Niche Partitioning Between Coastal and Offshore Shelf Waters Results in Differential Expression of Alkane and PAH Catabolic Pathways

In the wake of the Deepwater Horizon oil spill, the taxonomic response of marine microbial communities to oil and dispersants has been extensively studied. However, the functional metabolic response of these communities remains comparatively underexplored. Moreover, although marine oil spills can simultaneously impact thousands of square kilometers of both coastal and offshore environments, little information currently exists on how these responses vary spatially between different oceanic biomes. Using a combination of analytical chemistry, 16S rRNA amplicon, and metatranscriptomic sequencing, we provide a broad, comparative overview of the specific functional and ecological response of hydrocarbon degrading bacteria in marine surface waters over time between two oceanic biomes. We find evidence for the existence of different ecotypes of Alcanivorax, Marinobacter, and Cycloclasticus which behave differentially in coastal and offshore shelf waters despite being exposed to similar concentrations of oil, dispersants, and nutrients. This resulted in the differential expression of catabolic pathways for n-alkanes and polycyclic aromatic hydrocarbons (PAH)—the two major categories of compounds found in crude oil—with preferential expression of n-alkane degradation genes in coastal waters while offshore microbial communities trended more towards the expression of PAH degradation genes. This was unexpected as it contrasts with the generally held view that n-alkanes, being more labile, are attacked before the more refractory PAHs. Collectively, these results provide evidence for the existence of ecotone boundaries for hydrocarbon degrading taxa between neighboring oceanic regions.