Coral energy reserves and calcification in a high-CO2 world at two temperatures

Abstract

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.

Figure 1. Photos of representative coral fragments from (a) Acropora millepora, (b) Pocillopora damicornis, (c) Montipora monasteriata, and (d) Turbinaria reniformis.

Rectangles indicate subsamples taken from each fragment for lipid, protein/carbohydrate, and tissue biomass analyses. The remaining tissue was airbrushed for chlorophyll a and endosymbiont density measurements.

Figure 2. Average daily calcification rate during the first and the second half of the experiment for (a, b) Acropora millepora, (c, d) Pocillopora damicornis, (e, f) Montipora monasteriata, and (g, h) Turbinaria reniformis.

Averages ± 1 SE are shown for three pCO₂ levels and two temperature regimes (26.5, 29.0°C). Asterisks indicate significant differences between 26.5 and 29.0°C within a given pCO₂ level (determined by a posteriori slice tests). The letters a and b indicate results of the post hoc Tukey tests when there was a significant pCO₂ effect. Sample sizes ranged between 5 and 6. Statistical details can be found in Table S1.

Figure 3. Average chlorophyll a concentrations and symbiont density for (a, b) Acropora millepora, (c, d) Pocillopora damicornis, (e, f) Montipora monasteriata, and (g, h) Turbinaria reniformis.

Averages ± 1 SE are shown for three pCO₂ levels and two temperature regimes (26.5, 29.0°C). Asterisks indicate significant differences between 26.5 and 29.0°C within a specific pCO₂ level (determined by a posteriori slice tests). The letters a and b indicate results of the post hoc Tukey tests when there was a significant pCO₂ effect. Sample sizes ranged between 5 and 6. Statistical details can be found in Table S2.

Figure 4. Average lipid, protein, carbohydrate concentrations, and tissue biomass of (a–d) Acropora millepora, (e–h) Pocillopora damicornis, (i–l) Montipora monasteriata, and (m–p) Turbinaria reniformis.

Averages ± 1 SE are shown for three pCO₂ levels and two temperature regimes (26.5, 29.0°C). Asterisks indicate significant differences between 26.5 and 29.0°C within a specific pCO₂ level (determined by a posteriori slice tests). The letters a and b indicate results of the post hoc Tukey tests when there was a significant pCO₂ effect. Sample sizes ranged between 4 and 6. Statistical details can be found in Table S3.