Population differentiation of thermal physiology in the temperate coral Astrangia poculata

Abstract

Understanding the evolutionary and physiological mechanisms that drive thermal tolerance is crucial to supporting coral conservation and restoration. In the face of climate change, organisms can cope with variable environmental landscapes through adaptive genetic differentiation and phenotypic plasticity. In this study, we utilized a factorial common garden experiment to assess stress tolerance and thermal plasticity of symbiotic colonies of the temperate coral, Astrangia poculata. We collected coral colonies from sites in Massachusetts, Rhode Island, and North Carolina, representing a 7.5° latitudinal range and a 10 °C range in mean summer seawater temperatures. We measured how acclimation of the corals to temperatures representative of summer means at their origin site versus other latitudes impacted basal rates of cellular respiration and photosynthesis, as well as stress indicators including endosymbiont density, chlorophyll fluorescence (F_v/F_m), and chlorophyll a content. We found evidence of heat stress at 32 °C in photosynthetic endosymbionts from all populations, especially those from North Carolina. Metabolic performance of the coral holobiont appears optimized for native conditions (28 °C) in NC populations but appears to be plastic in RI corals, as incubation in warmer than native conditions (28 °C) led to elevated respiration in RI corals. Metabolic performance does not differ between acclimation treatments in MA populations, suggesting both population-specific responses and differential plasticity of metabolic responses to temperature changes. Overall, we see some minimal support for locally adapted thermal maxima in these populations and some evidence of thermal plasticity. Continued work to understand the ecological repercussions of a high capacity for phenotypic plasticity will be important to understanding the long-term tolerance of this species to elevated temperatures predicted with climate change.