Skeletal mineralogy of coral recruits under high temperature and <i>p</i>CO₂

Aragonite, which is the polymorph of CaCO₃ precipitated by modern corals during skeletal formation, has a higher solubility than the more stable polymorph calcite. This higher solubility may leave animals that produce aragonitic skeletons more vulnerable to anthropogenic ocean acidification. It is therefore important to determine whether scleractinian corals have the plasticity to adapt and produce calcite in their skeletons in response to changing environmental conditions. Both high <i>p</i>CO₂ and lower Mg ∕ Ca ratios in seawater are thought to have driven changes in the skeletal mineralogy of major marine calcifiers in the past ∼ 540 Ma. Experimentally reduced Mg ∕ Ca ratios in ambient seawater have been shown to induce some calcite precipitation in both adult and newly settled modern corals; however, the impact of high <i>p</i>CO₂ on the mineralogy of recruits is unknown. Here we determined the skeletal mineralogy of 1-month-old <i>Acropora spicifera</i> coral recruits grown under high temperature (+3 °C) and <i>p</i>CO₂ (∼ 900 µatm) conditions, using X-ray diffraction and Raman spectroscopy. We found that newly settled coral recruits produced entirely aragonitic skeletons regardless of the treatment. Our results show that elevated <i>p</i>CO₂ alone is unlikely to drive changes in the skeletal mineralogy of young corals. Not having an ability to switch from aragonite to calcite precipitation may leave corals and ultimately coral reef ecosystems more susceptible to predicted ocean acidification. An important area for prospective research would be the investigation of the combined impact of high <i>p</i>CO₂ and reduced Mg ∕ Ca ratio on coral skeletal mineralogy