Award: OCE-1437371

Project objective: The objective of the funded project was to employ a multipronged, comparative approach to measuring pH of the coral calcifying fluid (pH microelectrodes, boron isotopes) under normal and acidified seawater conditions. Funding from this award supported a wide range of experimental and field-based research projects in pursuit of this and related objectives, which are summarized below. (1) Conducted controlled laboratory experiments investigating the impact of ocean acidification (OA) and warming on net calcification rates of 4 species of scleractinian corals (3 tropical, 1 cold-water), showing that warming exacerbates the negative impacts of OA by reducing the symbionts whose photosynthesis would otherwise be enhanced under the high-CO2 conditions associated with OA. (2) Investigated changes in the carbonate chemistry of the calcifying fluid of 3 coral species in response to CO2-induced OA using boron isotopes and pH-sensitive microelectrodes, showing that corals maintain their calcifying fluids at elevated pH relative to external seawater (~8.5 - 9.5 vs. seawater pH of 8), thereby converting dissolved carbon within this fluid into CO32- for calcification. (3) Investigated the impact of OA and warming on gene expression of the coral Siderastrea siderea, showing that coral exposure to extreme thermal stress elicits a more adverse transcriptomic response than exposure to extreme OA. (4) Investigated the impact of warming and CO2-induced OA on the dissolution kinetics of dead shell material produced by corals and other reef-dwelling marine calcifiers, showing that dissolution rates of biogenic carbonates vary systematically with inorganic mineral solubility, that dissolution rates increase with temperature, and that all biogenic carbonates exhibit baseline dissolution even in highly oversaturated seawater. (5) Investigated the impact of CO2-induced OA on calcification and boron isotopic composition of 3 coccolithophore species, showing that coccolithophores respond to OA by changing their carbon source for photosynthesis. (6) Performed a forereef-backreef-nearshore reciprocal coral transplant experiment on the Meso-American Barrier Reef System (MBRS) to isolate the effects of reef-zone-specific genotypes on the response of S. siderea to warming. (7) Calibrated the relationship between CO2-induced OA and clumped isotope composition (Δ47CO2) of 5 bivalve and 1 coral species, showing that seawater pH can influence shell Δ47CO2 and that this should be accounted for in Δ47CO2-based reconstructions of paleo-seawater temperature for these species. (8) Investigated the impact of OA and warming on the δ18O composition of an urchin (Echinometra viridis), showing that δ18O was negatively correlated with seawater temperature and suggesting that fossil echinoid δ18O is a potential archive of past oceanic temperatures. (9) Performed multi-elemental, δ18O, and δ13C measurements of S. siderea cores obtained from the MBRS and S. siderea specimens reared under 25, 28, and 32 °C to explore the use of multi-elemental- and δ18O-paleothermometry for the S. siderea species. (10) CT-scanned S. siderea coral cores to identify annual growth bands and reconstruct annual growth histories, showing that growth histories over the past century vary by reef zone and latitude. (11) Developed novel microphotographic techniques for quantifying the impact of OA and warming on the corallite morphology of corals, showing that both OA and warming impair rate of coral growth, only OA affects corallite structure. (12) Analyzed the δ11B of a range of calcifying marine organisms cultured under a range of pCO2 conditions as a proxy of calcifying fluid pH, showing that a diverse range of marine calcifiers precipitate their CaCO3 from a discrete calcifying medium with a pH that is greater than external seawater pH and that elevation of calcifying fluid pH is a polyphyletic response to OA. (13) Analyzed the δ11B and δ13C composition of annually resolved coral cores from the MBRS as a proxy for past OA, showing that the evolution of the carbonate system on coral reefs over the last ca. 100 years was not controlled solely by anthropogenic CO2. (14) Investigated the impact of seawater temperature on the lithium isotopic composition (δ7Li) of various species of marine calcifiers, showing that δ7Li within brachiopods is the best proxy of seawater temperature, while δ7Li within calcitic mollusks is the worst. (15) Applied the empirically derived multi-elemental proxy of seawater temperature to cores of S. siderea to reconstruct MBRS water temperatures, showing that there are hot-spots and cool-spots to thermal trends, with the latter serving as potential thermal refugia for corals and other thermally sensitive reef species. The project supported the training of 11 undergraduate researchers, 3 Ph.D. students, 2 Master’s students, 2 research technicians, and 4 postdoctoral researchers. Research results have been disseminated through more than 8 invited presentations at universities, 14 invited presentations at other private or public institutions, more than 31 conference presentations. The funded research has resulted in 22 peer-reviewed publications (including 3 in review), 44 conference abstracts, 3 Ph.D. theses, and 2 Masters theses. Results were also widely disseminated to the public through various popular media outlets, the PI’s professional website (http://nuweb2.neu.edu/rieslab), and the BCO-DMO website (https://www.bco-dmo.org/project/562814, https://www.bco-dmo.org/award/562813). Last Modified: 01/16/2021 Submitted by: Justin B Ries