Combined effects of chemical dispersant and suspended minerals on the dispersion process of spilled oil.

The dispersion process of spilled oil is an important concern for the effective disposal of oil spills. The dispersed oil concentration and oil droplets size distribution were studied through a wave tank test under the application of chemical dispersant and suspended minerals. The results indicated that dispersant and minerals increased the dispersed oil concentration and the effect of dispersant was more significant, and they had a synergistic effect on oil dispersion. When dispersant and minerals were applied together, the volume mean diameter of oil droplets decreased in the first 30 min, then increased and reached a maximum value at 90-120 min, and decreased again. Moreover, suspended minerals could inhibit the coalescence of oil droplets. This study can afford data support for oil spill emergency response that occurs in inshore or estuaries.

Shrimp-waste based dispersant as oil spill treating agent: Biodegradation of dispersant and dispersed oil.

The emerging demand for the enhancement of biodegradation of persistent organic pollutants from marine oil spills using oil-treating agents to minimize the environmental impacts promotes the development of green dispersants. Shrimp waste is a potential raw material to generate green dispersants. The biodegradability of dispersed oil and dispersants themselves are key factors for the national consideration of the approval, stockpile, and usage of dispersants. However, it is unknown whether shrimp-waste-based dispersant (SWD) has high bioavailability or facilitates the biodegradation of dispersed oil. In this study, we tackled the biodegradation of oil dispersed by a purified SWD. Furthermore, the SWD biodegradability was evaluated by exploring the degradation genes via metagenomic sequencing, analyzing the enzymatic activities for dispersant biodegradation by molecular docking, and discussing the SWD toxicity. We discovered that the SWD facilitated the biodegradation of two crude oils (Alaska North Slope and Marine Fuel-No.6). The metagenomic analysis with molecular docking showed that fresh seawater had feasible enzymes to degrade the SWD to safety components. Additionally, the SWD was low toxic and high bioactive. The findings helped confirm that the purified SWD is an effective and eco-sustainable marine oil spill treating agent and tracked the biodegradation of dispersed oil and the SWD.

TRopical Oil Pollution Investigations in Coastal Systems [TROPICS]: A synopsis of impacts and recovery.

The TRopical Oil Pollution Investigations in Coastal Systems (TROPICS) experiment, conducted on the Caribbean coast of Panama, has become one of the most comprehensive field experiments examining the long-term impacts of oil and dispersed oil exposures in nearshore tropical marine environments. From the initial experiment through more than three decades of study and data collection visits, the intertidal and subtidal communities have exhibited significantly different impact and recovery regimes, depending on whether the sites were exposed to crude oil only or crude oil treated with a chemical dispersant. This review provides a synopsis of the original experiment and a cumulative summary of the results and observations, illustrating the environmental and ecosystem trade-offs of chemical dispersant use in mangrove, seagrass, and coral reef environments.

Dispersed Crude Oil Induces Dysbiosis in the Red Snapper Lutjanus campechanus External Microbiota.

The fish external microbiota competitively excludes primary pathogens and prevents the proliferation of opportunists. A shift from healthy microbiota composition, known as dysbiosis, may be triggered by environmental stressors and increases host susceptibility to disease. The Deepwater Horizon (DWH) oil spill was a significant stressor event in the Gulf of Mexico. Despite anecdotal reports of skin lesions on fishes following the oil spill, little information is available on the impact of dispersed oil on the fish external microbiota. In this study, juvenile red snapper (Lutjanus campechanus) were exposed to a chemically enhanced water-accommodated fraction (CEWAF) of Corexit 9500/DWH oil (CEWAF) and/or the bacterial pathogen Vibrio anguillarum in treatments designed to detect changes in and recovery of the external microbiota. In fish chronically exposed to CEWAF, immunoglobulin M (IgM) expression significantly decreased between 2 and 4 weeks of exposure, coinciding with elevated liver total polycyclic aromatic hydrocarbons (PAHs). Dysbiosis was detected on fish chronically exposed to CEWAF compared to seawater controls, and addition of a pathogen challenge altered the final microbiota composition. Dysbiosis was prevented by returning fish to clean seawater for 21 days after 1 week of CEWAF exposure. Four fish exhibited lesions during the trial, all of which were exposed to CEWAF but not all of which were exposed to V. anguillarum. This study indicates that month-long exposure to dispersed oil leads to dysbiosis in the external microbiota. As the microbiota is vital to host health, these effects should be considered when determining the total impacts of pollutants in aquatic ecosystems. IMPORTANCE Fish skin is an immunologically active tissue. It harbors a complex community of microorganisms vital to host homeostasis as, in healthy fish, they competitively exclude pathogens found in the surrounding aquatic environment. Crude oil exposure results in immunosuppression in marine animals, altering the relationship between the host and its microbial community. An alteration of the healthy microbiota, a condition known as dysbiosis, increases host susceptibility to pathogens. Despite reports of external lesions on fishes following the DWH oil spill and the importance of the external microbiota to fish health, there is little information on the effect of dispersed oil on the external microbiota of fishes. This research provides insight into the impact of a stressor event such as an oil spill on dysbiosis and enhances understanding of long-term sublethal effects of exposure to aid in regulatory decisions for protecting fish populations during recovery.

