Using natural abundance radiocarbon to trace the flux of petrocarbon to the seafloor following the Deepwater Horizon oil spill

Simulating oil-driven abundance changes in benthic marine invertebrates using an ecosystem model

Field studies showed that benthic macrofauna and meiofauna abundances increased with sediment oil concentration in areas affected by the Deepwater Horizon (DWH) oil spill. Benthic invertebrate biomass shows a dome-shaped relationship with respect to petrogenic hydrocarbon concentrations suggesting a positive effect on biomass at low-to-medium oil concentrations and a negative effect at high concentrations. If this is due to enrichment of the benthic food web, then this adds to an emerging picture of a food web response over a large spatial area with both abundance increases and decreases as a result of DWH. We would be obliged to consider long term multispecies effects beyond the initial pulse disturbance in modeling impacts and recovery of economically valuable species. An Atlantis ecosystem model of the Gulf of Mexico is used to simulate three mechanisms that could explain observed changes in the invertebrate community. Scenario 1 is that stimulation of surface primary productivity occurred as a result of nutrient loading caused by diversion of Mississippi River water into Barataria Bay (a mitigation action taken during the DWH oil spill). Scenario 2 is that enrichment of the benthos occurred due to detrital loading from marine oil snow sedimentation and flocculent accumulation (MOSSFA). Scenario 3 is that predator declines and/or avoidance of oiled areas caused a release of predation mortality on benthic invertebrates. Scenario 2 (MOSSFA) stimulated the detritus-driven food web and was best able to cause a net increase in invertebrate biomass despite a realistic amount of oil toxicity. Scenario 3 (predator release) plausibly could have contributed to changes in benthic invertebrates. Scenario 1 (nutrient loading) had little impact on the benthos suggesting the benthic food web is decoupled from local pelagic production sources.

A tradeoff between physical encounters and consumption determines an optimal droplet size for microbial degradation of dispersed oil

Immiscible hydrocarbons occur in the ocean water column as droplets of varying diameters. Although microbial oil degradation is a central process in the remediation of hydrocarbon pollution in marine environments, the relationship between droplet size distribution and oil degradation rates by bacteria remains unclear, with a conflicting history of laboratory studies. Despite this knowledge gap, the use of chemical dispersants in oil spill response and mitigation is based on the rationale that increasing the surface-area-to-volume ratio of droplets will enhance net bacterial biodegradation rates. We demonstrate that this intuitive argument does not apply to most natural marine environments, where the abundance of oil droplets is much lower than in laboratory experiments and droplet-bacteria encounters are the limiting factor. We present a mechanistic encounter-consumption model to predict the characteristic time for oil degradation by marine bacteria as a function of the initial oil concentration, the distribution of droplet sizes, and the initial abundance of oil-degrading bacteria. We find that the tradeoff between the encounter time and the consumption time leads to an optimal droplet size larger than the average size generated by the application of dispersants. Reducing droplet size below this optimum can increase the persistence of oil droplets in the environment from weeks to years. The new perspective granted by this biophysical model of biodegradation that explicitly accounts for oil-microbe encounters changes our understanding of biodegradation particularly in the deep ocean, where droplets are often small and oil concentrations low, and explains degradation rate discrepancies between laboratory and field studies.

Validation of oil fate and mass balance for the Deepwater Horizon oil spill: Evaluation of water column partitioning

Model predictions of oil transport and fate for the 2010 Deepwater Horizon oil spill (Gulf of Mexico) were compared to field observations and absolute and relative concentrations of oil compounds in samples from 900 to 1400 m depth <11 km from the well. Chemical partitioning analyses using quantitative indices support a bimodal droplet size distribution model for oil released during subsea dispersant applications in June with 74% of the mass in >1 mm droplets that surfaced near the spill site within a few hours, and 1-8% as <0.13 mm microdroplets that remained below 900 m. Analyses focused on 900-1400 m depth <11 km from the well indicate there was substantial biodegradation of dissolved components, some biodegradation in microdroplets, recirculation of weathered microdroplets into the wellhead area, and marine oil snow settling from above 900 m carrying more-weathered particulate oil into the deep plume.

Assessing the coastal sensitivity to oil spills from the perspective of ecosystem services: A case study for Canada's pacific coast

Coastal environment is one of the most important ecological and socioeconomic areas. However, increasing energy demand and economic development lead to a continuous gas and oil exploration, production, and traffics, which notably raise the risk of oil spill accidents in coastal areas. Sensitivity assessment aiming to determine the coastal features that would be severely impaired by spill incidents is a crucial part of the response planning. In this study, an innovative framework for coastal sensitivity mapping that incorporated ecosystem service (ES) valuation and multidimensional assessment was proposed. Sensitivity was computed by valuing physical, biological, and social-economical indicators from ES perspective and separating each indicator into specific coastal domains. For different ES typologies, provisioning services contributed most to the overall ES value followed by culture services, supporting services, and regulating services. For ES value in different coastal domains, the highest value was recorded in the water column followed by water surface, shoreline, and seabed. However, the shoreline ranked highest regarding the ES value per ha. Sensitivity assessment revealed that sensitive areas differed in different domains, both in distribution and extent. Compared with the scoring method, the ES valuation method showed more coincidence with Ecologically and Biologically Significant Areas (EBSA), representing a more precise and practical approach for sensitivity assessment. A three-dimensional (3D) oil spill model was also applied to generate maps of oil contamination probability in shoreline, water surface, and water column. The obtained results highlighted the significance of incorporating different coastal domains into oil spill responses, and the urgent demand to broaden and deepen our understanding of ecological processes across the vertical coastal zones.

Bioremediation perspectives and progress in petroleum pollution in the marine environment: a review

The marine environment is often affected by petroleum hydrocarbon pollution due to industrial activities and petroleum accidents. This pollution has recalcitrant and persistent compounds that pose a high risk to the ecological system and human health. For this reason, the world claims to seek to clean up these pollutants. Bioremediation is an attractive approach for removing petroleum pollution. It is considered a low-cost and highly effective approach with fewer side effects compared to chemical and physical techniques. This depends on the metabolic capability of microorganisms involved in the degradation of hydrocarbons through enzymatic reactions. Bioremediation activities mostly depend on environmental conditions such as temperature, pH, salinity, pressure, and nutrition availability. Understanding the effects of environmental conditions on microbial hydrocarbon degraders and microbial interactions with hydrocarbon compounds could be assessed for the successful degradation of petroleum pollution. The current review provides a critical view of petroleum pollution in seawater, the bioavailability of petroleum compounds, the contribution of microorganisms in petroleum degradation, and the mechanisms of degradation under aerobic and anaerobic conditions. We consider different biodegradation approaches such as biostimulation, bioaugmentation, and phytoremediation.

Oil fate and mass balance for the Deepwater Horizon oil spill

Based on oil fate modeling of the Deepwater Horizon spill through August 2010, during June and July 2010, ~89% of the oil surfaced, ~5% entered (by dissolving or as microdroplets) the deep plume (>900 m), and ~6% dissolved and biodegraded between 900 m and 40 m. Subsea dispersant application reduced surfacing oil by ~7% and evaporation of volatiles by ~26%. By July 2011, of the total oil, ~41% evaporated, ~15% was ashore and in nearshore (<10 m) sediments, ~3% was removed by responders, ~38.4% was in the water column (partially degraded; 29% shallower and 9.4% deeper than 40 m), and ~2.6% sedimented in waters >10 m (including 1.5% after August 2010). Volatile and soluble fractions that did not evaporate biodegraded by the end of August 2010, leaving residual oil to disperse and potentially settle. Model estimates were validated by comparison to field observations of floating oil and atmospheric emissions.

