The effects of seawater temperature-induced coral bleaching on the aragonite structure and material properties of massive Porites lutea coral skeletons

Abstract

The coral skeletons that contribute to tropical reef structures are biominerals, composed of inorganic aragonite and organic biomolecules. The biomolecules influence the aragonite structure and material properties of the skeleton. We collected massive Porites lutea skeletons from Phuket, Thailand, in 1991, approximately one month into a temperature-induced bleaching event. Some specimens had expelled their Symbiodiniaceae in response to the increased water temperatures (bleached), while other corals appeared unaffected (unbleached). We investigate the effect of Symbiodiniaceae loss on the amino acid composition, aragonite structure, and Vickers hardness of the coral skeletons. We observe no significant difference in the amino acid content or composition of the outermost 1 mm of skeleton (representing 0.5 to 2 months growth) between bleached and unbleached specimens. The full width half maximum of the Raman spectrum ʋ1 band, an indicator of disorder around the CO3 group in the aragonite lattice, varies significantly between some corals in the outermost 200 µm of skeleton, but these differences are not attributable to the bleaching status of the coral colonies. Similarly, Vickers hardness varies significantly between some colonies, but this is not related to coral bleaching. This is a positive finding, suggesting that bleaching, from which corals recover, does not adversely affect the coral skeletal structure.