Different synthetic chemical dispersants elicit distinct bacterial community responses to crude oil

Background: Dispersants are commonly used worldwide as a primary response tool to treat oil spills at sea, yet their use is debated due to their toxicological effects and potential to affect oil biodegradation rates. We examined the effect of three globally stockpiled synthetic chemical dispersants (Superdispersant 25, Slickgone EW, Slickgone NS) on the microbial response to crude oil and its biodegradation in a subarctic marine environment during 2015 and 2017 using 16S rRNA gene amplicon sequencing. Results: Across both years, communities were dominated by known hydrocarbon-degrading bacteria, including Alcanivorax , Alteromonas , and Pseudoalteromonas . Superdispersant 25 enriched for Dokdonia , Thalassospira , and Roseobacter , genera with demonstrated alkane and PAH degradation pathways. Slickgone EW and NS selected Marinomonas , Colwellia , Psychromonas , and Alcanivorax . Samples without dispersant also contained hydrocarbon degraders, however, the community composition was altered. In 2017, we quantified hydrocarbon degradation using GC-FID/MS for aliphatic and aromatic hydrocarbons respectively. Hydrocarbon biomarker ratios showed n-alkane depletion (Pr/C17 and Ph/C18 ratios increased 3–12 fold) in dispersant treatments. Selective weathering was evident in all treatments. 1MP/9MP ratios indicated limited or variable effects on aromatic hydrocarbon biodegradation, particularly in the presence of dispersant. We integrated aromatic hydrocarbon concentrations with microbial community data using DIABLO (Data Integration Analysis for Biomarker discovery using Latent variable approaches for Omics studies). Several taxa were negatively correlated to specific aromatic hydrocarbons – i.e. they increased when the hydrocarbon was reduced suggesting these may have served carbon sources. Taxa included Aquibacter , Hyphomonas , Thalassospira , Alteromonas , Sphingopyxis , and Paraperlucidibaca . Marine oil snow (MOS) formed in all oil-amended treatments and showed high microbial colonisation, whereas marine dispersant snow showed little colonisation. Conclusion: This study provides evidence that different dispersants affect different microbial responses to crude oil contamination, including the enrichment of key oil-degrading bacterial taxa. However, this did not correlate with enhanced aromatic hydrocarbon degradation. MOS formation with high microbial colonisation suggests natural aggregation processes may provide an effective biodegradation pathway. These findings question the functional benefit of synthetic chemical dispersants and highlight that the marine environment naturally harbours hydrocarbon-degrading microbial communities with the potential to respond to oil spills.