From Nano-Gels to Marine Snow: A Synthesis of Gel Formation Processes and Modeling Efforts Involved with Particle Flux in the Ocean

Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Mapping spatial and temporal variation of seafloor organic matter Δ14C and δ13C in the Northern Gulf of Mexico following the Deepwater Horizon Oil Spill

Following the Deepwater Horizon oil spill of 2010, large amounts of biodegraded oil (petrocarbon) sank to the seafloor. Our objectives were to 1) determine post-spill isotopic values as the sediments approached a new baseline and 2) track the recovery of affected sediments. Sediment organic carbon δ¹³C and Δ¹⁴C reached a post-spill baseline averaging -21.2 ± 0.9‰ (n = 129) and -220 ± 66‰ (n = 95). Spatial variations in seafloor organic carbon baseline isotopic values, ¹³C and ¹⁴C, were influenced by river discharge and hydrocarbon seepage, respectively. Inverse Distance Weighting of surface sediment Δ¹⁴C values away from seep sites showed a 50% decrease in the total mass of petrocarbon, from 2010 to 2014. We estimated a rate of loss of -2 × 10⁹ g of petrocarbon-C/year, 2-11% of the degradation rates in surface slicks. Despite the observed recovery in sediments, lingering residual material in the surface sediments was evident seven years following the blowout.

The Complexity of Spills: The Fate of the Deepwater Horizon Oil

The Deepwater Horizon oil spill was the largest, longest-lasting, and deepest oil accident to date in US waters. As oil and natural gas jetted from release points at 1,500-m depth in the northern Gulf of Mexico, entrainment of the surrounding ocean water into a buoyant plume, rich in soluble hydrocarbons and dispersed microdroplets of oil, created a deep (1,000-m) intrusion layer. Larger droplets of liquid oil rose to the surface, forming a slick of mostly insoluble, hydrocarbon-type compounds. A variety of physical, chemical, and biological mechanisms helped to transform, remove, and redisperse the oil and gas that was released. Biodegradation removed up to 60% of the oil in the intrusion layer but was less efficient in the surface slick, due to nutrient limitation. Photochemical processes altered up to 50% (by mass) of the floating oil. The surface oil expression changed daily due to wind and currents, whereas the intrusion layer flowed southwestward. A portion of the weathered surface oil stranded along shorelines. Oil from both surface and intrusion layers were deposited onto the seafloor via sinking marine oil snow. The biodegradation rates of stranded or sedimented oil were low, with resuspension and redistribution transiently increasing biodegradation. The subsequent research efforts increased our understanding of the fate of spilled oil immensely, with novel insights focusing on the importance of photooxidation, the microbial communities driving biodegradation, and the formation of marine oil snow that transports oil to the seafloor.

A review on the sinking mechanisms for oil and successful response technologies

A rise in the shipping of heavier hydrocarbon products increases the potential for an oil to sink after a spill. Further, sunken oil is difficult to locate and recover, and appropriate response technologies depend on the sinking mechanism. In this review, principal sinking mechanisms for oil are described and appropriate response technologies are suggested. Then, models appropriate for tracking sunken oil are compared. Oil may sink as burn residue, microscopic oil-particle aggregates (OPAs) or macroscopic oil-sediment mixtures (OSMs), marine oil snow during a MOSSFA event, or due to its high density. The most common mechanism is by sediment entrainment, and in such scenarios manual recovery has been reported as a successful response option. Among oil tracking models, trajectory models and Bayesian oil search models are compared for sunken oil capabilities. Many oil spill models require hydrodynamic inputs, whereas Bayesian models infer parameters based on available field concentration and bathymetric data.

Beyond Thresholds: A Holistic Approach to Impact Assessment Is Needed to Enable Accurate Predictions of Environmental Risk from Oil Spills

The risk assessment for the environmental impact of oil spills in Australia is often conducted in part using a combination of spill mapping and toxicological thresholds derived from laboratory studies. While this process is useful in planning operational responses, such as where to position equipment stockpiles and whether to disperse oil, and can be used to identify areas near the spill site where impacts are likely to occur, it cannot accurately predict the environmental consequences of an oil spill or the ecosystem recovery times. Evidence of this disconnect between model predictions and observed impacts is the lack of a profound effect of the Deepwater Horizon wellhead blowout on recruitment to fisheries in the northern Gulf of Mexico, contrary to the predictions made in the Natural Resources Damage Assessment and despite the occurrence of impacts of the spill on marine mammals, marshes, and deep water ecosystems. The incongruity between predictions made with the current approach using threshold monitoring and impacts measured in the field results from some of the assumptions included in the oil spill models. The incorrect assumptions include that toxicity is acute, results from dissolved phase exposure, and would be readily reversible. The toxicity tests from which threshold models are derived use members of the ecosystem that are easily studied in the lab but may not represent the ecosystem as a whole. The test species are typically highly abundant plankton or planktonic life stages, and they have life histories that account for rapid changes in environmental conditions. As a consequence, these organisms recover quickly from an oil spill. The interdependence of ecosystem components, including the reliance of organisms on their microbiomes, is often overlooked. Additional research to assess these data gaps conducted using economically and ecologically relevant species, especially in Australia and other understudied areas of the world, and the use of population dynamic models, will improve the accuracy of environmental risk assessment for oil spills. Integr Environ Assess Manag 2020;16:813-830. © 2020 SETAC.

Marine Snow Aggregates are Enriched in Polycyclic Aromatic Hydrocarbons (PAHs) in Oil Contaminated Waters: Insights from a Mesocosm Study

Marine snow was implicated in the transport of oil to the seafloor during the Deepwater Horizon oil spill, but the exact processes remain controversial. In this study, we investigated the concentrations and distributions of the 16 USEPA priority polycyclic aromatic hydrocarbons (PAHs) in marine snow aggregates collected during a mesocosm experiment. Seawater only, oil in a water accommodated fraction (WAF), and Corexit-enhanced WAF (DCEWAF) were incubated for 16 d. Both WAF and DCEWAF aggregates were enriched in heavy molecular weight PAHs but depleted in naphthalene. DCEWAF aggregates had 2.6 times more total 16 PAHs than the WAF (20.5 vs. 7.8 µg/g). Aggregates in the WAF and DCEWAF incorporated 4.4% and 19.3%, respectively of the total PAHs in the mesocosm tanks. Our results revealed that marine snow sorbed and scavenged heavy molecular weight PAHs in the water column and the application of Corexit enhanced the incorporation of PAHs into the sinking aggregates.

Formation and fate of oil-related aggregates (ORAs) in seawater at different temperatures

In this study, the formation and fate of oil-related aggregates (ORAs) from chemically dispersed oil in seawater (SW) were investigated at different temperatures (5 °C, 13 °C, 20 °C). Experiments in natural SW alone, and in SW amended with typical marine snow constituents (phytoplankton and mineral particles), showed that the presence of algae stimulated the formation of large ORAs, while high SW temperature resulted in faster aggregate formation. The ORAs formed at 5 °C and 13 °C required mineral particles for sinking, while the aggregates also sank in the absence of mineral particles at 20°. Early in the experimental periods, oil compound accumulation in ORAs was faster than biodegradation, particularly in aggregates with algae, followed by rapid biodegradation. High abundances of bacteria associated with hydrocarbon biodegradation were determined in the ORAs, together with algae-associated bacteria, while clustering analyses showed separation between bacterial communities in experiments with oil alone and oil with algae/mineral particles.

Formation, Detection, and Modeling of Submerged Oil: A Review

Submerged oil, oil in the water column (neither at the surface nor on the bottom), was found in the form of oil droplet layers in the mid depths between 900–1300 m in the Gulf of Mexico during and following the Deepwater Horizon oil spill. The subsurface peeling layers of submerged oil droplets were released from the well blowout plume and moved along constant density layers (also known as isopycnals) in the ocean. The submerged oil layers were a challenge to locate during the oil spill response. To better understand and find submerged oil layers, we review the mechanisms of submerged oil formation, along with detection methods and modeling techniques. The principle formation mechanisms under stratified and cross-current conditions and the concepts for determining the depths of the submerged oil layers are reviewed. Real-time in situ detection methods and various sensors were used to reveal submerged oil characteristics, e.g., colored dissolved organic matter and dissolved oxygen levels. Models are used to locate and to predict the trajectories and concentrations of submerged oil. These include deterministic models based on hydrodynamical theory, and probabilistic models exploiting statistical theory. The theoretical foundations, model inputs and the applicability of these models during the Deepwater Horizon oil spill are reviewed, including the pros and cons of these two types of models. Deterministic models provide a comprehensive prediction on the concentrations of the submerged oil and may be calibrated using the field data. Probabilistic models utilize the field observations but only provide the relative concentrations of the submerged oil and potential future locations. We find that the combination of a probabilistic integration of real-time detection with trajectory model output appears to be a promising approach to support emergency response efforts in locating and tracking submerged oil in the field.

