Description: Coral tissue chlorophyll a concentrations were measured to assess how corals in the Kimberley region, NW Australia, were impacted by and recovered from the 2016 mass bleaching event documented at this location. The corals were collected at Shell island (Shenton Bluff), Cygnet Bay, in both the intertidal and subtidal reef zone. Tissue samples were collected from tagged colonies of the dominant coral species at this location, Acropora aspera, in April 2016 (peak bleaching) and 7 months after peak bleaching in October 2016. The health status of all tagged colonies was assessed in April 2016 and after 7 months of recovery in November 2016 using the Coral Watch Coral Health Chart where a change of two units in brightness indicates a significant change in symbiont density and chlorophyll a content (Siebeck et al., 2006). Colonies were considered either “healthy” (brightness scale, 3.6–6) or “bleached” (brightness, 1–3.5). Corals were stored at -80°C prior to processing. To quantify bleaching, chlorophyll a concentration was determined spectrophotometrically (Jeffrey and Humphrey, 1975; doi:10.1016/S0015-3796(17)30778-3) and used as a proxy for bleaching susceptibility. Tissue was removed from a branch tip using an airbrush and separated into animal and symbiont fraction via centrifugation (2 x 10 min at 3,000 g). Chlorophyll a from the symbiont fraction was extracted in 100% acetone in the dark at 4°C for 24 h and the concentration determined spectrophotometrically (Jeffrey and Humphrey, 1975) and then standardized to surface area. Surface area was calculated using the relationship between skeletal mass (x, in g) and the respective computer tomography (CT)- determined surface area (y, in cm2) of A. aspera skeletons from our study site (y = 9.4871x0.7729, n = 6, R2 = 0.99).

Description: Ocean acidification (OA) is a pressing threat to reef-building corals, but it remains poorly understood how coral calcification is inhibited by OA and whether corals could acclimatize and/or adapt to OA. Using a novel geochemical approach, we reconstructed the carbonate chemistry of the calcifying fluid in two coral species using both a pH and dissolved inorganic carbon (DIC) proxy (delta 11B and B/Ca, respectively). To address the potential for adaptive responses, both species were collected from two sites spanning a natural gradient in seawater pH and temperature, and then subjected to three pHT levels (8.04, 7.88, 7.71) crossed by two temperatures (control, +1.5°C) for 14 weeks. Corals from the site with naturally lower seawater pH calcified faster and maintained growth better under simulated OA than corals from the higher-pH site. This ability was consistently linked to higher pH yet lower DIC values in the calcifying fluid, suggesting that these differences are the result of long-term acclimatization and/or local adaptation to naturally lower seawater pH. Nevertheless, all corals elevated both pH and DIC significantly over seawater values, even under OA. This implies that high pH upregulation combined with moderate levels of DIC upregulation promote resistance and adaptive responses of coral calcification to OA.