbial community and new implications for dispersant application after marine oil spills

The usage of dispersant presents a difficult choice: the uncertainty of beneficial biodegradation of spilled oil against what could be a greater environmental impact from the finely dispersed oil. Here, we evaluated the effect of dispersant on the microbial community at a high-precision concentration (0.1–20 % (v/v), 10 gradients) to elucidate the uncertainty of beneficial biodegradation and proposed the associated mechanism. Results at the phylum, class, and genus level revealed no significant changes in microbial diversity and structure at low-concentration dispersants (0.1–3 %), but significant changes at high-concentration dispersants (5–20 %). For beneficial biodegradation, 4 oil-degrading bacteria and 3 non-oil-degrading bacteria exhibited strong positive (R² > 0.72) and negative (R² > 0.73) correlations at 0.1–3 % low-concentrations. More importantly, oil-degrading bacteria abundance exceeded 75.9 % in the total abundance proportion (70.1 %) of them. This indicated low-concentration dispersants can promote biodegradation. In the high-concentrations, similar but opposite results were shown. Therefore, dispersant concentration dominates biodegradation of spilled oil. Finally, we proposed the associated “bacterial peaking” mechanism and clarified that biodegradation increases under the drive of the affected bacteria and reach a peak at low-concentration dispersants, but decline rapidly at high-concentrations. This study provided a new insight to understand the usage of dispersant on biodegradation of spilled oil in the sea.