Physiological acclimation of Porites panamensis (Scleractinia: Poritidae) under high-latitude marginal conditions

Hermatypic corals living at high latitudes face suboptimal environmental conditions associated with seasonal changes. In the central Gulf of California, the coral Porites panamensis is acclimated to eutrophication, low light availability, and a wide range of seasonal fluctuations in sea surface temperature (SST). The physiological adjustments of its resistance thresholds are associated with phenotypic plasticity. This study evaluated the interannual acclimation responses of P. panamensis to the warm and cold seasons of 2022 and 2023 using the physiological markers of endosymbiont density, chlorophyll a (Chl a) concentration, and the total lipid content in coral tissue. In addition, the abiotic variables of SST, Chl a, particulate organic carbon (POC), and the diffuse attenuation coefficient (Kd490) were compared between seasons. The results indicated a significant difference in endosymbiont density between seasons (cold season: ~4 × 106 cell·cm–2; warm season: ~2 × 106 cell·cm–2), and an increase in the Chl a concentration during the warm season of 2023. We also observed a significant increase in total lipid content in the warm season of 2023. However, seasonal changes did not negatively affect lipid content, likely due to the high concentrations of Chl a and POC throughout the year (2022: 4.47 ± 1.75 mg·m–3; 2023: 403.3 ± 132.2 mg·m–3), suggesting the existence of a potential year-round food source for P. panamensis. Our results indicate that P. panamensis acclimates to seasonal changes in temperature and turbidity. We suggest that regulating mixotrophy could be a key nutritional strategy for P. panamensis to withstand fluctuating environmental conditions. The ability to alternate between different nutritional pathways according to seasonal environmental conditions may allow P. panamensis to distribute throughout the Eastern Tropical Pacific, even inhabiting suboptimal regions for reef development.