Impacts of Frazil Ice on the Effectiveness of Oil Dispersion and Migration of Dispersed Oil.

Oil spills in the Arctic have drawn dramatic attention in recent years. Frazil ice, as the essential formation of sea ice, may affect the effectiveness of dispersants during oil spill response and the associated behaviors of dispersed oil. However, these impacts remain poorly understood, limiting the appropriate usage of dispersants in ice-covered regions. Herein this work explored the effects of frazil ice on the dispersion effectiveness of two dispersants (Corexit 9500A and hydrolyzed shrimp waste) and the migration of dispersed oil within frazil ice. We discovered that frazil ice inhibited dispersion effectiveness by attenuating water velocity. Permeable frazil ice encapsulated 11-30% of dispersed oil, implying a lower oil bioavailability. We thus proposed and verified a microscopic mechanism to unravel the migration of dispersed oil toward permeable constrictions in frazil ice. We predicted the concentration of dispersed oil encapsulated in frazil ice using bed filtration theory and verified the prediction through experiments. Furthermore, the presence of frazil ice can lead to the breakup and coalescence of dispersed oil. Overall, our findings would facilitate the appropriate planning and decision-making of dispersant-based oil spill response and a better understanding of the fate of dispersed oil in the frazil ice-infested ocean.

Influence of Dispersed Oil on the Remote Sensing Reflectance-Field Experiment in the Baltic Sea.

Remote sensing techniques currently used to detect oil spills have not yet demonstrated their applicability to dispersed forms of oil. However, oil droplets dispersed in seawater are known to modify the local optical properties and, consequently, the upwelling light flux. Theoretically possible, passive remote detection of oil droplets was never tested in the offshore conditions. This study presents a field experiment which demonstrates the capability of commercially available sensors to detect significant changes in the remote sensing reflectance Rrs of seawater polluted by six types of dispersed oils (two crude oils, cylinder lubricant, biodiesel, and two marine gear lubricants). The experiment was based on the comparison of the upwelling radiance Lu measured in a transparent tank floating in full immersion in seawater in the Southern Baltic Sea. The tank was first filled with natural seawater and then polluted by dispersed oils in five consecutive concentrations of 1-15 ppm. After addition of dispersed oils, spectra of Rrs noticeably increased and the maximal increase varied from 40% to over three-fold at the highest oil droplet concentration. Moreover, the most affected Rrs band ratios and band differences were analyzed and are discussed in the context of future construction of algorithms for dispersed oil detection.

Biodegradation of weathered crude oil by microbial communities in solid and melted sea ice.

Oil spilled in the Arctic may drift into ice-covered areas and become trapped until the ice melts. To determine if exposure to oil during freezing may have a priming effect on degradation of the oil, weathered dispersed oil (2-3 mg/L) was frozen into solid ice for 200 days at -10 °C, then melted and incubated for 64 days at 4 °C. No degradation was measured in oil frozen into ice prior to melting. Both total amount of oil and target compounds were biotransformed by the microbial community from the melted ice. However, oil released from melted ice was degraded at a slower rate than oil incubated in fresh seawater at the same temperature (4 °C), and by a different microbial community. These data suggest negligible biodegradation of oil frozen in sea ice, while oil-degrading bacteria surviving in the ice may contribute to biodegradation when the ice melts.

