Osmolyte composition and skeletal matrix polymorph and crystalline structure analysis of salinity- and acid-resistant Yucatan corals

Increasing atmospheric carbon dioxide is causing drastic changes in seawater chemistry, making seawater more acidic, lowering aragonite saturation state (ΩA) and salinity, and increasing oceanic temperature. Acidic springs, or ojos de agua, off of the Yucatan peninsula mirror many of these future environmental changes and are uninhabitable to most marine organisms with the exception of three species of coral: Porites astreoides, Porites divaricata, and Siderastrea radians. Samples from P. astreoides growing in ojo and normal seawater were analyzed for their osmolyte content, which revealed an increasing concentration of proline betaine as pH, ΩA, and salinity decreased, while the concentration of all other osmolytes decreased. This suggests that proline betaine is not acting primarily as an osmolyte, but instead may be functioning to help P. astreoides survive the harsh ojo conditions. We also looked for abnormalities in the calcium carbonate matrix to see if the lowered pH caused a shift from the aragonite polymorph to the less soluble calcite polymorph, thus altering the crystalline structure of the calcium carbonate. We found no such switching of polymorphs in ojo corals, indicating that they are effectively keeping internal pH low enough to precipitate calcium carbonate skeletons and protecting these matrices from dissolution in the acidic water, perhaps with a thin tissue layer.