Dissolved Organic Matter (DOM) is one of the largest and least understood pools of carbon in the ocean, and ~10-25% of the total oceanic DOM is composed of biopolymers. A large proportion of DOM is produced from the synthesis and release of exopolysaccharides (EPS) from bacteria and eukaryotic phytoplankton. This has led to research into the potentially significant role of EPS, particularly in bacteria, in the degradation of hydrocarbon pollutants in the ocean. There has been much interest in the fate of the oil released following the explosion and sinking of the Deepwater Horizon oil rig in the Gulf of Mexico in 2010, the worst accidental marine oil spill in the history of the oil and gas industry. The fate of the oil from the ecological disaster is poorly known, but Gutierrez et al. stress the importance of understanding the capacity of EPS produced by marine bacteria to affect the dissolution of aromatic hydrocarbons to enhance the bioavailability of these compounds for indigenous microbial communities and hence the rate of degradation.
EPS-producing bacteria enriched during the oil spill were thought to have contributed to the formation of abundant oil aggregates on the surface and within deep water oil plumes, and the authors demonstrated this enrichment both in laboratory experiments and in the field. This study used one type of EPS-producing bacteria, the Halomonas species strain TG39, as a model to predict the potential influence of EPS on the oil, which was collected from surface waters during the active phase of the spill. This strain produces large amounts of EPS which display amphiphilic, or biosurfactant-like, properties, and can lead to the emulsification of aromatic hydrocarbons, crude oils or refined petroleum products by interacting with hydrophobic substrates on their surfaces and solubilising them to enhance their biodegradation.
An additional analysis of pyrosequence data from surface water samples collected during the spill revealed other distinct Halomonas strains which could produce EPS. Some of these strains had high (97%-100%) 16S rRNA sequence identity to strain TG39. Clusters of other EPS-producing bacterial communities, such as Alteromonas, Colwellia and Pseudoalteromonas, were also enriched in surface and deep waters, and could also emulsify the oil and form aggregates, which may have supplemented the bioavailability of EPS in the Gulf. They also used a roller bottle incubation experimental design with one of the other Halomonas isolates, TGOS-10, which exhibited an independent ability to emulsify the oil, as the aggregates directly formed on the extracellular cell surface of the strain.
Evidence from the results showed that the EPS produced by Halomonas species strain TG39 can facilitate the dissolution of poorly-soluble aromatic hydrocarbons, which enhanced the bioavailability to indigenous populations of microorganisms present during the active phase of the spill, particularly in the surface waters. The enrichment of EPS-producing bacteria, together with their capacity to produce amphiphilic EPS, is likely to have contributed to the eventual removal of the oil and to the formation of oil aggregates.
The authors focus on the enrichment and dominance of EPS in the Halomonas strain to influence the degradation of the oil in the surface waters, as well as other EPS-producing bacteria in the deep water oil plumes, but it would be interesting to see whether the rate of degradation differs in the surface waters compared to the deep water plumes and whether it is based on the level of enrichment alone. In addition, as this experiment was carried out during the active phase of the oil spill, further research should look at the after-effects to see how the abundance of the DOM pool changed once the oil had been degraded, and the effect it may have had on the carbon cycle in this region of the Gulf.
Gutierrez, T., Berry, D., Yang, T., Mishamandani, S., McKay, L., Teske, A., and Aitken, M.D. (2013) Role of Bacterial Exopolysaccharides (EPS) in the Fate of the Oil Released during the Deepwater Horizon Oil Spill. PLoS one, 8 (6), e67717