Zooxanthellae ultrastructure affected in bleached corals

Results & Discussion Most reef-building corals are hermatypic : they harbour endosymbiotic dinoflagellates, (zooxanthellae, unicellular algae), within gastroderm cells. Bleaching, loss of colour due to loss of these symbiotic algae and/or their photosynthetic pigments, on a large scale, appears to increase by intensity and geographic extent, certainly related to increasing sea temperatures. A lot of tools are still needed to study the mechanisms of this phenomenon. To observe what are morphological changes appearing in zooxanthellae during bleaching, we sampled three coral species from environment and one coral heat-shocked in experimental aquarium. For each species, healthy and bleached fragments were cut and fixed in 2.5% glutaraldehyde. Coral skeletons were dissolved in 0.2 M EDTA. Samples were further fixed in 2.5% glutaraldehyde solution and post-fixed in 1% OsO4, before embedding in epoxy resin according to a routine procedure (ethanol/epoxypropane dehydratation). Ultra-thin sections (~70 nm thick) were performed with a diamond knife on an ultramicrotome, contrasted with uranyl acetate and lead citrate, and observed on a Jeol JEM 100-SX transmission electron microscope (TEM) at 80 kV of accelerating voltage.