Characterization of the differential expressed genes and transcriptomic pathway analysis in the liver of sub-adult red drum (Sciaenops ocellatus) exposed to Deepwater Horizon chemically dispersed oil.

The Deepwater Horizon blowout resulted in the second-largest quantity of chemical dispersants used as a countermeasure for an open water oil spill in the Gulf of Mexico. Of which, the efficacy of dispersant as a mitigation strategy and its toxic effects on aquatic fauna remains controversial. To enhance our understanding of potential sub-lethal effects of exposure to chemically dispersed-oil, sub-adult red drum (Sciaenops ocellatus) were continuously exposed to a Corexit 9500: DWH crude oil chemically enhanced water accommodated fraction (CEWAF) for 3-days and transcriptomic responses were assessed in the liver. Differential expressed gene (DEG) analysis demonstrated that 63 genes were significantly impacted in the CEWAF exposed fish. Of these, 37 were upregulated and 26 downregulated. The upregulated genes were primarily involved in metabolism and oxidative stress, whereas several immune genes were downregulated. Quantitative real-time RT-PCR further confirmed upregulation of cytochrome P450 and glutathione S-transferase, along with downregulation of fucolectin 2 and chemokine C-C motif ligand 20. Ingenuity Pathway Analysis (IPA) predicted 120 pathways significantly altered in the CEWAF exposed red drum. The aryl hydrocarbon receptor pathway was significantly activated, while pathways associated with immune and cellular homeostasis were primarily suppressed. The results of this study indicate that CEWAF exposure significantly affects gene expression and alters signaling of biological pathways important in detoxification, immunity, and normal cellular physiology, which can have potential consequences on organismal fitness.

The influences of phytoplankton species, mineral particles and concentrations of dispersed oil on the formation and fate of marine oil-related aggregates.

The formation and fallout of oil-related marine snow have been associated with interactions between dispersed oil and small marine particles, like phytoplankton and mineral particles. In these studies, the influences of phytoplankton species, mineral particle concentration, and oil concentration on the aggregation of oil in seawater (SW) were investigated. The experiments were performed in a low-turbidity carousel incubation system, using natural SW at 13 °C. Aggregation was measured by silhouette camera analyses, and oil compound group distribution and depletion by gas chromatography (GC-FID or GC-MS). Aggregates with median sizes larger than 500 µm in diameter were measured in the presence of dispersed oil and the phytoplankton species Thalassiosira rotula, Phaeocystis globosa, Skeletonema pseudocostatum, but not with the microalgae Micromonas pusilla. When mineral particles (diatomaceous earth) were incubated at different concentrations (5-30 mg/L) with dispersed oil and S. pseudocostatum, the largest aggregates were measured at the lower mineral particle concentration (5 mg/L). Since dispersed oil rapidly dilutes in the marine water column, experiments were performed with oil concentrations of from 10 mg/L to 0.01 mg/L in the presence of S. pseudocostatum and diatomaceous earth. Aggregates larger than 500 µm was measured only at the highest oil concentrations (10 mg/L). However, oil attachment to the marine particles were also measured at low oil concentrations (≤1 mg/L). Depletion of oil compound groups (n-alkanes, naphthalenes, PAHs, decalins) were measured at all oil concentrations, both in aggregate and water phases, with biodegradation as the expected main depletion process. These results showed that oil concentration may be important for oil-related marine snow formation, but that even oil droplets at low concentrations may attach to the particles and be transported by prevailing currents.

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.

Sorption of dispersed petroleum hydrocarbons by activated charcoals: Effects of oil dispersants.

Marine oil spill often causes contamination of drinking water sources in coastal areas. As the use of oil dispersants has become one of the main practices in remediation of oil spill, the effect of oil dispersants on the treatment effectiveness remains unexplored. Specifically, little is known on the removal of dispersed oil from contaminated water using conventional adsorbents. This study investigated sorption behavior of three prototype activated charcoals (ACs) of different particle sizes (4-12, 12-20 and 100 mesh) for removal of dispersed oil hydrocarbons, and effects of two model oil dispersants (Corexit EC9500A and Corexit EC9527A). The oil content was measured as n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons (TPHs). Characterization results showed that the smallest AC (PAC100) offered the highest BET surface area of 889 m2/g and pore volume of 0.95 cm3/g (pHPZC = 6.1). Sorption kinetic data revealed that all three ACs can efficiently adsorb Corexit EC9500A and oil dispersed by the two dispersants (DWAO-I and DWAO-II), and the adsorption capacity followed the trend: PAC100 > GAC12 × 20 > GAC4 × 12. Sorption isotherms confirmed PAC100 showed the highest adsorption capacity for dispersed oil in DWAO-I with a Freundlich KF value of 10.90 mg/g∙(L/mg)1/n (n = 1.38). Furthermore, the presence of Corexit EC9500A showed two contrasting effects on the oil sorption, i.e., adsolubilization and solubilization depending on the dispersant concentration. Increasing solution pH from 6.0 to 9.0 and salinity from 2 to 8 wt% showed only modest effect on the sorption. The results are useful for effective treatment of dispersed oil in contaminated water and for understanding roles of oil dispersants.

