Crude oil impairs immune function and increases susceptibility to pathogenic bacteria in southern flounder.

Exposure to crude oil or its individual constituents can have detrimental impacts on fish species, including impairment of the immune response. Increased observations of skin lesions in northern Gulf of Mexico fish during the 2010 Deepwater Horizon oil spill indicated the possibility of oil-induced immunocompromisation resulting in bacterial or viral infection. This study used a full factorial design of oil exposure and bacterial challenge to examine how oil exposure impairs southern flounder (Paralichthys lethostigma) immune function and increases susceptibility to the bacteria Vibrio anguillarum, a causative agent of vibriosis. Fish exposed to oil prior to bacterial challenge exhibited 94.4% mortality within 48 hours of bacterial exposure. Flounder challenged with V. anguillarum without prior oil exposure had <10% mortality. Exposure resulted in taxonomically distinct gill and intestine bacterial communities. Mortality strongly correlated with V. anguillarum levels, where it comprised a significantly higher percentage of the microbiome in Oil/Pathogen challenged fish and was nearly non-existent in the No Oil/Pathogen challenged fish bacterial community. Elevated V. anguillarum levels were a direct result of oil exposure-induced immunosuppression. Oil-exposure reduced expression of immunoglobulin M, the major systemic fish antibody, and resulted in an overall downregulation in transcriptome response, particularly in genes related to immune function, response to stimulus and hemostasis. Ultimately, sediment-borne oil exposure impairs immune function, leading to increased incidences of bacterial infections. This type of sediment-borne exposure may result in long-term marine ecosystem effects, as oil-bound sediment in the northern Gulf of Mexico will likely remain a contamination source for years to come.

Changes in free-living bacterial community diversity reflect the magnitude of environmental variability.

A 2-year study was undertaken to compare patterns in the diversity of free-living bacteria in a river-dominated estuary and offshore, on the shelf, to determine whether changes in the free-living bacterial community could be related to differences in environmental seasonality and variability. Although the environmental conditions inshore were significantly more variable than those on the shelf and demonstrated clear seasonal patterns, there were no significant differences in the alpha diversity of the communities based on richness, evenness, or phylogenetic diversity. Comparison of communities using Bray-Curtis similarity indicated no significant differences in the magnitude of change between sequential samples from inshore and on the shelf. Seasonal differences were detected both inshore and on the shelf. However, analysis using the weighted UniFrac distance indicated significantly lower overall change between shelf samples with no significant seasonal differences. These findings suggest different patterns of change between the two sites. Inshore, changes in the relative abundance of distantly related bacterial species reflect the larger environmental variability, while on the shelf, changes in the relative abundance of closely related bacterial species or strains may result in a more functionally stable community. Thus, the magnitude of environmental change can alter patterns of bacterial diversity in marine systems.