Responses of branching reef corals Acropora digitifera and Montipora digitata to elevated temperature and pCO₂

Abstract

Anthropogenic emission of CO2 into the atmosphere has been increasing exponentially, causing ocean acidification (OA) and ocean warming (OW). The “business-as-usual” scenario predicts that the atmospheric concentration of CO2 may exceed 1,000 µatm and seawater temperature may increase by up to 3 °C by the end of the 21st century. Increases in OA and OW may negatively affect the growth and survival of reef corals. In the present study, we separately examined the effects of OW and OA on the corals Acropora digitifera and Montipora digitata, which are dominant coral species occurring along the Ryukyu Archipelago, Japan, at three temperatures (28 °C, 30 °C, and 32 °C) and following four pCO2 treatments (400, 600, 800, and 1,000 µatm) in aquarium experiments. In the OW experiment, the calcification rate (p = 0.02), endosymbiont density, and maximum photosynthetic efficiency (Fv/Fm) (both p < 0.0001) decreased significantly at the highest temperature (32 °C) compared to those at the lower temperatures (28 °C and 30 °C) in both species. In the OA experiment, the calcification rate decreased significantly as pCO2 increased (p < 0.0001), whereas endosymbiont density, chlorophyll content, and Fv/Fm were not affected. The calcification rate of A. digitifera showed greater decreases from 30 °C to 32 °C than that of M. digitata. The calcification of the two species responded differently to OW and OA. These results suggest that A. digitifera is more sensitive to OW than M. digitata, whereas M. digitata is more sensitive to OA. Thus, differences in the sensitivity of the two coral species to OW and OA might be attributed to differences in the endosymbiont species and high calcification rates, respectively.

Individual and Interactive Effects of Ocean Warming and Acidification on Adult Favites colemani

“…This enabled coral explants to recover from handling stress and acclimate to the conditions in the experiment control tanks. Similar to other studies, coral explants had positive growth, no mortality and no bleaching under relatively lower light intensity than the natural environment (Ohki et al, 2013;Kavousi et al, 2015;Da-Anoy et al, 2019;Manullang et al, 2020).…”

Humic substances mitigate adverse effects of elevated temperature with potentially critical repercussions for coral reef resilience

“…In the present study, we found that elevated temperatures were associated with significantly lower Fv/Fm ratios in the hard corals M. digitata and M. capricornis, but not in the soft coral S. glaucum. The results for both hard coral species confirm previous laboratory studies and in situ observations (Higuchi et al, 2013;Manullang et al, 2020) and suggest that the photosynthetic machinery of the coral symbionts were damaged by the high-temperature treatment (Downs et al, 2002). In contrast to our results, another study found a significant reduction in Fv/Fm ratios in S. glaucum, following exposure to a laboratory-induced heatwave (Travesso et al, 2023).…”

Near-future levels of <i>p</i>CO₂ impact skeletal weights of coral primary polyps (<i>Acropora digitifera</i>)

“…We used this established system called AICAL (Acidification Impact on CALcifiers). AICAL system measures the pCO 2 of the seawater and adjusted the pCO 2 by controlling a feedback loop to achieve the desired pCO 2 level (Manullang et al 2020). This system has built in pCO 2 analyzer (non-dispersive infrared (NDIR) analyzer) and recorded pCO 2 every 90 minutes.…”