Selective solvent filters for non-aqueous phase liquid separation from water

Injectable filters permeable to water but impermeable to non-polar solvents were developed to contain non-aqueous phase liquids (NAPL) in contaminated aquifers, hence protecting downstream receptors during NAPL remediation. Filters were produced by injecting aqueous solutions of 0.01% chitosan, hydroxyethylcellulose and quaternized hydroxyethylcellulose into sand columns, followed by rinsing with water. Polymer sorption onto silica was verified using a quartz-crystal microbalance with dissipation monitoring. Fluorescence and gas chromatography mass spectroscopy showed low ppm range concentrations of non-polar solvents (e.g., hexane and toluene) in water eluted from the filters (in the absence of emulsifiers). The contact angles between polymer-coated surfaces and hexane or toluene were > 90°, indicating surface oleophobicity. Organic, polar solvents (e.g. tetrahydrofuran and tetrachloroethylene, TCE) were not separated from water. The contact angles between polymer-coated surfaces and TCE was also > 90°. However, the contact area with polymer coated surfaces was greater for TCE than non-polar solvents, suggesting higher affinity between TCE and the surfaces. Emulsifiers can be used to facilitate NAPL extraction from aquifers. Emulsion separation efficiency depended on the emulsifier used. Emulsions were not separated with classical surfactants (e.g. Tween 20 and oleic acid) or alkaline zein solutions. Partial emulsion separation was achieved with humic acids and zein particles.

Diatom aggregation when exposed to crude oil and chemical dispersant: Potential impacts of ocean acidification

Diatoms play a key role in the marine carbon cycle with their high primary productivity and release of exudates such as extracellular polymeric substances (EPS) and transparent exopolymeric particles (TEP). These exudates contribute to aggregates (marine snow) that rapidly transport organic material to the seafloor, potentially capturing contaminants like petroleum components. Ocean acidification (OA) impacts marine organisms, especially those that utilize inorganic carbon for photosynthesis and EPS production. Here we investigated the response of the diatom Thalassiosira pseudonana grown to present day and future ocean conditions in the presence of a water accommodated fraction (WAF and OAWAF) of oil and a diluted chemically enhanced WAF (DCEWAF and OADCEWAF). T. pseudonana responded to WAF/DCEWAF but not OA and no multiplicative effect of the two factors (i.e., OA and oil/dispersant) was observed. T. pseudonana released more colloidal EPS (< 0.7 μm to > 3 kDa) in the presence of WAF/DCEWAF/OAWAF/OADCEWAF than in the corresponding Controls. Colloidal EPS and particulate EPS in the oil/dispersant treatments have higher protein-to-carbohydrate ratios than those in the control treatments, and thus are likely stickier and have a greater potential to form aggregates of marine oil snow. More TEP was produced in response to WAF than in Controls; OA did not influence its production. Polyaromatic hydrocarbon (PAH) concentrations and distributions were significantly impacted by the presence of dispersants but not OA. PAHs especially Phenanthrenes, Anthracenes, Chrysenes, Fluorenes, Fluoranthenes, Pyrenes, Dibenzothiophenes and 1-Methylphenanthrene show major variations in the aggregate and surrounding seawater fraction of oil and oil plus dispersant treatments. Studies like this add to the current knowledge of the combined effects of aggregation, marine snow formation, and the potential impacts of oil spills under ocean acidification scenarios.

The expanded footprint of the Deepwater Horizon oil spill in the Gulf of Mexico deep-sea benthos

The 2010 Deepwater Horizon blowout off the coast of Louisiana caused the largest marine oil spill on record. Samples were collected 2–3 months after the Macondo well was capped to assess damage to macrofauna and meiofauna communities. An earlier analysis of 58 stations demonstrated severe and moderate damage to an area of 148 km². An additional 58 archived stations have been analyzed to enhance the resolution of that assessment and determine if impacts occurred further afield. Impacts included high levels of total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAH) in the sediment, low diversity, low evenness, and low taxonomic richness of the infauna communities. High nematode to copepod ratios corroborated the severe disturbance of meiofauna communities. Additionally, barium levels near the wellhead were very high because of drilling activities prior to the accident. A principal component analysis (PCA) was used to summarize oil spill impacts at stations near the Macondo well, and the benthic footprint of the DWH oil spill was estimated using Empirical Bayesian Kriging (EBK) interpolation. An area of approximately 263 km² around the wellhead was affected, which is 78% higher than the original estimate. Particularly severe damages to benthic communities were found in an area of 58 km², which is 142% higher than the original estimate. The addition of the new stations extended the area of the benthic footprint map to about twice as large as originally thought and improved the resolution of the spatial interpolation. In the future, increasing the spatial extent of sampling should be a top priority for designing assessment studies.

The southern Gulf of Mexico: A baseline radiocarbon isoscape of surface sediments and isotopic excursions at depth

The southern Gulf of Mexico (sGoM) is home to an extensive oil recovery and development infrastructure. In addition, the basin harbors sites of submarine hydrocarbon seepage and receives terrestrial inputs from bordering rivers. We used stable carbon, nitrogen, and radiocarbon analyses of bulk sediment organic matter to define the current baseline isoscapes of surface sediments in the sGoM and determined which factors might influence them. These baseline surface isoscapes will be useful for accessing future environmental impacts. We also examined the region for influence of hydrocarbon deposition in the sedimentary record that might be associated with hydrocarbon recovery, spillage and seepage, as was found in the northern Gulf of Mexico (nGoM) following the Deepwater Horizon (DWH) oil spill in 2010. In 1979, the sGoM experienced a major oil spill, Ixtoc 1. Surface sediment δ13C values ranged from -22.4‰ to -19.9‰, while Δ14C values ranged from -337.1‰ to -69.2‰. Sediment δ15N values ranged from 2.8‰ to 7.2‰, while the %C on a carbonate-free basis ranged in value of 0.65% to 3.89% and %N ranged in value of 0.09% to 0.49%. Spatial trends for δ13C and Δ14C were driven by water depth and distance from the coastline, while spatial trends for δ15N were driven by location (latitude and longitude). Location and distance from the coastline were significantly correlated with %C and %N. At depth in two of twenty (10%) core profiles, we found negative δ13C and Δ14C excursions from baseline values in bulk sedimentary organic material, consistent with either oil-residue deposition or terrestrial inputs, but likely the latter. We then used 210Pb dating on those two profiles to determine the time in which the excursion-containing horizons were deposited. Despite the large spill in 1979, no evidence of hydrocarbon residue remained in the sediments from this specific time period.

Oil-mineral flocculation and settling velocity in saline water

Cohesive particles in aquatic systems can play an important role in determining the fate of spilled oil via the generation of Oil-Mineral Aggregates (OMAs). Series of laboratory experiments have been conducted aiming at filling the knowledge gap regarding how cohesive clay particles influence the accumulation of petroleum through forming different aggregate structures and their resulting settling velocity. OMAs have been successfully created in a stirring jar with artificial sea-water, crude oil and two types of most common cohesive minerals, Kaolinite and Bentonite clay. With the magnetic stirrer adjusted to 490 rpm to provide a high level homogeneous flow turbulence (Turbulence dissipation ε estimated to be about 0.02 m²⋅s^-3), droplet OMAs and flake/solid OMAs were obtained in oil-Kaolinite sample and oil-Bentonite sample, respectively. Kaolinite clay with relatively low flocculation rate (R_f = 0.13 min^-1) tends to physically attach around the surface of oil droplets. With the lower density of oil, these oil-Kaolinite droplet OMAs generally show lower settling velocity comparing to pure mineral Kaolinite flocs. Differently, Bentonite clay with higher flocculation rate (R_f = 0.66 min^-1) produces more porous flocs that can absorb or be absorbed by the oil and form compact flake/solid OMAs with higher density and settling velocity than pure Bentonite flocs. In the mixed Kaolinite-Bentonite sample (1:1 in weight), oil is observed to preferably interacting with Bentonite and increase settling velocity especially in larger floc size classes.

