The explosion of the Deepwater Horizon oil platform
on April 20, 2010 resulted in the largest release of oil in history, entering
the northern Gulf of Mexico causing massive amounts of ecological and economical
damage to the ecosystem. Up to 1773 km of coastline from Louisiana to Florida experienced
some form of oil exposure. The application of dispersant via aircraft, was
intended to remove much of the oil from the ocean surface which resulted in a
large portion of sinking to sediments. Exposure to crude oil and its individual
constituents can have detrimental impacts on fish species, including impairment
of the immune response.
In this study, they examined the effects of exposure
to high concentrations of hydrocarbons in crude oil on immune function and increased
susceptibility to pathogen infection, in the southern flounder (Paralichthys lethostigm), based on the
increased incidents of external lesions/ sores on fish species that are
indicative of bacterial infections such as vibriosis.
The experiment consisted of a fully factorial
exposure design with contaminated sediment and pathogen exposure as main
factors.
Juvenile flounder were exposed to oil-contaminate
sediment for 7 days, then challenged with a known fish pathogen for 1 hour, Vibrio anguillarum, that is prevalent in
the Gulf of Mexico. V. anguillarum is
the causative agent of vibriosis, a haemorrhagic disease responsible for severe
economic losses, especially in aquaculture fisheries. The juvenile fish were sampled
at 24 hours post bacterial challenge with two
fish then being sacrificed from each tank, in which the liver, spleen, kidney,
intestine and gill samples were removed and preserved for further processing.
DNA was extracted from samples using PowerSoil DNA
Isolation Kit and the relationship between microbial communities in intestine
and gill tissues of oil, pathogen and co-exposed fish was determined by
amplification and sequencing of the gene encoding 16S rRNA. After denoising and
chimera checking, sequence data were separated into operational taxonomic units
(OTUs) and predictive metagenomic analysis was performed.
Fish in several oil exposed treatment tanks were observed swimming to
the surface and gulping for air, despite oxygen levels being monitored and
adequate at those times in comparison to fish from the control tanks, which did
not exhibit these behaviours. A lowered haemoglobin concentration
has been linked to oil exposure in which down-regulation of β- haemoglobin gene expression, a gene important for erythrocyte
production and oxygen transport, occurred suggesting a highly negative impact
of hydrocarbons on fish health.
Oil exposure also resulted in reduced expression of IgM mRNA, the
primary systemic fish antibody and typically one of the first to respond to
bacterial infection. In several fish species, a reduced ability to
defend against pathogen infection occurred after exposure to oil or its components.
The results suggest that the lesions observed in nGOM fish after Deep-Water
Horizon oil spill are the result of an immunotoxic response to oil exposure, resulting
in an increased prevalence of pathogenic infections.
Oil exposure resulted in consistent
and clear shifts in overall taxonomically distinct bacterial community
composition in gills and intestines of fish, with Alcanivorax being found
in increased quantities in marine sediments and impacted by Deepwater Horizon
oil where it has been demonstrated that, when supplied with adequate nutrients
(nitrogen and phosphorus), it will become the predominant bacterial species in
seawater containing petroleum. Imbalances in these communities can
have detrimental physiological effects, important for organismal
homeostasis.
This study provides evidence that an
imbalance or shift in gut microbial communities can result in the successful
establishment of lethal infections by pathogenic organisms, leading to possible disastrous impacts on the aquaculture industry and the overall diversity of the Gulf of Mexico.
Bayha, K. M., Ortell, N., Ryan, C.N.,
Griffitt, K.J., Krasnec, M., Sena, J., Ramaraj, T., Takeshita, R., Mayer, G.D., Schilkey, F. & Griffitt, R.J (2017) Crude oil impairs immune
function and increases susceptibility to pathogenic bacteria in southern
flounder. PLoS ONE 12(5): e0176559. https://doi.org/10.1371/journal.pone.0176559
Hi Sophie,
ReplyDeleteI also find it very interesting to trace the aftermath of the Deepwater Horizon incident and its impact on the marine environment. Most studies, which are researching on the influence of the event on the local biota, are focused on the effects of singular stressors, which can be misleading, as different factors can interact. I found this paper quite intriguing, because it looks into the combined effects of the stress imposed on a fish-species by spilled oil and a common pathogen.
There is another very interesting publication on the subject from Rodgers et al.(2018), which studies the effects of oil and V. anguillarum on another fish common for the Gulf of Mexico - the red snapper. They also reported a decrease in 5 immune-related genes (including Igm) shortly after exposure to oil and/or V. anguillarum. I recommend the read!
Best,
Yana