Heating rate modulates the metabolic response of the staghorn coral Acropora cervicornis (Lamarck, 1816)

Corals are relatively stenothermic organisms and highly sensitive to thermal stress. To understand how heating rate modulates the holobiont response to thermal challenge, we compared the effects of acute heat shock and cumulative thermal exposures on Acropora cervicornis from Southeast Florida (26 degrees 06 N, 80 degrees 05 W). In March of 2017, maximum dark-adapted yield of photosystem II (F-V/F-M), rates of dark respiration (R), and rates of gross photosynthesis (P-g) were measured at temperatures spanning 25-36 degrees C. Thermal performance curves of each response variable were constructed as a function of temperature and thermal dose (i.e., degree heating minutes). Acute exposure (i.e., instantaneously increasing temperature by between 2.5 and 10 degrees C) resulted in a more marked decline in physiology despite a lower thermal dose, compared to the cumulative exposure (2.5 degrees C h(-1)). Apparent deficits in coral holobiont metabolism were observed as a function of both temperature and thermal dose, but examination of holobiont metabolism in a dose-context revealed more pronounced differences between acute and cumulative exposures. Subtle physiological differences may be more easily identified and better cross-study comparisons of cnidarian thermal tolerance may be achieved by placing temperature exposures in degree heating times. In addition, the ratio of daily P-g:R fell below 1 when temperatures exceeded 32 degrees C, corroborating prior observations that P-g:R may be a means of identifying physiological deficits in advance of visual signs of dysbiosis, such as bleaching.

Automated summary

The study found that acute heat exposure leads to a more pronounced physiological decline in corals despite lower thermal doses, while placing temperature exposures in degree heating times can better compare differences in physiological metabolism. When temperatures exceed 32 degrees Celsius, the ratio of daily gross photosynthesis to respiration fell below 1 in corals.