Description: Abiotic data were collected to assess the environmental conditions that coincided with the 2016 mass bleaching event documented at Shell island (Shenton Bluff), Cygnet Bay, Kimberley region, northwestern Australia. Temperature is of particular importance because heat stress is one of the key drivers of coral bleaching. Water temperature was recorded every 15 min by HOBO U22 v2 temperature loggers (±0.2°C) in both intertidal and subtidal environments from September 2015 until October 2016.

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: Abiotic data were collected to assess the environmental conditions that coincided with the 2016 mass bleaching event documented at Shell island (Shenton Bluff), Cygnet Bay, Kimberley region, northwestern Australia. Water level was recorded in both the intertidal and subtidal reef zone since this region has the world's largest tropical tides, with ~ 8m tidal range at the study site. Thus, intertidal corals regularly get exposed to air during low tide. Water level was monitored continuously from September 2015 to October 2016 at both sites using HOBO U20-001-02-Ti water level loggers (±0.05%) and RBR virtuoso water level loggers (±0.05%). Temporal resolution varied from 5 to 30 min.

Description: Coral bleaching surveys were conducted to assess how a coral reef in the Kimberley region, NW Australia, recovered from the 2016 mass bleaching event documented at this location. The surveys were conducted at Shell island (Shenton Bluff), Cygnet Bay, in both the intertidal and subtidal reef zone. To quantify coral recovery and mortality, reef-wide coral health surveys were conducted at Shell Island 6 months after peak bleaching from 18 to 21 October 2016 using the same methods that were used by Le Nohaïc et al. (2017, doi: 10.1038/s41598-017-14794-y) to assess coral health prior to and during peak bleaching (January 13–17 and April 6–9, 2016, respectively). Surveys were conducted along six randomly positioned, 15 m transects. High-resolution photos of a 50-cm x 50-cm quadrat were taken every 0.5–1 m along the transect line. Photo-quadrats were analyzed using the software (Trygonis and Sini, 2012; doi: 10.1016/j.jembe.2012.04.018). All species of hard corals encountered in the photo-quadrats were scored using the following four health categories as a categorical bleaching score (McClanahan et al., 2004; doi: 10.1016/j.marpolbul.2003.08.024): unbleached/healthy (H), moderately bleached (M: 〈50% of the colony bleached or colony pale), severely bleached (S: 〉50% bleached), and dead (D).

Description: Rising atmospheric CO2 concentrations threaten coral reefs globally by causing ocean acidification (OA) and warming. Yet, the combined effects of elevated pCO2 and temperature on coral physiology and resilience remain poorly understood. While coral calcification and energy reserves are important health indicators, no studies to date have measured energy reserve pools (i.e., lipid, protein, and carbohydrate) together with calcification under OA conditions under different temperature scenarios. Four coral species, Acropora millepora, Montipora monasteriata, Pocillopora damicornis, Turbinaria reniformis, were reared under a total of six conditions for 3.5 weeks, representing three pCO2 levels (382, 607, 741 µatm), and two temperature regimes (26.5, 29.0°C) within each pCO2 level. After one month under experimental conditions, only A. millepora decreased calcification (-53%) in response to seawater pCO2 expected by the end of this century, whereas the other three species maintained calcification rates even when both pCO2 and temperature were elevated. Coral energy reserves showed mixed responses to elevated pCO2 and temperature, and were either unaffected or displayed nonlinear responses with both the lowest and highest concentrations often observed at the mid-pCO2 level of 607 µatm. Biweekly feeding may have helped corals maintain calcification rates and energy reserves under these conditions. Temperature often modulated the response of many aspects of coral physiology to OA, and both mitigated and worsened pCO2 effects. This demonstrates for the first time that coral energy reserves are generally not metabolized to sustain calcification under OA, which has important implications for coral health and bleaching resilience in a high-CO2 world. Overall, these findings suggest that some corals could be more resistant to simultaneously warming and acidifying oceans than previously expected.

Description: High-latitude coral reefs provide natural laboratories for investigating the mechanisms and limits of coral calcification. While the calcification processes of tropical corals have been studied intensively, little is known about how their temperate counterparts grow under much lower temperature and light conditions. Here, we report the results of a long-term (2-year) study of seasonal changes in calcification rates, photo-physiology and calcifying fluid (cf) chemistry (using boron isotope systematics and Raman spectroscopy) for the coral Turbinaria reniformis growing near its latitudinal limits (34.5° S) along the southern coast of Western Australia. In contrast with tropical corals, calcification rates were found to be threefold higher during winter (16 to 17° C) compared with summer (approx. 21° C), and negatively correlated with light, but lacking any correlation with temperature. These unexpected findings are attributed to a combination of higher chlorophyll a, and hence increased heterotrophy during winter compared with summer, together with the corals' ability to seasonally modulate pHcf, with carbonate ion concentration [CO32-]cf being the main controller of calcification rates. Conversely, calcium ion concentration [Ca2+]cf declined with increasing calcification rates, resulting in aragonite saturation states Ωcf that were stable yet elevated fourfold above seawater values. Our results show that corals growing near their latitudinal limits exert strong physiological control over their cf in order to maintain year-round calcification rates that are insensitive to the unfavourable temperature regimes typical of high-latitude reefs.

Description: The physiological response to individual and combined stressors of elevated temperature and pCO2 were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15, and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein, and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins, and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.

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.