Coral reefs are extremely vulnerable to climate change and suffer as ocean temperatures rise. One common sign of an unhealthy coral reef is coral bleaching. Bleaching happens when algal symbionts flee the coral host after experiencing stress, which can be deadly to the coral.
But bleaching may not be the only consequence of climate change on coral health. Other cellular processes, such as metabolism and pH balance (also called acid-base homeostasis), can deteriorate as organisms increase their defenses against high temperatures. The balance of cellular processes is critical for long-term health, and these processes in corals may be important indicators of worsening coral function in response to climate change.
Biologists from the University of Pennsylvania recently studied bleaching and cellular process changes in corals in Hawai'i during marine heat waves. In a 2015 heat wave, they characterized the corals as either “bleaching resistant,” where the corals did not bleach during the heat wave, or “bleaching susceptible,” where the corals did bleach. During a 2019 heat wave, they also measured metabolism and acid-base homeostasis in both coral types.
The responses of the two coral types to stress were consistent across the heat waves, meaning bleaching resistant corals in 2015 were also resistant in 2019, and bleaching susceptible corals bleached during both heat waves. Surprisingly, they found that both bleaching resistant and bleaching susceptible corals experienced metabolic depression during the 2019 heat wave. Metabolic depression occurs when animals lower their metabolic rate to conserve energy, often in response to stress. Both coral types also had imbalances in acid-base homeostasis and could not return to normal pH levels after encountering low pH conditions.
Even though the bleaching resistant corals appeared to be healthy, all corals suffered on the cellular level during the heat wave. These changes in cellular processes, over time, can lead to poor coral health and can make corals more vulnerable to future climate changes, especially marine heat waves. This research warns that only studying coral bleaching may lead us to underestimate the impacts of climate change on coral reefs, and that the damage may be more severe than previously understood.