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Reunion Island: cyanobacterial communities subject to environmental change

In the blue waters of the Indian Ocean, on the west coast of Reunion Island, INRA scientists from Versailles-Grignon and their colleagues have revealed that the composition of the cyanobacterial communities typical of tropical lagoons is being affected by global and local changes.

West coat of Reunion island.
M. Grugger heads the laboratory dedicated to cyanobacteria at the Pasteur Institute (Paris, France).. © Muriel Grugger, Muriel Gugger
Updated on 11/18/2015
Published on 03/19/2015

Tempted by a trip to the tropics?  Sun, sea and cyanobacteria.  You are invited to participate in this dream programme by INRA scientists from Versailles-Grignon and their colleagues  who have explored the microbial communities that develop on sand, corals or rocks in the form of biofilms in the water that lap the shores of Reunion Island.

Making use of macroscopic and microscopic observations and the most recent genetics technologies, the scientists were able to analyse 187 samples from these biofilms, in which they focused in particular on the presence of cyanobacteria.  The samples were collected between September 2009 and April 2010 along the west coast of the island in the warm waters of the shallow lagoons that extend over 25 km within a coral barrier reef.

 

Cyanobacterial biofilms of considerable diversity, characteristic of tropical regions.

 A hazy film, a felted composition or a gelatinous assembly; these colonies are organised in a variety of structures  that are mainly made up of filamentous cyanobacteria associated with other bacteria.  The scientists observed a very broad diversity among these cyanobacteria, as they were able to list more than 67 operational taxonomic units, or in other words groups of individuals whose genetic material (i.e. their 16S rRNA sequences) presented a similarity of more than 98%.  These taxonomic units belonged to the Oscillatoria and Nostocale orders, and several of them - never previously described - probably constituted new species.

 

Cyanobacterial biofilms subject to environmental factors

Furthermore, in light of the results obtained during other studies, the scientists were able to observe that the cyanobacterial biofilms of Reunion Island and those of other lagoons (Raivavae in French Polynesia, the Bahamas in the Caribbean, etc.) have the common trait of harbouring a broad diversity of cyanobacterial species which in most cases are shared.  By contrast, biofilms sampled in the Arctic and Antarctic Oceans are much less diversified and contain different species.  These findings suggest the existence in cyanobacterial biofilms that have a biogeographical structure, and that their composition is influenced by factors and processes operating at a large geographical scale (for example, climatic regions).

At a local scale, the scientists showed that the seasonal variations which determine the climate of Reunion Island exert a strong influence on the cyanobacterial composition of biofilms.  The samples collected during the cold and dry season from September to December displayed less diversity than those harvested during the hot and humid months from January to April.  A difference that the scientists explained in particular by the effects of the rainfall associated with high temperatures on the nutrient availability and water quality in the lagoons.  Account must also be taken of the fact that the changes occurring within the cyanobacterial communities of Reunion Island might also have a health impact - during a study carried out in the Caribbean on the Île de Raivavae, researchers had shown that certain species were endowed with considerable potential toxicity.

Overall, these results obtained during a very extensive sampling campaign highlighted the fact that the structure and composition of the biofilms dominated by cyanobacteria in the lagoons of Reunion Island are markedly influenced by selection pressures acting at different geographical scales.  More generally, they raise questions as to the influence of anthropogenic pressures exerted at a global scale (e.g. global warming) or a local level (pollution by nutrients) and their effects on biofilm composition.  Perspectives that are now encouraging the scientists to consider other continents and other aquatic environments.

 

This work was carried out in the context of the ARISTOCYA project - Analysis of the toxic risks linked to the development of marine benthic cyanobacteria in tropical regions.

Find out more

Echenique-Subiabre I., Villeneuve A., Golubic S., Turquet J., Humbert J.F. and Gugger M. 2015. Influence of local and global environmental parameters on the composition of cyanobacterial mats in a tropical lagoon. Microb. Ecol. 69: 234.

Villeneuve A. et al. 2012. Diversity and potential toxicity of cyanobacterial mats in two tropical lagoons. J. Phycol. 48: 275.

Cyanobacteria in a few words

Cyanobacteria came into existence 2.5-3 billion years ago and have contributed to the expansion of existing forms of life on Earth due to their ability to produce oxygen by photosynthesis. They also play a major role in the functioning of ecosystems due to their ability to utilize atmospheric carbon and nitrogen.

Present in fresh or salt water, on the surface of the soil, in cold or hot areas, these bacteria are also pioneer organisms in desert environments and other extreme environments.

Known to proliferate sometimes significantly, cyanobacteria can then cause many problems including the production of toxins potentially dangerous for humans and animals.