Oil degradation by Ebro Delta microbial mats

The application of natural microbial communities for the cleanup of contaminated ecosystems after oil spills is of particular contemporary interest. Cyanobacterial mats contain phototrophic and heterotrophic bacteria and are thought to be promising bioremediation agents due to their capability to colonize contaminated coastal zones and their high metabolic diversity within a few millimeters depth. Llirós et al. (2008) aimed to determine the effect of two types of crude oil, Casablanca and Maya, on Ebro Delta microbial mat communities (a pristine marine benthic ecosystem, found at the sediment-water interface) in experimental microcosms. Casablanca has a low sulphur content, is similar to Arabian light and spreads easily and swiftly while Maya has a high sulphur content and viscosity and is similar to Aegyptian oil. Five microcosms were used, each with mechanical movement simulating a tidal cycle and a diel light cycle. Microcosms consisted of a control containing just the microbial mat (unpolluted); microbial mat and Casablanca oil (Casablanca); microbial mat and Maya oil (Maya); just Casablanca and just Maya (mechanical controls). They determined chlorophyll a (chl a) concentrations as a measure of oxygenic phototrophic biomass and protein content as an indicator of total biomass and cyanobacterial identification and total cultivable heterotrophic bacteria counts were carried out under aerobic and anaerobic conditions.

Llirós et al. (2008) found that the unpolluted microcosm showed no qualitative significant changes in diversity, chl a or protein content over time and at least seven cyanobacterial morphotypes were found. These were filamentous (Microcoleus chthonoplastes-like, Phormidium-like and two types of Oscillatoria-like morphotypes), ovoid (Synechococcus-like morphotypes) or spherical (Synechocystis-like or Gloeocapsa-like morphotypes). After 28 days the Casablanca microcosm showed diminished abundance of the filamentous forms and the disappearance of Gloeocapsa-like morphotypes, however there were no marked changes in Synechococcus- or Synechocystis-like morphotypes. Only the Gloeocapsa-like morphotypes were not present after 28 days exposure to Maya, but by the end of the experiment (50 days) the only filamentous cyanobacteria present was the M. chthonoplastes-like morphotypes and there was a noticeable decrease in the thickness of the photosynthetic layer of the mat. There were differences in counts of anaerobic heterotrophic bacteria between the polluted and unpolluted microcosms; significantly higher counts were found in Casablanca and significantly lower in Maya. While there were fluctuations, there were no significant differences in aerobic heterotrophic bacteria, oxygenic phototrophic biomass or total biomass. Lower molecular weight constituents of the oils were degraded to a higher extent by the microbial mats than higher weight.

These results show that microbial mat communities have a significant potential to degrade the components of oils, however different types of crude oils have distinct effects on the community structure. Gloeocapsa-like morphotypes were found to be the most sensitive to both types of oil, and M. chthonoplastes-like morphotypes show the highest tolerance for both, with a previous study showing that they are capable of surviving anaerobically on Maya as a sole carbon source. It would be interesting for further study to look at the effects of the individual components of crude oil on the microbial community however this shows great potential for microbial bioremediation of oil spills.

Llirós, M., Gaju, N., García de Oteyza, C., Grimalt, J., Esteve, I. & Martínez-Alonso, M. (2008) Microcosm experiments of oil degradation by microbial mats. II. The changes in microbial species. The Science of the Total Environment. 393, 39-49