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Is Global Coral Bleaching Only Half as Bad as Has Been Believed?

Paper Reviewed
Cruz, I.C.S., Leal, M.C., Mendes, C.R., Kikuchi, R.K.P., Rosa, R., Soares, A.M.V.M., Serodio, J., Calado, R. and Rocha, R.J.M. 2015. White but not bleached: photo-physiological evidence from white Montastraea cavernosa reveals potential overestimation of coral bleaching. Marine Biology 162: 889-899.

Introducing their truly intriguing study, Cruz et al. (2015) write that "coral bleaching is a physiological mechanism triggered by environmental stress, such as elevated temperature, changes in salinity, high solar radiation, pollutants or diseases, whereby symbiotic corals lose notable numbers of photosynthetic endosymbiotic dinoflagellates (Symbiodinium spp.) and/or their photosynthetic pigments," which events "increase coral susceptibility to secondary stressors that may induce mortality and, ultimately, lead to the collapse of coral reef ecosystems." But they note that "although bleaching leaves the coral skeleton visible under its transparent tissue, not all white coral colonies display this feature," which "raises the question as to whether all 'white'-shaded colonies are indeed bleached."

Seeking to answer this question, the nine researchers studied whether specimens of the coral Montastraea cavernosa with different colorations -- which they collected from Todos os Santos Bay on the east coast of Brazil -- displayed similar photo-physiological features, such as maximum quantum yield of PSII, photosynthetic pigments composition, spectral reflectance and, last of all, Symbiodinium density. And this work revealed that white M. cavernosa were not bleached but were physiologically healthy compared to dark and light-brown colonies.

Consequently, as Cruz et al. write in the concluding paragraph of their paper, "it is clear that white colonies are physiologically healthy and that the number of white M. cavernosa occurring in the surveyed reefs is high enough to notably contribute to an over-estimation of coral bleaching." And they thus conclude, in the final paragraph of their paper's abstract, that "video transects from reef monitoring surveys at Todos os Santos Bay revealed that the proportion of bleached and white colonies is similar, thus suggesting that current coral reef surveys may be overestimating the bleaching of M. cavernosa by nearly twofold," which may possibly hold true for other coral species as well.

Posted 6 July 2015