Changes in Reef Fish Community Structure Following the Deepwater Horizon Oil Spill

Large-scale anthropogenic disturbances can have direct and indirect effects on marine communities, with direct effects often taking the form of widespread injury or mortality and indirect effects manifesting as changes in food web structure. Here, we report a time series that captures both direct and indirect effects of the Deepwater Horizon Oil Spill (DWH) on northern Gulf of Mexico (nGoM) reef fish communities. We observed significant changes in community structure immediately following the DWH, with a 38% decline in species richness and 26% decline in Shannon-Weiner diversity. Initial shifts were driven by widespread declines across a range of trophic guilds, with subsequent recovery unevenly distributed among guilds and taxa. For example, densities of small demersal invertivores, small demersal browsers, generalist carnivores, and piscivores remained persistently low with little indication of recovery seven years after the DWH. Initial declines among these guilds occurred prior to the arrival of the now-widespread, invasive lionfish (Pterois spp.), but their lack of recovery suggests lionfish predation may be affecting recovery. Factors affecting persistently low densities of generalist carnivores and piscivores are not well understood but warrant further study given the myriad ecosystem services provided by nGoM reef fishes.

Bacteria forming drag-increasing streamers on a drop implicates complementary fates of rising deep-sea oil droplets

Competing time scales involved in rapid rising micro-droplets in comparison to substantially slower biodegradation processes at oil-water interfaces highlights a perplexing question: how do biotic processes occur and alter the fates of oil micro-droplets (<500 μm) in the 400 m thick Deepwater Horizon deep-sea plume? For instance, a 200 μm droplet traverses the plume in ~48 h, while known biodegradation processes require weeks to complete. Using a microfluidic platform allowing microcosm observations of a droplet passing through a bacterial suspension at ecologically relevant length and time scales, we discover that within minutes bacteria attach onto an oil droplet and extrude polymeric streamers that rapidly bundle into an elongated aggregate, drastically increasing drag that consequently slows droplet rising velocity. Results provide a key mechanism bridging competing scales and establish a potential pathway to biodegradation and sedimentations as well as substantially alter physical transport of droplets during a deep-sea oil spill with dispersant.

Deep-sea versus shallow conditions: a comparative ecobarotoxicological study

In the context of new oil exploration/production areas, knowledge of the biological impact of dispersed oil in the deep-sea environment is essential. Hence, the aim of this study was to perform a comparison, at atmospheric pressure (0.1 MPa) and at a high hydrostatic pressure corresponding to 1000 m depth (10.1 MPa), of lethal concentrations (LC) on a model fish, Scophthalmus maximus, exposed to chemically dispersed oil. Fish were exposed concomitantly at 0.1 and 10.1 MPa using two exposure tanks connected to the same source tank thanks to a closed circuit. Acute toxicity was evaluated at 24 h through the determination of LC₁₀ and LC₅₀ (respectively, 10 and 50% of mortality) calculated from measured total petroleum hydrocarbon concentrations in the water. No statistical differences were observed between the LC₁₀ at 0.1 MPa (46.1 mg L^- 1) and the LC₁₀ at 10.1 MPa (31.0 mg L^- 1), whereas the LC₅₀ of fish exposed to 0.1 MPa (90.8 mg L^- 1) was significantly higher than the LC₅₀ at 10.1 MPa (50.9 mg L^- 1). These results clearly show an increase in oil toxicity under high hydrostatic pressure. This effect may be due to synergistic effects of pressure and oil contamination on fish energetic metabolism.

A Mesocosm Assessment of the Effect of Bioturbation by the Ghost Shrimp (Lepidophthalmus louisianensis) on the Fate of Petroleum Hydrocarbons in the Intertidal Zone

To fully assess the long-term impacts of oil spills like the 2010 Deepwater Horizon incident in the northern Gulf of Mexico, the potential for organisms other than microbes to affect the fate and distribution of the oil may have to be considered. This influence could be substantial for abundant bioturbating benthic animals like the ghost shrimp Lepidophthalmus louisianensis. An assessment of the influence of these ghost shrimp on petroleum hydrocarbons was conducted in laboratory micro- and mesocosms containing coastal Gulf of Mexico sediment, seawater, and oil or the polynuclear aromatic hydrocarbon (PAH) pyrene. In an experiment with pyrene added to the water column, the ghost shrimp presence lowered water-column pyrene concentrations. In an experiment with oil added to the sediment surface, the ghost shrimp presence decreased PAH concentrations in the sediment surface layer but increased these in the water column and subsurface sediment. A companion study and a mass-balance analysis indicated a net loss of PAHs through an enhancement of microbial degradation. In an experiment in which oil was added as a narrow subsurface layer in the sediment, the ghost shrimp presence appeared to broaden the oil's depth distribution. Taken together, these results demonstrate that ghost shrimp can significantly influence the biodegradation and distribution of spilled oil in coastal ecosystems. Environ Toxicol Chem 2020;39:637-647. © 2019 SETAC.

Effect of Toluene and Hexane Sorption on the Rheology and Interfacial Properties of Lecithin-Based Emulsion Gels

A novel sorbent material consisting of a gel made from canola oil and water, emulsified with lecithin, was used to remove two model solvents from water. Sorption capacity was quantified through small-scale batch experiments. The structure and the mechanical properties of the gel were compared with and without added solvent to assess their cohesiveness upon removing contaminants from water. Confocal microscopy showed that the initial gel consisted of water droplets clustered in a canola oil continuous phase. The G' of the gels increased with solvent absorption to a maximum at 33% (v/v) hexane or 24% (v/v) toluene. Larger absorbed volumes led to decreases in G' of the gel. G' for solvent mixtures of 50% toluene and 50% hexane was intermediate between G' measured for the same volumes of pure solvents. Confocal microscopy suggests that the decrease of G' upon addition of large solvent volumes was due to a simple dilution effect. It is hypothesized that the initial increase in storage modulus was caused by changes in the structure of the lecithin films formed at the oil-water interfaces. This hypothesis was evaluated through measurements of interfacial tension, visualization of the interface with optical microscopy, force measurements of a single droplet under compression using a cantilevered-capillary force apparatus, compressional isotherm measurements conducted using a Langmuir trough. The cantilevered-capillary force apparatus and Langmuir trough experiments demonstrated that lecithin films at the canola oil-water interface were rigidified by toluene and hexane addition.

A ribosomal sequence-based oil sensitivity index for phytoplankton groups

Species-level variability has made it difficult to determine the relative sensitivity of phytoplankton to oil and mixtures of oil and dispersant. Here we develop a phytoplankton group sensitivity index using ribosome sequence data that we apply to a mesocosm experiment in which a natural microbial community was exposed to oil and two oil-dispersant mixtures. The relative sensitivity of four phytoplankton taxonomic groups, diatoms, dinoflagellates, green algae, and Chrysophytes, was computed using the log of the ratio of the number of species that increase to the number that decrease in relative abundance in the treatment relative to the control. The index indicates that dinoflagellates are the most sensitive group to oil and oil-dispersant treatments while the Chrysophytes benefit under oil exposure compared to the other groups examined. The phytoplankton group sensitivity index can be generally applied to quantify and rank the relative sensitivity of diverse microbial groups to environmental conditions and pollutants.

