Oil biodegradation and marine oil snow formation in Arctic seawater microcosms at 4 °C

Hudson Bay is a sub-Arctic inland sea characterized by seasonal sea-ice cover, which governs marine circulation, biological productivity, and ecosystem structure. In recent decades, the duration of sea-ice cover has declined, resulting in longer ice-free seasons that are expected to elevate marine activity and environmental exposure to anthropogenic disturbances. As such, the potential for oil spill incidents associated with increased marine operations is expected to rise. To understand microbial biodegradation dynamics associated with marine oil snow (MOS), seawater microcosms amended with diesel fuel were incubated at 4 {degrees}C for 6 weeks. We amended all our microcosms with UV-weathered Churchill diesel fuel at 1% (v/v). 16S rRNA sequencing of the incubations revealed a decline in overall microbial diversity and a dramatic increase in specialized oil-degrading taxa, with Pseudomonas and Pseudoalteromonas being the main taxa involved, as both effectively degrade hydrocarbons under ambient and stimulated conditions. Biostimulation with nutrients (NaNO3, KH2PO4) and an alternative dispersant at 0.1% (v/v) resulted in a greater abundance of MOS aggregates and rapid enrichment of oil-degraders. Simple n-alkanes exhibited the highest biodegradation loss relative to complex polycyclic aromatic hydrocarbons (PAHs); however, biostimulation did not substantially enhance their degradation. Additionally, UV weathering resulted in a 22% loss of PAHs, which is three times higher than the UV loss observed for n-alkanes. Biostimulated samples exhibited an elevated transfer of high molecular weight alkanes and PAHs from the oil into the aqueous phase, accompanied by a rapid enrichment of oil-degrading taxa.