Variable photosystem II thermal stress responses of reef-building corals Pocillopora indiania and Heliopora coerulea across latitudes from the Mascarene Plateau, Indian Ocean

The persistence of remote reef sites invariably depends on the ability of their corals to better respond and adapt to a continuing global environmental change, by way of their protection from additional and direct stressors as a result of their remoteness; such reefs are generally referred to as coral refugia. This study investigated the latitudinal thermal thresholds of two commonly occurring corals from reefs both remote from and close to human population centres, in the understudied Mascarene Plateau (MP), Central Indian Ocean. Thermal experimental trials at 26, 29 and 32 °C for a period of 22h focused on reef-building conspecifics: the regionally-endemic hard coral Pocillopora indiania, and heat-tolerant photo-physiologically understudied and threatened octocoral Heliopora coerulea from three sites of varying degrees of remoteness and human impact (Saya de Malha (SDM), St Brandon (SB) and Mauritius Island (MAU)), each with varying mean sea surface temperature (SST). Imaging Pulse-Amplitude-Modulation (Imaging-PAM) fluorometry was used to determine how the photosynthetic performances at the Photosystem II (PSII), in terms of effective quantum yield (Φ_PSII), of the conspecifics responded after exposure to these different temperature regimes. SST anomaly over the past 28 years showed an increment of 1.2 °C with no significant latitudinal variation within the MP. Moreover, the experimental results displayed significant differences in intra- and interspecific Φ_PSII when corals were exposed to 32 °C (p < 0.0001); P. indiania from MAU suffered severe bleaching (over 90% surface area), with the highest degree of tissue sloughing leading to coral mortality, while conspecifics from SDM and SB exhibited better thermal threshold by being photosynthetically active, with 23% and 33% Φ_PSII, respectively. In contrast, H. coerulea from MAU thrived through the experiment indicating site-specific thermal threshold by displaying the lowest Φ_PSII (24%) as compared to conspecific from SDM (48%) and SB (58%). Although limited to Photosystem II responses, this study provides a first insight on the latitudinal variability in thermal vulnerability of conspecific corals from the MP and suggesting that corals from SDM and SB might be more resilient to the impacts of future warming events than corals at other sites.