The tropical corals’ pH microenvironment examined under changing seawater pCO2 conditions

Our current understanding of how corals calcify and maintain calcification suggests that internal pH regulation is a critical step and strongly controlled. Here we examined changes in the corals’ internal (calcifying fluid between tissue and skeleton and within the tissue) and external (diffuse boundary layer) microenvironment using microsensors. Galaxea spp. corals cultured under present-day conditions were consecutively exposed to two elevated pCO₂ conditions (800 µatm and 1300 µatm), and instantaneous changes in their internal and external environment monitored. The pH within the diffusive boundary layer decreased immediately with increasing seawater pCO_2. In contrast, the change in internal pH was not always observed. In cases where internal pH decreased, this change was delayed and often corals up-regulated the internal pH to remain similar to internal pH values measured under present-day pCO₂. Even after 4 weeks of incubation at 3 different pCO₂ levels (present-day, 800 µatm and 1300 µatm), illuminated corals maintained a high internal pH irrespective of culturing conditions. In the dark, the internal pH was always slightly lower than the external seawater pH. Dark calcification decreased from 0.18 µM cm^-2 h^-1 (22% of light calcification) at present-day conditions to zero under the highest pCO₂ condition. We conclude that the external pH together with the capacity for internal pH regulation as well as the strong differences in dark and light internal pH microenvironment will affect the corals’ response to future elevated pCO₂ conditions. An improved understanding of differences in light and dark pH microenvironment as well as their effects on the holobionts’ metabolic processes (photosynthesis, respiration, calcification) is critical to better predict the responses of tropical corals to a future high CO₂ world.