Impacts of dispersants on microbial communities and ecological systems.

Accidental oil spills can result in catastrophic ecological insults and therefore require rapid intervention to mitigate the potential impacts to aquatic ecosystems. One of the largest oil spills, known as the Deepwater Horizon oil spill, occurred in the Spring of 2010 near the coast of Louisiana (USA) due to an explosion during oil drilling activities. Millions of gallons of oil were released into the Gulf of Mexico, impacting thousands of ocean miles and coastal areas linked to the gulf. Among the actions taken during the remediation efforts was the unprecedented large use of Corexit dispersants, including at the subsurface to prevent oil from reaching the surface. While there is evidence that dispersants can accelerate the biodegradation of oil, reports on their potential toxicity to aquatic biota and to microbial functions have also been documented. In this review, we will examine the most recent literature on the impact of dispersants on microbial communities implicated in oil degradation and overall ecological networks. The primary focus will be on studies using Corexit but other dispersants will be discussed if data are available. We will share the literature gaps identified and discuss future work that is needed to reconcile some of the discrepancies found on the effectiveness of dispersants on oil degradation and their potential toxicity. KEY POINTS: • Chemical dispersants have been applied as a chemical response measure for oil spills. • The effects of chemical dispersants on microbial communities have been the subject of substantial research. • This work seeks to review recent work on the impact of chemical dispersants on oil biodegradation, microbial communities, and ecosystems.

Potential for commercial PlanetScope satellites in oil response monitoring.

Extraction of petroleum oil resources may result in oil spills in the aquatic environment. Active and passive satellites are generally limited in either spatial coverage, temporal revisit periods, or spatial resolution when tracking surface oil slicks. PlanetScope passive satellites are reported to have near daily global coverage at a resolution of 3.5 m at nadir. These satellites may complement monitoring and fill temporal gaps by leveraging sun glint caused by the nadir viewing angle. Here, we demonstrate potential for PlanetScope satellite usage by investigating overpass timing and sun glint intensity. The United States potential for use was greatest during summer solstice and at lower latitudes. When combined with other high-resolution active and passive satellites, PlanetScope coverage added an average of 86.3 days each year from January 2018 through December 2020, as demonstrated at the Mississippi Canyon Block 20 Saratoga Platform site in the Gulf of Mexico.

Fluorescence-estimated oil concentration (Foil) in the Deepwater Horizon subsea oil plume.

Tracking the subsea oil plume during the 2010 Deepwater Horizon Oil Spill (DWH) was conducted using in situ fluorescence via vertical profilers (n = 1157) and discrete sample chemical analyses (n = 7665). During monitoring efforts, discrete samples provided a coarse picture of the oil plume footprint, but the majority of the samples were below standard analytical detection limits for petroleum hydrocarbons. In situ fluorescence data improved the spatial and temporal resolution of the subsea oil plume characterization. Here we synthesized millions of continuous fluorescence data points from hundreds of contemporaneously discrete samples collected to demonstrate how fluorescence could serve as a proxy for Benzene-Toluene-Ethylbenzene-Xylene (BTEX) concentration. Data mined from Gulf Science Data repository were well correlated, and geographically and temporally aligned to provide direct comparisons. Described here are the methods used to calibrate the fluorescence data and to spatially approximate the three-dimensional geographic extent of the oil plume.

Analysis of emissions and residue from methods to improve efficiency of at-sea, in situ oil spill burns.

The combustion efficiency of simulated at-sea surface oil burns (in situ burns) was determined in a 63 m3 tank while testing varied boom configurations and air-assist nozzles in the presence and absence of waves. Combustion efficiencies of Alaska North Slope oil based on unburned carbon in the plume emissions ranged from 85% to 93% while values based on oil mass loss ranged from 89% to 99%. A four-fold variation in PM2.5 emission factors was observed from the test conditions. The most effective burns in terms of reduced emissions and post-burn residue concentration of total petroleum hydrocarbons were those that had high length to width boom ratios resulting in higher flame front surface area exposure to ambient air. The amount of oil mass lost was not related to any combustion efficiency parameters measured in the plume, representing a potential tradeoff between unburnt oil and air pollution.

Toxicity of oil spill response agents and crude oils to five aquatic test species.

