Thermal stress reduces Pocilloporid coral resilience to ocean acidification by impairing control over calcifying fluid chemistry: A combined boron geochemistry and pH microelectrode study

Robert Eagle1, Maxence Guillermic2, Louise Cameron3, Ilian Antoine DeCorte4, Sambuddha Misra5, Jelle Bijma6, Dirk de Beer7, Claire Reymond8, Hildegard Westphal8,9 and Justin B Ries10, (1)University of California Los Angeles, Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, Los Angeles, CA, United States, (2)University of California Los Angeles, Earth, Planetary and Space Sciences, Los Angeles, United States, (3)Northeastern University, Boston, MA, United States, (4)University of California - Los Angeles, Atmospheric and Oceanic Sciences, Los Angeles, CA, United States, (5)Indian Institute of Science, Bangalore, Centre for Earth Sciences, Bangalore, India, (6)Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, Bremerhaven, Germany, (7)Max Planck Institute for Marine Microbiology, Bremen, Germany, (8)Leibniz Center for Tropical Marine Ecology, Bremen, Germany, (9)MAGILL, SA, Australia, (10)Northeastern University, Department of Marine and Environmental Sciences, Nahant, MA, United States
Recent studies have examined the effect of ocean acidification (OA) on corals’ growth rates and ability to maintain control of their calcification fluid (cf) chemistry but, to our knowledge, no study has examined the joint effect of OA and thermal stress . The δ11B of coral aragonite is a proxy for pH of the coral’s calcification fluid (pHcf) and, when combined with B/Ca, can yield estimates of other carbonate system parameters within the calcifying fluid, including [CO32-]cf and DICcf. Specimens of Pocillipora damicornis and Stylophora pistillata were previously cultured under a range of controlled pH (7.6 to 8.2) and temperatures (28, 31°C) (Cameron et al., preparation). Here, for the same specimens, we compare boron-derived estimates of pHcf to microelectrode pH measurements of fluid pockets beneath the coral’s calicoblastic epithelium, where aragonite crystals of the coral skeleton are known to nucleate and grow. Both techniques reveal the important, yet previously unappreciated, observation that thermal stress, in addition to ocean acidification, causes a reduction in pHcf. The comparison between pH estimates from both techniques also reveals information on the mechanisms of coral biomineralization by linking the microenvironment of aragonite crystalization recorded by boron geochemistry to the bulk chemistry of calicoblastic epithelium fluid pockets recorded by microelectrodes. Our The data also show that both species cultured under controlled temperature conditions of 28°C exhibited positive net calcification rates under the complete range of pHsw examined (7.6 - 8.2), and that both species increased pHcf, DICcf, and [CO32-]cf to maintain an aragonite saturation state (Ωcf) at the site of calcification that is significantly greater than that of surrounding seawater in support of calcification. However, at 31°C, control over calcification fluid chemistry, include pHcf, is to some extent lost, apparently resulting in the observed reduction in net calcification under these conditions. Thus, this study reveals one of the mechanisms by which the deleterious impacts of ocean acidification on Pocilloporid corals are compounded by thermal stress.