Role of EPS, Dispersant and Nutrients on the Microbial Response and MOS Formation in the Subarctic Northeast Atlantic

Abstract

In this study we report the formation of marine oil snow (MOS), its associated microbial community, the factors influencing its formation, and the microbial response to crude oil in surface waters of the Faroe-Shetland Channel (FSC). The FSC is a subarctic region that is hydrodynamically complex located in the northeast Atlantic where oil extraction is currently occurring and where exploration is likely to expand into its deeper waters (>500 m). A major oil spill in this region may mirror the aftermath that ensued following the Deepwater Horizon (DWH) blowout in the Gulf of Mexico, where the massive influx of Macondo crude oil triggered the formation of copious quantities of rapidly sinking MOS and successional blooms of opportunistic oil-degrading bacteria. In laboratory experiments, we simulated environmental conditions in sea surface waters of the FSC using water collected from this site during the winter of 2015. We demonstrated that the presence of dispersant triggers the formation of MOS, and that nutrient amendments magnify this. Illumina MiSeq sequencing revealed the enrichment on MOS of associated oil-degrading (Cycloclasticus, Thalassolituus, Marinobacter) and EPS-producing (Halomonas, Pseudoalteromonas, Alteromonas) bacteria, and included major representation by Psychrobacter and Cobetia with putative oil-degrading/EPS-producing qualities. The formation of marine snow, in the absence of crude oil and dispersant, in seawater amended with nutrients alone indicated that the de novo synthesis of bacterial EPS is a key factor in MOS formation, and the glycoprotein composition of the MOS aggregates confirmed that its amorphous biopolymeric matrix was of microbial (likely bacterial) origin. The presence of dispersants and crude oil with/without nutrients resulted in distinct microbial responses marked by intermittent, and in some cases short-lived, blooms of opportunistic heterotrophs, principally obligate hydrocarbonoclastic (Alcanivorax, Cycloclasticus, Thalassolituus, Marinobacter) and EPS-producing (Halomonas, Alteromonas, Pseudoalteromonas) bacteria. Interestingly, members of the Vibrionales (principally the genus Vibrio) were strongly enriched by crude oil (with/without dispersant or nutrients), highlighting a putative importance for these organisms in crude oil biodegradation in the FSC. Our findings mirror those observed at DWH and hence underscore their broad relevance.

Keywords: Deepwater Horizon; Faroe-shetland channel; crude oil; hydrocarbon-degrading bacteria; marine environment; marine oil snow (MOS).

FIGURE 1

Map representing the stations studied along the FIM section of the Faroe Shetland channel. The solid point shows the station sampled in this study.

FIGURE 2

Marine oil snow aggregates shown floating at the surface of the CEWAF (A) and CEWAF+N (B) roller-bottle incubations, and Marine Dispersant Snow (MDS) aggregates shown settled at the bottom of the bottles in the SW+D treatments (C).

FIGURE 3

Formation of MOS and marine snow in the roller bottle incubations. Under the epifluorenscence microscope after staining with acridine orange, MOS (A) which formed in the CEWAF (±nutrients) treatments was populated with associated prokaryotic cells (small green dots) and oil droplets (larger green spherical/irregular blobs). Marine snow (B) that formed in the SW+N treatments contained few associated prokaryotic cells. Under the light microscope, MOS stained with coomassie brilliant blue G (C) and Alcian Blue (D). Bar, 10 μm.

FIGURE 4

Bacterial community composition at family-level classification of MOS compared to that in the surrounding seawater in the CEWAF+N treatment at weeks 2.5 and 4.

FIGURE 5

Prokaryotic (bacterial and archaeal) cell numbers from roller-bottle incubations of the different treatments with sea surface water from the FSC amended with or without nutrients, dispersant and/or crude oil (as WAF). SW, seawater; SW+N, seawater with nutrients; SW+D, seawater with dispersant; WAF, water-accomodated fraction; CEWAF, chemically enhanced WAF; CEWAF+N, chemically enhanced WAF with nutrients.

FIGURE 6

Bacterial community composition at family level classification for each of the different treatments over the 6 week incubation period that the roller-bottle experiments were run. SW, seawater; SW+N, seawater with nutrients; SW+D, seawater with dispersant; WAF, water-accomodated fraction; CEWAF, chemically enhanced WAF; CEWAF+N, chemically enhanced WAF with nutrients.

FIGURE 7

Non-metric multidimensional scaling (NMDS) plot showing the similarity of each sample. The stress achieved is indicated in the top right of the plot. Symbol colors signify the original treatment: SW (red); SW+N (blue); SW+D (green); WAF (purple); CEWAF (black); CEWAF+N (gray). Each time point is represented by a different symbol: T₀ (square); T₁ (open circle); T₂ (triangle); T₄ (diamond); T₆ (closed circle). Ellipses shown surrounding symbols represent grouping the treatment types with 95% confidence interval.