White but not bleached: photophysiological evidence from white Montastraea cavernosa reveals potential overestimation of coral bleaching

Abstract

Climate change and other types of environmental stress are known to increase corals’ vulnerability to bleaching, a process whereby colonies lose their colour either due to the loss of photosynthetic symbionts or their pigments. Although bleaching leaves the coral skeleton visible under its transparent tissue, not all white coral colonies display this feature. This raises the question as to whether all ‘white’-shaded colonies are indeed bleached. Within this context, Montastraea cavernosa colonies of different colour types (dark brown, light brown, bleached and white) were sampled for photobiological evaluation. Here, we show that, while the conventional spectral reflectance techniques failed to discriminate white from bleached colonies, chlorophyll fluorescence, photosynthetic pigment profile and Symbiodinium density enabled a clear distinction between these shades. Subsequently, video transects from reef monitoring surveys at Todos os Santos Bay (Brazil) 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.