Biological response to dissolved versus dispersed oil.

The water-soluble compounds of oil (e.g. low molecular weight PAHs) dissolve as a function of their physicochemical properties and environmental conditions, while the non-soluble compounds exist as dispersed droplets. Both the chemical and physical form of oil will affect the biological response. We present data from a mesocosm study comparing the microbial response to the water-soluble fraction (WSF), versus a water-accommodated fraction of oil (WAF), which contains both dispersed and dissolved oil components. WAF and WSF contained similar concentrations of low molecular weight PAHs, but concentrations of 4- and 5-ring PAHs were higher in WAF compared to WSF. Microbial communities were significantly different between WSF and WAF treatments, primary productivity was reduced more in WSF than in WAF, and concentrations of transparent exopolymeric particles were highest in WSF and lowest in the controls. These differences highlight the importance of dosing strategy for mesocosm and toxicity tests.

Differentiation of weathered chemically dispersed oil from weathered crude oil.

Oil fingerprinting is a crucial technology to trace the sources and behaviors of spilled oil. The use of dispersants enhances the stay of dispersed oil in a water column and changes the important properties of spilled oil. In case of fingerprinting of dispersed oil driven by dispersants, the fate and behaviors of biomarkers may be affected by the application of dispersants. Limited studies have investigated the statistical difference between fingerprinting of dispersed oil and non-dispersed oil using biomarkers, and the possible influence of the differences, if present. This study applied several principal component analyses (PCA) to differentiate weathered chemically dispersed oil from weathered crude (non-dispersed) oil using 103 diagnostic ratios of the same type of biomarkers and those of two types of biomarkers as input data. It showed that weathered chemically dispersed oil (CDO) can be differentiated from weathered crude oil (WCO) using specific diagnostic ratios that are affected by weathering. PCA analyses indicated the effects of the application of dispersants and weathering duration on weathering of biomarkers in CDO and WCO.

The physiological effects of oil, dispersant and dispersed oil on the bay mussel, Mytilus trossulus, in Arctic/Subarctic conditions.

Increasing oil development around Alaska and other Arctic regions elevates the risk for another oil spill. Dispersants are used to mitigate the impact of an oil spill by accelerating natural degradation processes, but the reduced hydrophobicity of dispersed oil may increase its bioavailability to marine organisms. There is limited research on the effect of dispersed oil on cold water species and ecosystems. Therefore, spiked exposure tests were conducted with bay mussels (Mytilus trossulus) in seawater with non-dispersed oil, Corexit 9500 and oil dispersed with different concentrations of Corexit 9500. After three weeks of exposure, acute and chronic physiological impacts were determined. The majority of physiological responses occurred during the first seven days of exposure, with mussels exhibiting significant cytochrome P450 activity, superoxide dismutase activity and heat shock protein levels. Mussels exposed to non-dispersed oil also experienced immune suppression, reduced transcription and higher levels of mortality. After 21 days, mussels in all treatments exhibited evidence of genetic damage, tissue loss and a continued stress response. Bay mussels are useful as indicators of ecosystem health and recovery, and this study was an important step in understanding how non-dispersed oil, dispersant and dispersed oil affect the physiology of this sentinel species in Arctic/subarctic conditions.

Chemically dispersed oil is cytotoxic and genotoxic to sperm whale skin cells.