The interplay of extracellular polymeric substances and oil/Corexit to affect the petroleum incorporation into sinking marine oil snow in four mesocosms

Large amounts of oil containing mucous-like marine snow formed in surface waters adjacent to the Deepwater Horizon spill that was implicated in oil delivery to the seafloor. However, whether chemical dispersants that were used increased or decreased the oil incorporation and sedimentation efficiency, and how exopolymeric substances (EPS) are involved in this process remains unresolved. To investigate the microbial responses to oil and dispersants in different oceanic settings, indicated by EPS production, petro- and non-petro carbon sedimentation, four mesocosm (M) experiments were conducted: 1) nearshore seawater with a natural microbial consortia (M2); 2) offshore seawater with f/20 nutrients (M3); 3) coastal seawater with f/20 nutrients (M4); 4) nearshore seawater with a natural microbial consortia for a longer duration (M5). Four treatments were conducted in M2, M3 and M4 whereas only three in M5: 1) a water accommodated fraction of oil (WAF), 2) a chemically-enhanced WAF prepared with Corexit (CEWAF, not in M5), 3) a 10-fold diluted CEWAF (DCEWAF); and 4) controls. Overall, oil and dispersants input, nutrient and microbial biomass addition enhanced EPS production. Dispersant addition tended to induce the production of EPS with higher protein/carbohydrate (P/C) ratios, irrespective of oceanic regions. EPS produced in M4 was generally more hydrophobic than that produced in M3. The P/C ratio of EPS in both the aggregate and the colloidal fraction was a key factor that regulated oil contribution to sinking aggregates, based on the close correlation with %petro-carbon in these fractions. In the short term (4-5 days), both the petro and non-petro carbon sedimentation efficiencies showed decreasing trends when oil/dispersants were present. In comparison, in the longer-term (16 days), petro-carbon sedimentation efficiency was less influenced by dispersants, possibly due to biological and physicochemical changes of the components of the oil-EPS-mineral phase system, which cooperatively controlled the sinking velocities of the aggregates.

A new ecology-on-a-chip microfluidic platform to study interactions of microbes with a rising oil droplet

Advances in microfluidics technology has enabled many discoveries on microbial mechanisms and phenotypes owing to its exquisite controls over biological and chemical environments. However, emulating accurate ecologically relevant flow environments (e.g. microbes around a rising oil droplet) in microfluidics remains challenging. Here, we present a microfluidic platform, i.e. ecology-on-a-chip (eChip), that simulates environmental conditions around an oil droplet rising through ocean water as commonly occurred during a deep-sea oil spill or a natural seep, and enables detailed observations of microbe-oil interactions at scales relevant to marine ecology (i.e. spatial scales of individual bacterium in a dense suspension and temporal scales from milliseconds to weeks or months). Owing to the unique capabilities, we present unprecedented observations of polymeric microbial aggregates formed on rising oil droplets and their associated hydrodynamic impacts including flow fields and momentum budgets. Using the platform with Pseudomonas, Marinobacter, and Alcarnivorax, we have shown that polymeric aggregates formed by them present significant differences in morphology, growth rates, and hydrodynamic impacts. This platform enables us to investigate unexplored array of microbial interactions with oil drops.

Enhanced effectiveness of oil dispersants in destabilizing water-in-oil emulsions

Oil impacting the northern Gulf of Mexico shoreline from the 2010 Deepwater Horizon accident was predominantly in the form of water-in-oil emulsions (WOE), a chemically weathered, highly viscous, neutrally buoyant material. Once formed, WOE are extremely difficult to destabilize. Commercially-available oil dispersants are largely ineffective de-emulsifiers as a result of the inability of dispersant surfactants to displace asphaltenes stabilizing the oil-water interface. This study investigated the effectiveness of the commercially-available dispersant Corexit 9500A, modified to enhance its polar fraction, in destabilizing WOE. Results suggest that Corexit modified to include between 20-60% fractional amount of either polar additive (1-octanol or hexylamine) will produce a modest increase in WOE instability, with a Corexit to hexylamine ratio of approximately 80/20 providing the most effective enhanced destabilization. Results support the hypothesis that modifying the fraction of polar constituents in commercial dispersants will increase asphaltene solubility, decrease oil-water interface stability, and enhance WOE instability.

Oiling of the continental shelf and coastal marshes over eight years after the 2010 Deepwater Horizon oil spill

We measured the temporal and spatial trajectory of oiling from the April, 2010, Deepwater Horizon oil spill in water from Louisiana's continental shelf, the estuarine waters of Barataria Bay, and in coastal marsh sediments. The concentrations of 28 target alkanes and 43 target polycyclic aromatic hydrocarbons were determined in water samples collected on 10 offshore cruises, in 19 water samples collected monthly one km offshore at 13 inshore stations in 2010 and 2013, and in 16-60 surficial marsh sediment samples collected on each of 26 trips. The concentration of total aromatics in offshore waters peaked in late summer, 2010, at 100 times above the May, 2010 values, which were already slightly contaminated. There were no differences in surface or bottom water samples. The concentration of total aromatics declined at a rate of 73% y^-1 to 1/1000th of the May 2010 values by summer 2016. The concentrations inside the estuary were proportional to those one km offshore, but were 10-30% lower. The oil concentrations in sediments were initially different at 1 and 10 m distance into the marsh, but became equal after 2 years. Thus, the distinction between oiled and unoiled sites became blurred, if not non-existent then, and oiling had spread over an area wider than was visible initially. The concentrations of oil in sediments were 100-1000 times above the May 2010 values, and dropped to 10 times higher after 8 years, thereafter, demonstrating a long-term contamination by oil or oil residues that will remain for decades. The chemical signature of the oil residues offshore compared to in the marsh reflects the more aerobic offshore conditions and water-soluble tendencies of the dissolved components, whereas the anaerobic marsh sediments will retain the heavier molecular components for a long time, and have a consequential effect on the ecosystems.

Effect of spatial origin and hydrocarbon composition on bacterial consortia community structure and hydrocarbon biodegradation rates

Oil reserves in deep-sea sediments are currently subject to intense exploration, with associated risks of oil spills. Previous research suggests that microbial communities from deep-sea sediment (>1000m) can degrade hydrocarbons (HCs), but have a lower degradation ability than shallow (<200m) communities, probably due to in situ temperature. This study aimed to assess the effect of marine origin on microbial HC degradation potential while separating the influence of temperature, and to characterise associated HC-degrading bacterial communities. Microbial communities from 135 and 1000 m deep sediments were selectively enriched on crude oil at in situ temperatures and both consortia were subsequently incubated for 42 days at 20°C with two HC mixtures: diesel fuel or model oil. Significant HC biodegradation occurred rapidly in the presence of both consortia, especially of low molecular weight HCs and was concomitant with microbial community changes. Further, oil degradation was higher with the shallow consortium than with the deep one. Dominant HC-degrading bacteria differed based on both spatial origin of the consortia and supplemented HC types. This study provides evidence for influence of sediment spatial origin and HC composition on the selection and activity of marine HC-degrading bacterial communities and is relevant for future bioremediationdevelopments.

Combining electrokinetic transport and bioremediation for enhanced removal of crude oil from contaminated marine sediments: Results of a long-term, mesocosm-scale experiment

Marine sediments represent an important sink of harmful petroleum hydrocarbons after an accidental oil spill. Electrobioremediation techniques, which combine electrokinetic transport and biodegradation processes, represent an emerging technological platform for a sustainable remediation of contaminated sediments. Here, we describe the results of a long-term mesocosm-scale electrobioremediation experiment for the treatment of marine sediments contaminated by crude oil. A dimensionally stable anode and a stainless-steel mesh cathode were employed to drive seawater electrolysis at a fixed current density of 11 A/m². This approach allowed establishing conditions conducive to contaminants biodegradation, as confirmed by the enrichment of Alcanivorax borkumensis cells harboring the alkB-gene and other aerobic hydrocarbonoclastic bacteria. Oil chemistry analyses indicated that aromatic hydrocarbons were primarily removed from the sediment via electroosmosis and low molecular weight alkanes (nC₆ to nC₁₀) via biodegradation.