The majority of aquatic toxicity data for petroleum products has been limited to a few intensively studied crude oils and Corexit chemical dispersants, and acute toxicity testing in two standard estuarine test species: mysids (Americamysis bahia) and inland silversides (Menidia beryllina). This study compared the toxicity of two chemical dispersants commonly stock piled for spill response (Corexit EC9500A®, Finasol®OSR 52), three less studied agents (Accell Clean®DWD dispersant; CytoSol® surface washing agent; Gelco200® solidifier), and three crude oils differing in hydrocarbon composition (Dorado, Endicott, Alaska North Slope). Consistent with listings on the U.S. National Contingency Plan Product Schedule, general rank order toxicity was greatest for dispersants and lowest for the solidifier. The results indicate that freshwater species can have similar sensitivity as the conventionally tested mysids and silversides, and that the sea urchin (Arbacia punctulata) appears to be a reasonable addition to increase taxa diversity in standardized oil agent testing.

Toxicity of Cold Lake Blend and Western Canadian Select dilbits to standard aquatic test species.

Dilbits are blends of bitumen and natural gas condensates or crude oils with only limited toxicity data. Two dilbits, Cold Lake Blend and Western Canadian Select, were tested as either unweathered or weathered oils for acute and chronic toxicity to standard freshwater and estuarine organisms. Water accommodated fractions of the dilbits were characterized for total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), and monoaromatics (BTEX). Acute toxicity of unweathered and weathered dilbits ranged from 4 to 16 mg/L TPH, 8 to 40 µg/L total PAHs, and 0.7 to 16 mg/L BTEX in Ceriodaphnia dubia, Pimephales promelas, Americamysis bahia, and Menidia beryllina. Concentrations of weathered dilbits causing impaired growth (A. bahia) and reproduction (C. dubia) ranged from 0.8 to 3.5 mg/L TPH and 6 to 16 µg/L PAHs. The two dilbits had generally similar acute and short term chronic toxicity expressed as TPH or total PAHs as other crude oils and other petroleum products.

Corexit 9500 Enhances Oil Biodegradation and Changes Active Bacterial Community Structure of Oil-Enriched Microcosms.

To better understand the impacts of Corexit 9500 on the structure and activity levels of hydrocarbon-degrading microbial communities, we analyzed next-generation 16S rRNA gene sequencing libraries of hydrocarbon enrichments grown at 5 and 25°C using both DNA and RNA extracts as the sequencing templates. Oil biodegradation patterns in both 5 and 25°C enrichments were consistent with those reported in the literature (i.e., aliphatics were degraded faster than aromatics). Slight increases in biodegradation were observed in the presence of Corexit at both temperatures. Differences in community structure were observed between treatment conditions in the DNA-based libraries. The 25°C consortia were dominated by Vibrio, Idiomarina, Marinobacter, Alcanivorax, and Thalassospira species, while the 5°C consortia were dominated by several species of the genera Flavobacterium, Alcanivorax, and Oleispira Most of these genera have been linked to hydrocarbon degradation and have been observed after oil spills. Colwellia and Cycloclasticus, known aromatic degraders, were also found in these enrichments. The addition of Corexit did not have an effect on the active bacterial community structure of the 5°C consortia, while at 25°C, a decrease in the relative abundance of Marinobacter was observed. At 25°C, Thalassospira, Marinobacter, and Idiomarina were present at higher relative abundances in the RNA than DNA libraries, suggesting that they were active in degradation. Similarly, Oleispira was greatly stimulated by the addition of oil at 5°C.IMPORTANCE While dispersants such as Corexit 9500 can be used to treat oil spills, there is still debate on the effectiveness on enhancing oil biodegradation and its potential toxic effect on oil-degrading microbial communities. The results of this study provide some insights on the microbial dynamics of hydrocarbon-degrading bacterial populations in the presence of Corexit 9500. Operational taxonomic unit (OTU) analyses indicated that several OTUs were inhibited by the addition of Corexit. Conversely, a number of OTUs were stimulated by the addition of the dispersant, many of which were identified as known hydrocarbon-degrading bacteria. The results highlight the value of using RNA-based methods to further understand the impact of dispersant on the overall activity of different hydrocarbon-degrading bacterial groups.

Characterization of emissions and residues from simulations of the Deepwater Horizon surface oil burns.

The surface oil burns conducted by the U.S. Coast Guard from April to July 2010 during the Deepwater Horizon disaster in the Gulf of Mexico were simulated by small scale burns to characterize the pollutants, determine emission factors, and gather particulate matter for subsequent toxicity testing. A representative crude oil was burned in ocean-salinity seawater, and emissions were collected from the plume by means of a crane-suspended sampling platform. Emissions included particulate matter, aromatic hydrocarbons, polychlorinated dibenzodioxins/dibenzofurans, elements, and others, the sum of which accounted for over 92% by mass of the combustion products. The unburned oil mass was 29% of the original crude oil mass, significantly higher than typically reported. Analysis of alkanes, elements, and PAHs in the floating residual oil and water accounted for over 51% of the gathered mass. These emission factors, along with toxicity data, will be important toward examining impacts of future spill burning operations.