Two major oil crises in United States history, the 1989 Exxon-Valdez oil spill in Alaska and the 2010 Deepwater Horizon Oil Rig explosion in the Gulf of Mexico, drew attention to the need for toxicological experiments on oil and chemically dispersed oil. We are still learning the effects these spills had on wildlife. However, little data is known about the toxicity of these substances in marine mammals. The objective of this study is to determine the toxicity of Alaskan oil, as well as chemically dispersed oil. Oil experiments were performed using the water accommodated fraction of Alaskan oil (WAF) and the chemically enhanced water accommodated fraction of Alaskan oil (CEWAF). The Alaskan WAF is not cytotoxic to sperm whale skin cells though it did induce chromosome damage; S9-mediated metabolism did not affect the cytotoxicity of WAF but did increase the levels of chromosome damage. Alaskan CEWAF is more cytotoxic and genotoxic than the WAF; S9 mediated metabolism increased both cytotoxicity and genotoxicity of CEWAF. Analysis of the PAH content of Alaskan WAF and CEWAF revealed a forty-fold increase in the total levels of PAHs in CEWAF compared to WAF. These findings show that chemically dispersed oil leads to higher levels of PAH exposure which are more toxic and likely to lead to longer and more persistent health effects.

Exposure of European sea bass (Dicentrarchus labrax) to chemically dispersed oil has a chronic residual effect on hypoxia tolerance but not aerobic scope.

We tested the hypothesis that the chronic residual effects of an acute exposure of European sea bass (Dicentrarchus labrax) to chemically dispersed crude oil is manifest in indices of hypoxic performance rather than aerobic performance. Sea bass were pre-screened with a hypoxia challenge test to establish their incipient lethal oxygen saturation (ILOS), but on discovering a wide breadth for individual ILOS values (2.6-11.0% O2 saturation), fish were subsequently subdivided into either hypoxia sensitive (HS) or hypoxia tolerant (HT) phenotypes, traits that were shown to be experimentally repeatable. The HT phenotype had a lower ILOS and critical oxygen saturation (O2crit) compared with the HS phenotype and switched to glycolytic metabolism at a lower dissolved oxygen, even though both phenotypes accumulated lactate and glucose to the same plasma concentrations at ILOS. As initially hypothesized, and regardless of the phenotype considered, we found no residual effect of oil on any of the indices of aerobic performance. Contrary to our hypothesis, however, oil exposure had no residual effect on any of the indices of hypoxic performance in the HS phenotype. In the HT phenotype, on the other hand, oil exposure had residual effects as illustrated by the impaired repeatability of hypoxia tolerance and also by the 24% increase in O2crit, the 40% increase in scope for oxygen deficit, the 17% increase in factorial scope for oxygen deficit and the 57% increase in accumulated oxygen deficit. Thus, sea bass with a HT phenotype remained chronically impaired for a minimum of 167days following an acute 24-h oil exposure while the HS phenotypes did not. We reasoned that impaired oxygen extraction at gill due to oil exposure activates glycolytic metabolism at a higher dissolved oxygen, conferring on the HT phenotype an inferior hypoxia resistance that might eventually compromise their ability to survive hypoxic episodes.

A new perspective of particle adsorption: Dispersed oil and granular materials interactions in simulated coastal environment.

This study, adsorption behaviors of dispersed oil in seawaters by granular materials were explored in simulation environment. We quantitatively demonstrated the dispersed oil adsorbed by granular materials were both dissolved petroleum hydrocarbons (DPHs) and oil droplets. Furthermore, DPHs were accounted for 42.5%, 63.4%, and 85.2% (35.5% was emulsion adsorption) in the adsorption of dispersed oil by coastal rocks, sediments, and bacterial strain particles respectively. Effects of controlling parameters, such as temperature, particle size and concentration on adsorption of petroleum hydrocarbons were described in detail. Most strikingly, adsorption concentration was followed a decreasing order of bacterial strain (0.5-2µm)>sediments (0.005-0.625mm)>coastal rocks (0.2-1cm). With particle concentration or temperature increased, adsorption concentration increased for coastal rocks particle but decreased for sediments particle. Besides, particle adsorption rate of petroleum hydrocarbons (n-alkanes and PAHs) was different among granular materials during 60 days.

Sensitivity of the deep-sea amphipod Eurythenes gryllus to chemically dispersed oil.