Mercury bioaccumulation in tilefish from the northeastern Gulf of Mexico 2 years after the Deepwater Horizon oil spill: Insights from Hg, C, N and S stable isotopes

Mercury (Hg) concentration in fish of the Gulf of the Mexico (GoM) is a major concern due to the importance of the GoM for U.S. fisheries. The Deepwater Horizon (DWH) oil spill in April 2010 in the northern GoM resulted in large amounts of oil and dispersant released to the water column, which potentially modified Hg bioaccumulation patterns in affected areas. We measured Hg species (methylmercury (MMHg) and inorganic Hg (IHg)) concentrations, and light (C, N and S) and Hg stable isotopes in muscle and liver tissues from tilefish (Lopholatilus chamaleonticeps) sampled in 2012 and 2013 along the shelf break of the northeastern GoM. Fish located close to the mouth of the Mississippi River (MR) and northwest of the DWH well-head (47 km) showed significantly lower Hg levels in muscle and liver than fish located further northeast of the DWH (>109 km), where 98% of tilefish had Hg levels in the muscle above US consumption advisory thresholds (50% for tilefish close to the DWH). Differences in light and Hg stable isotopes signatures were observed between these two areas, showing higher δ¹⁵N, and lower δ²⁰²Hg, Δ¹⁹⁹Hg and δ³⁴S in fish close to the DWH/MR. This suggests that suspended particles from the MR reduces Hg bioavailability at the base of the GoM food chains. This phenomenon can be locally enhanced by the DWH that resulted in increased particles in the water column as evidenced by the marine snow layer in the sediments. On the other hand, freshly deposited Hg associated with organic matter in more oligotrophic marine waters enhanced Hg bioaccumulation in local food webs. Comparing Hg isotopic composition in liver and muscle of fish indicates specific metabolic response in fish having accumulated high levels of MMHg.

Rapid Degradation of Oil in Mesocosm Simulations of Marine Oil Snow Events

Following the Deepwater Horizon oil spill in the Gulf of Mexico, natural marine snow interacted with oil and dispersants forming marine oil snow (MOS) that sank from the water column to sediments. Mesocosm simulations demonstrate that Macondo surrogate oil incorporates into MOS and can be isolated, extracted, and analyzed via Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Up to 47% of the FTICR-MS signal from MOS extracts can be attributed to formulas also found in Macondo surrogate oil demonstrating extensive oil incorporation. Additionally, oxygenation patterns for MOS extracts provide evidence for degraded oil compounds. Formulas having similar double bond equivalents but higher oxygen content (MOS CHO: CHO_2-9, DBE_2-16, MOS CHON: CHO_0-7N₁, DBE_9-18; Macondo CHO: CHO_1-4, DBE_2-15, CHON: CHO_0-3N₁, DBE_9-21) were found in MOS extracts generating isoabundance distributions similar to those of environmentally aged oil. Such shifts in molecular composition are consistent with the transformation of high DBE oil components, unobservable by FTICR-MS until oxygenation in the mesocosms. Low light conditions and the rapid proliferation of hydrocarbon-degraders observed in parallel studies suggest biological activity as the primary cause of oil degradation. MOS may thus represent an important microenvironment for oil degradation especially during its long transit below the euphotic zone to sediments.

Ecotoxicological benthic impacts of experimental oil-contaminated marine snow deposition

Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) can pose serious threats to the marine benthic ecosystem as it results in a deposition of oil contaminated marine snow on the sediment surface. In a microcosm experiment we investigated the effects of oil in combination with artificial marine snow or kaolin clay on two benthic invertebrate species and benthic meiofauna. The amphipod showed a dose-dependent decrease in survival for both oil-contaminated clay and oil-contaminated marine snow. The gastropod was only affected by the highest concentration of oil-contaminated marine snow and had internal concentrations of PAHs with a similar distribution as oil-contaminated marine snow. Benthic copepods showed higher survival in presence of marine snow. This study revealed that marine snow on the sediment after oil spills affects organisms in a trait-dependent way and that it can be a vector for introducing oil into the food web.

Fate of particle-bound polycyclic aromatic hydrocarbons in the river-influenced continental margin of the northern Gulf of Mexico

This study utilizes suspended particles and seafloor sediments collected from the northern Gulf of Mexico (GOM) continental margin to study the fate, transport, residence times and accumulation rates of particle-bound polycyclic aromatic hydrocarbons (PAHs). Total particulate-PAHs and particulate organic carbon (POC) varied between 0.9 and 7.0 ng/L, and 4-131 μg/L, respectively. Particulate-PAHs were positively correlated with POC, while both particulate-PAHs and POC were negatively correlated with salinity (P-value < 0.05). These results show that the river-derived particle influx and associated POC are important vectors for transport and fate of particulate-PAHs in the river-dominated northern GOM continental ecosystems. The composition of underlying seafloor sediment-PAHs were not correlated to the water column particulate-PAHs, which is attributed to re-mineralization, sediment resuspension/redistribution and different timescales of comparison. The ²¹⁰Pb-derived residence time of particles and associated particulate-PAHs in water column varied between 2 and 39 days. Residence times of particulate-PAHs were significantly correlated with seafloor sediment-PAHs accumulation rates, shorter water column residence times leading to higher PAHs accumulation rates.

Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico

Hydrocarbons released during the Deepwater Horizon (DWH) oil spill weathered due to exposure to oxygen, light, and microbes. During weathering, the hydrocarbons' reactivity and lability was altered, but it remained identifiable as "petrocarbon" due to its retention of the distinctive isotope signatures (14C and 13C) of petroleum. Relative to the initial estimates of the quantity of oil-residue deposited in Gulf sediments based on 2010-2011 data, the overall coverage and quantity of the fossil carbon on the seafloor has been attenuated. To analyze recovery of oil contaminated deep-sea sediments in the northern Gulf of Mexico we tracked the carbon isotopic composition (13C and 14C, radiocarbon) of bulk sedimentary organic carbon through time at 4 sites. Using ramped pyrolysis/oxidation, we determined the thermochemical stability of sediment organic matter at 5 sites, two of these in time series. There were clear differences between crude oil (which decomposed at a lower temperature during ramped oxidation), natural hydrocarbon seep sediment (decomposing at a higher temperature; Δ14C = -912‰) and our control site (decomposing at a moderate temperature; Δ14C = -189‰), in both the stability (ability to withstand ramped temperatures in oxic conditions) and carbon isotope signatures. We observed recovery toward our control site bulk Δ14C composition at sites further from the wellhead in ~4 years, whereas sites in closer proximity had longer recovery times. The thermographs also indicated temporal changes in the composition of contaminated sediment, with shifts towards higher temperature CO2 evolution over time at a site near the wellhead, and loss of higher temperature CO2 peaks at a more distant site.

Anaerobic degradation of hexadecane and phenanthrene coupled to sulfate reduction by enriched consortia from northern Gulf of Mexico seafloor sediment

To advance understanding of the fate of hydrocarbons released from the Deepwater Horizon oil spill and deposited in marine sediments, this study characterized the microbial populations capable of anaerobic hydrocarbon degradation coupled with sulfate reduction in non-seep sediments of the northern Gulf of Mexico. Anaerobic, sediment-free enrichment cultures were obtained with either hexadecane or phenanthrene as sole carbon source and sulfate as a terminal electron acceptor. Phylogenetic analysis revealed that enriched microbial populations differed by hydrocarbon substrate, with abundant SSU rRNA gene amplicon sequences from hexadecane cultures showing high sequence identity (up to 98%) to Desulfatibacillum alkenivorans (family Desulfobacteraceae), while phenanthrene-enriched populations were most closely related to Desulfatiglans spp. (up to 95% sequence identity; family Desulfarculaceae). Assuming complete oxidation to CO₂, observed stoichiometric ratios closely resembled the theoretical ratios of 12.25:1 for hexadecane and 8.25:1 for phenanthrene degradation coupled to sulfate reduction. Phenanthrene carboxylic acid was detected in the phenanthrene-degrading enrichment cultures, providing evidence to indicate carboxylation as an activation mechanism for phenanthrene degradation. Metagenome-assembled genomes (MAGs) revealed that phenanthrene degradation is likely mediated by novel genera or families of sulfate-reducing bacteria along with their fermentative syntrophic partners, and candidate genes linked to the degradation of aromatic hydrocarbons were detected for future study.