Impact of mixing time and energy on the dispersion effectiveness and droplets size of oil.

The effects of mixing time and energy on Alaska Northern Slope (ANS) and diluted bitumen Cold Lake Blend (CLB) were investigated using EPA baffled flask test. Dispersion effectiveness and droplet size distribution were measured after 5-120 min. A modeling method to predict the mean droplet size was introduced for the first time to tentatively elucidate the droplet size breakup mechanism. The ANS dispersion effectiveness greatly increased with dispersant and mixing energy. However, little CLB dispersion was noted at small energy input (ε = 0.02 Watt/kg). With dispersant, the ANS droplet size distribution reached quasi-equilibrium within 10 min, but that of CLB seems to reach quasi-equilibrium after 120 min. Dispersants are assumed ineffective on high viscosity oils because dispersants do not penetrate them. We provide an alternative explanation based on the elongation time of the droplets and its residence in high intensity zones. When mixing energy is small, CLB did not disperse after 120 min, long enough to allow the surfactant penetration. Our findings suggest that dispersants may disperse high viscosity oils at a rougher sea state and a longer time. The latter could determine how far offshore one can intervene for effective responses to a high viscosity oil spill offshore.

Reynolds number scaling to predict droplet size distribution in dispersed and undispersed subsurface oil releases.

This study was aimed at testing the applicability of modified Weber number scaling with Alaska North Slope (ANS) crude oil, and developing a Reynolds number scaling approach for oil droplet size prediction for high viscosity oils. Dispersant to oil ratio and empirical coefficients were also quantified. Finally, a two-step Rosin-Rammler scheme was introduced for the determination of droplet size distribution. This new approach appeared more advantageous in avoiding the inconsistency in interfacial tension measurements, and consequently delivered concise droplet size prediction. Calculated and observed data correlated well based on Reynolds number scaling. The relation indicated that chemical dispersant played an important role in reducing the droplet size of ANS under different seasonal conditions. The proposed Reynolds number scaling and two-step Rosin-Rammler approaches provide a concise, reliable way to predict droplet size distribution, supporting decision making in chemical dispersant application during an offshore oil spill.

Effect of dispersants on the biodegradation of South Louisiana crude oil at 5 and 25 °C.

This article reports biodegradation rates for a commercial dispersant, JD-2000, South Louisiana crude oil (SLC) alone, and SLC dispersed with JD-2000 at 5 and 25 °C. Results from the biodegradation experiments revealed that Component X, a chemical marker for JD-2000, rapidly degraded at both temperatures. The application of JD-2000 decreased by half the overall biodegradation rate of aliphatic compounds at 25 °C. At 5 °C, a residual fraction consisting of iso- and n-alkanes (C29-C35) persisted after 56 d. The combination of dispersant and higher temperature resulted in faster removal rates for 2- and 3-ring polycyclic aromatic hydrocarbons. When compared with Corexit 9500, our results suggest that the chemistry of the surfactant (or surfactants) in JD-2000 might have favored oil dissolution (substrate transport to the aqueous phase) as an uptake mechanism over adhesion, which requires direct contact of the biomass with the oil.

Submersible optical sensors exposed to chemically dispersed crude oil: wave tank simulations for improved oil spill monitoring.

In situ fluorometers were deployed during the Deepwater Horizon (DWH) Gulf of Mexico oil spill to track the subsea oil plume. Uncertainties regarding instrument specifications and capabilities necessitated performance testing of sensors exposed to simulated, dispersed oil plumes. Dynamic ranges of the Chelsea Technologies Group AQUAtracka, Turner Designs Cyclops, Satlantic SUNA and WET Labs, Inc. ECO, exposed to fresh and artificially weathered crude oil, were determined. Sensors were standardized against known oil volumes and total petroleum hydrocarbons and benzene-toluene-ethylbenzene-xylene measurements-both collected during spills, providing oil estimates during wave tank dilution experiments. All sensors estimated oil concentrations down to 300 ppb oil, refuting previous reports. Sensor performance results assist interpretation of DWH oil spill data and formulating future protocols.