In the context of an oil spill accident and the following oil spill response, much attention is given to the use of dispersants. Dispersants are used to disperse an oil slick from the sea surface into the water column generating a cloud of dispersed oil droplets. The main consequence is an increasing of the sea water-oil interface which induces an increase of the oil biodegradation. Hence, the use of dispersants can be effective in preventing oiling of sensitive coastal environments. Also, in case of an oil blowout from the seabed, subsea injection of dispersants may offer some benefits compared to containment and recovery of the oil or in situ burning operation at the sea surface. However, biological effects of dispersed oil are poorly understood for deep-sea species. Most effects studies on dispersed oil and also other oil-related compounds have been focusing on more shallow water species. This is the first approach to assess the sensitivity of a macro-benthic deep-sea organism to dispersed oil. This paper describes a toxicity test which was performed on the macro-benthic deep-sea amphipod (Eurythenes gryllus) to determine the concentration causing lethality to 50% of test individuals (LC50) after an exposure to dispersed Brut Arabian Light (BAL) oil. The LC50 (24 h) was 101 and 24 mg L(-1) after 72 h and 12 mg L(-1) at 96 h. Based on EPA scale of toxicity categories to aquatic organisms, an LC50 (96 h) of 12 mg L(-1) indicates that the dispersed oil was slightly to moderately toxic to E. gryllus. As an attempt to compare our results to others, a literature study was performed. Due to limited amount of data available for dispersed oil and amphipods, information on other crustacean species and other oil-related compounds was also collected. Only one study on dispersed oil and amphipods was found, the LC50 value in this study was similar to the LC50 value of E. gryllus in our study. Since toxicity data are important input to risk assessment and net environmental benefit analyses, and since such data are generally lacking on deep-sea species, the data set produced in this study is of interest to the industry, stakeholders, environmental management, and ecotoxicologists. However, studies including more deep-sea species covering different functional groups are needed to evaluate the sensitivity of the deep-sea compartments to dispersed oil relative to other environmental compartments.

The effect of dispersed Petrobaltic oil droplet size on photosynthetically active radiation in marine environment.

Oil pollution in seawater, primarily visible on sea surface, becomes dispersed as an effect of wave mixing as well as chemical dispersant treatment, and forms spherical oil droplets. In this study, we examined the influence of oil droplet size of highly dispersed Petrobaltic crude on the underwater visible light flux and the inherent optical properties (IOPs) of seawater, including absorption, scattering, backscattering and attenuation coefficients. On the basis of measured data and Mie theory, we calculated the IOPs of dispersed Petrobaltic crude oil in constant concentration, but different log-normal size distributions. We also performed a radiative transfer analysis, in order to evaluate the influence on the downwelling irradiance Ed, remote sensing reflectance Rrs and diffuse reflectance R, using in situ data from the Baltic Sea. We found that during dispersion, there occurs a boundary size distribution characterized by a peak diameter d0 = 0.3 µm causing a maximum E d increase of 40% within 0.5-m depth, and the maximum Ed decrease of 100% at depths below 5 m. Moreover, we showed that the impact of size distribution on the "blue to green" ratios of Rrs and R varies from 24% increase to 27% decrease at the same crude oil concentration.

Synergistic effect of crude oil plus dispersant on bacterial community in a louisiana salt marsh sediment.

A combined effect of crude oil plus dispersant (Corexit 9500A) significantly altered indigenous bacterial communities in a Louisiana salt marsh sediment after 30 days of incubation; the crude oil and/or Corexit 9500A treatments triggered shifts in bacterial communities and the shifted bacterial structure by crude oil plus Corexit 9500A was considerably different from those by either crude oil or Corexit 9500A. However, the synergistic effect of crude oil plus Corexit 9500A was not observed after 7 days of incubation; the bacterial community was slightly shifted by Corexit 9500A and the crude oil did not trigger any bacterial community shift after 7 days of incubation. DNA sequencing data indicated that Chromobacterium species was enriched in the Corexit 9500A microcosms after 7 days of incubation, while Pseudomonas, Advenella, Acidocella and Dyella spp. were enriched after 30 days of incubation. Parvibaculum was a dominant species in the crude oil microcosms after 30 days of incubation. Rhodanobacter, Dyella and Frateuria spp. were dominant in crude oil plus Corexit 9500A microcosms after 30 days of incubation. Our data show that the effect of crude oil plus Corexit 9500A on bacterial community is synergistic, and thus the dispersant effect should be considered with the spilled oil to correctly evaluate the environmental impact.