Response of natural phytoplankton communities exposed to crude oil and chemical dispersants during a mesocosm experiment

During the 2010 Deepwater Horizon oil spill, the chemical dispersant Corexit was applied over vast areas of the Gulf of Mexico. Marine phytoplankton play a key role in aggregate formation through the production of extracellular polymeric materials (EPS), an important step in the biological carbon pump. This study examined the impacts of oil and dispersants on the composition and physiology of natural marine phytoplankton communities from the Gulf of Mexico during a 72-hour mesocosm experiment and consequences to carbon export. The communities were treated using the water accommodated fraction (WAF) of oil, which was produced by adding Macondo surrogate oil to natural seawater and mixed for 24 h in the dark. A chemically enhanced WAF (CEWAF) was made in a similar manner, but using a mixture of oil and the dispersant Corexit in a 20:1 ratio as well as a diluted CEWAF (DCEWAF). Phytoplankton communities exposed to WAF showed no significant changes in PSII quantum yield (F_v/F_m) or electron transfer rates (ETR_max) compared to Control communities. In contrast, both F_v/F_m and ETR_max declined rapidly in communities treated with either CEWAF or DCEWAF. Analysis of other photophysiological parameters showed that photosystem II (PSII) antenna size and PSII connectivity factor were not altered by exposure to DCEWAF, suggesting that processes downstream of PSII were affected. The eukaryote community composition in each experimental tank was characterized at the end of the 72 h exposure time using 18S rRNA sequencing. Diatoms dominated the communities in both the control and WAF treatments (52 and 56% relative abundance respectively), while in CEWAF and DCEWAF treatments were dominated by heterotrophic Euglenozoa (51 and 84% respectively). Diatoms made up the largest relative contribution to the autotrophic eukaryote community in all treatments. EPS concentration was four times higher in CEWAF tanks compared to other treatments. Changes in particle size distributions (a proxy for aggregates) over time indicated that a higher degree of particle aggregation occurred in both the CEWAF and DCEWAF treatments than the WAF or Controls. Our results demonstrate that chemically dispersed oil has more negative impacts on photophysiology, phytoplankton community structure and aggregation dynamics than oil alone, with potential implications for export processes that affect the distribution and turnover of carbon and oil in the water column.

Effect of Deepwater Horizon Crude Oil Water Accommodated Fraction on Olfactory Function in the Atlantic Stingray, Hypanus sabinus

The Deepwater Horizon oil spill was the largest accidental marine oil spill in history, releasing nearly 5 million barrels of crude oil. Crude oil causes both lethal and sublethal effects on marine organisms, and sensory systems have the potential to be strongly affected. Marine fishes rely upon the effective functioning of their sensory systems for detection of prey, mates, and predators. However, despite the obvious importance of sensory systems, the impact of crude oil exposure upon sensory function remains largely unexplored. Here we show that olfactory organ responses to amino acids are significantly depressed in oil exposed stingrays. We found that the response magnitude of the electro-olfactogram (EOG) to 1 mM amino acids decreased by an average of 45.8% after 48 h of exposure to an oil concentration replicating that measured in coastal areas. Additionally, in oil exposed individuals, the EOG response onset was significantly slower, and the clearing time was protracted. This study is the first to employ an electrophysiological assay to demonstrate crude oil impairment of the olfactory system in a marine fish. We show that stingrays inhabiting an area impacted by an oil spill experience reduced olfactory function, which would detrimentally impact fitness, could lead to premature death, and could cause additional cascading effects through lower trophic levels.

Decadal Assessment of Polycyclic Aromatic Hydrocarbons in Mesopelagic Fishes from the Gulf of Mexico Reveals Exposure to Oil-Derived Sources

This study characterizes a decadal assessment of polycyclic aromatic hydrocarbons (PAHs) in the muscle tissues of mesopelagic fish species as indicators of the environmental health of the Gulf of Mexico (GoM) deep-pelagic ecosystem. Mesopelagic fishes were collected prior to the Deepwater Horizon (DWH) oil spill (2007), immediately post-spill (2010), 1 year after the spill (2011), and 5-6 years post-spill (2015-2016) to assess if the mesopelagic ecosystem was exposed to, and retained, PAH compounds from the DWH spill. Results indicated that a 7- to 10-fold increase in PAHs in fish muscle tissues occurred in 2010-2011 (4972 ± 1477 ng/g) compared to 2007 (630 ± 236 ng/g). In 2015-2016, PAH concentrations decreased close to the levels measured in 2007 samples (827 ± 138 ng/g); however, the composition of PAHs still resembles a petrogenic source similar to samples collected in 2010-2011. PAH composition in muscle samples indicated that natural sources (e.g., Mississippi River and natural seeps) or spatial variability within the GoM do not explain the temporal variability of PAHs observed from 2007 to 2016. Furthermore, analysis of different fish tissues indicated the dietary intake and maternal transfer of PAHs as the primary mechanisms for bioaccumulation in 2015-2016, explaining the elevated levels and composition of PAHs in ovarian eggs.

A critical review of marine snow in the context of oil spills and oil spill dispersant treatment with focus on the Deepwater Horizon oil spill

Natural marine snow (NMS) is defined as the "shower" of particle aggregates formed by processes that occur in the world's oceans, consisting of macroscopic aggregates of detritus, living organisms and inorganic matter. Recent studies from the Deepwater Horizon oil spill suggest that marine snow is also formed in association with oil spills and was an important factor for the transport of oil to the seabed. This review summarizes the research and literature on MS, mainly from the DWH oil spill, with a focus on the relation between the use of oil spill dispersants and the formation and fate of oil-related marine snow (ORMS). Studies are still required to determine ORMS processes at oil concentrations as relevant as possible for chemically dispersed oil.

Hydrocarbon-degrading bacteria in deep-water subarctic sediments (Faroe-Shetland Channel)

AIMS

The aim of this study was the baseline description of oil-degrading sediment bacteria along a depth transect in the Faroe-Shetland Channel (FSC) and the identification of biomarker taxa for the detection of oil contamination in FSC sediments.

METHODS AND RESULTS

Oil-degrading sediment bacteria from 135, 500 and 1000 m were enriched in cultures with crude oil as the sole carbon source (at 12, 5 and 0°C respectively). The enriched communities were studied using culture-dependent and culture-independent (clone libraries) techniques. Isolated bacterial strains were tested for hydrocarbon degradation capability. Bacterial isolates included well-known oil-degrading taxa and several that are reported in that capacity for the first time (Sulfitobacter, Ahrensia, Belliella, Chryseobacterium). The orders Oceanospirillales and Alteromonadales dominated clone libraries in all stations but significant differences occurred at genus level particularly between the shallow and the deep, cold-water stations. Alcanivorax constituted 64% of clones at FSC135 but was absent at deeper stations. Pseudoalteromonas and Oleispira dominated the bacterial community at 500 and 1000 m.

CONCLUSIONS

The genus Oleispira emerged as a major player in the early stages of crude oil degradation in deep-sea sediments of the FSC particularly at subzero temperatures. This finding is offering a direction for future research into biomonitoring tools for the detection of low levels of crude oil contamination in the deep FSC, and possibly high latitude cold waters in general.

SIGNIFICANCE AND IMPACT OF THE STUDY

Oil and gas exploration in the FSC occurs at depths >1000 m but baseline environmental data necessary for the assessment of ecosystem recovery to prespill conditions in the event of an oil spill are lacking. This study will contribute to our ability to assess the impact of oil release in the FSC and guide the direction of bioremediation strategies tailored to the area.

Hydrocarbon-degradation and MOS-formation capabilities of the dominant bacteria enriched in sea surface oil slicks during the Deepwater Horizon oil spill

A distinctive feature of the Deepwater Horizon (DwH) oil spill was the formation of significant quantities of marine oil snow (MOS), for which the mechanism(s) underlying its formation remain unresolved. Here, we show that Alteromonas strain TK-46(2), Pseudoalteromonas strain TK-105 and Cycloclasticus TK-8 - organisms that became enriched in sea surface oil slicks during the spill - contributed to the formation of MOS and/or dispersion of the oil. In roller-bottle incubations, Alteromonas cells and their produced EPS yielded MOS, whereas Pseudoalteromonas and Cycloclasticus did not. Interestingly, the Cycloclasticus strain was able to degrade n-alkanes concomitantly with aromatics within the complex oil mixture, which is atypical for members of this genus. Our findings, for the first time, provide direct evidence on the hydrocarbon-degrading capabilities for these bacteria enriched during the DwH spill, and that bacterial cells of certain species and their produced EPS played a direct role in MOS formation.

Advective pore-water transport of hydrocarbons in North East Scotland coastal sands

Oil spills in the marine environment can cause ecosystem-level impacts. Dispersant application as an oil spill response measure leads to the widespread distribution of hydrocarbons in the water column and marine sediments. The North Sea is an area of intense hydrocarbon production and is at risk of oil spills, which are of concern to its benthic ecosystem due to its sediments' high permeability. Here, entrainment of hydrocarbons via pore-water advection into permeable North Sea sands and the associated effect of Superdispersant-25, a commercial oil dispersant, were evaluated in a laboratory. Centrally stirred chambers that induce advective pore-water fluxes in sediments were filled with sediment, seawater and mixtures of oil and Superdispersant-25. Dispersant application had contrasting effects on hydrocarbon interactions with sediment: (1) it reduced accumulation of hydrocarbons in surface sediments and (2) facilitated the entrainment of hydrocarbons up to 8 cm deep into sediments by increasing hydrocarbon solubility in seawater and its subsequent washout or pumping into sediment by pore-water movement. Results here show that dispersant application can have counter-intuitive effects on hydrocarbon interactions with marine sediments and highlight the need for further research in this area to make better informed decision in an oil spill response scenario.

The influence of pressure on crude oil biodegradation in shallow and deep Gulf of Mexico sediments

A significant portion of oil released during the Deepwater Horizon disaster reached the Gulf of Mexico (GOM) seafloor. Predicting the long-term fate of this oil is hindered by a lack of data about the combined influences of pressure, temperature, and sediment composition on microbial hydrocarbon remineralization in deep-sea sediments. To investigate crude oil biodegradation by native GOM microbial communities, we incubated core-top sediments from 13 GOM sites at water depths from 60–1500 m with crude oil under simulated aerobic seafloor conditions. Biodegradation occurred in all samples and followed a predictable compound class sequence dictated by molecular weight and structure. 45 to ~100% of total n-alkane and 3 to 60% of total polycyclic aromatic hydrocarbons (PAH) were depleted. In reactors incubated at 4°C and at pressures of 6–15 MPa, the depletion in total n-alkane was inversely correlated to pressure (R² ~ 0.85), equivalent to a 4% decrease in total n-alkane depletion for every 1 MPa increase. Our results indicated a modest inhibitory effect of pressure on biodegradation over our experimental range. However, the expansion of oil exploration to deeper waters (e.g., 5000 m) opens the risk of spills at conditions at which pressure might have a more pronounced effect.

The impact of the Deepwater Horizon blowout on historic shipwreck-associated sediment microbiomes in the northern Gulf of Mexico

More than 2,000 historic shipwrecks spanning 500 years of history, rest on the Gulf of Mexico seafloor. Shipwrecks serve as artificial reefs and hotspots of biodiversity by providing hard substrate, something rare in deep ocean regions. The Deepwater Horizon (DWH) spill discharged crude oil into the deep Gulf. Because of physical, biological, and chemical interactions, DWH oil was deposited on the seafloor, where historic shipwrecks are present. This study examined sediment microbiomes at seven historic shipwrecks. Steel-hulled, World War II-era shipwrecks and wooden-hulled, 19^th century shipwrecks within and outside of the surface oiled area and subsurface plume were examined. Analysis of 16S rRNA sequence libraries, sediment radiocarbon age data, sedimentation rates, and hydrocarbons revealed that the German U-boat U-166 and the wooden-hulled sailing vessel known as the Mardi Gras Wreck, both in the Mississippi Canyon leasing area, were exposed to deposited oil during a rapid sedimentation event. Impacts to shipwreck microbiomes included a significant increase in Piscirickettsiaceae-related sequences in surface sediments, and reduced biodiversity relative to unimpacted sites. This study is the first to address the impact of the spill on shipwreck-associated microbiomes, and to explore how shipwrecks themselves influence microbiome diversity in the deep sea.

Hydrocarbon degradation and response of seafloor sediment bacterial community in the northern Gulf of Mexico to light Louisiana sweet crude oil

The Deepwater Horizon (DWH) blowout resulted in the deposition to the seafloor of up to 4.9% of 200 million gallons of oil released into the Gulf of Mexico. The petroleum hydrocarbon concentrations near the wellhead were high immediately after the spill, but returned to background levels a few years after the spill. Microbial communities in the seafloor are thought to be responsible for the degradation of hydrocarbons, however, our knowledge is primarily based upon gene diversity surveys and hydrocarbon concentration in field sediment samples. Here, we investigated the oil degradation potential and changes in bacterial community by amending seafloor sediment collected near the DWH site with crude oil and both oil and Corexit dispersant. Polycyclic aromatic hydrocarbons were rapidly degraded during the first 30 days of incubation, while alkanes were degraded more slowly. With the degradation of hydrocarbons, the relative abundances of Colwelliaceae, Alteromonadaceae, Methylococales, Alcanivorax, Bacteriovorax, and Phaeobacter increased remarkably. However, the abundances of oil-degrading bacteria changed with oil chemistry. Colwelliaceae decreased with increasing oil degradation, whereas Alcanivorax and Methylococcales increased considerably. We assembled seven genomes from the metagenome, including ones belonging to Colwellia, Alteromonadaceae, Rhodobacteraceae, the newly reported genus Woeseia, and candidate phylum NC10, all of which possess a repertoire of genes for hydrocarbon degradation. Moreover, genes related to hydrocarbon degradation were highly enriched in the oiled treatment, suggesting that the hydrocarbons were biodegraded, and that the indigenous microflora have a remarkable potential for the natural attenuation of spilled oil in the deep-sea surface sediment.

Characterization and flux of marine oil snow settling toward the seafloor in the northern Gulf of Mexico during the Deepwater Horizon incident: Evidence for input from surface oil and impact on shallow shelf sediments

Sediment trap samples from the shelf edge area (400-450m water depth), 58km northeast of the failed Macondo well, were collected before, during and after the Deepwater Horizon oil spill. Detailed chemical analyses of particulates revealed that fluxes of spill-derived TPH (2356μg/m²/day), total PAH (5.4μg/m²/day), and hopane (0.89μg/m²/day) settling to the seafloor directly beneath the surface-plume were 19- to 44-times higher during the active spill than pre- and post-spill background values. The oil was variably biodegraded, evaporated and photo-oxidized indicating that it derived from the sinking of surface oil. The hopane-based oil flux that we calculate (10bbl/km²) indicates that at least 76,000bbl of Macondo oil that reached the ocean surface subsequently sank over an area of approximately 7600km². We explore how this flux of sunken surface oil contributed to the total volume of oil deposited on the seafloor following the Deepwater Horizon incident.

Current status of deepwater oil spill modelling in the Faroe-Shetland Channel, Northeast Atlantic, and future challenges

As oil reserves in established basins become depleted, exploration and production moves towards relatively unexploited areas, such as deep waters off the continental shelf. The Faroe-Shetland Channel (FSC, NE Atlantic) and adjacent areas have been subject to increased focus by the oil industry. In addition to extreme depths, metocean conditions in this region characterise an environment with high waves and strong winds, strong currents, complex circulation patterns, sharp density gradients, and large small- and mesoscale variability. These conditions pose operational challenges to oil spill response and question the suitability of current oil spill modelling frameworks (oil spill models and their forcing data) to adequately simulate the behaviour of a potential oil spill in the area. This article reviews the state of knowledge relevant to deepwater oil spill modelling for the FSC area and identifies knowledge gaps and research priorities. Our analysis should be relevant to other areas of complex oceanography.