Photoacclimation and photoregulation strategies of <i>Corallina</i> (Corallinales, Rhodophyta) across the NE Atlantic

This study characterizes the photoacclimation and photoregulation mechanisms that allow calcified macroalgae of the genus <i>Corallina</i> (Corallinales, Rhodophyta) to dominate rock pool habitats across the NE Atlantic despite the highly variable irradiance regimes experienced. Rapid light curves (RLCs) were performed with pulse amplitude modulation (PAM) fluorometry <i>in situ</i> across a full seasonal cycle in the UK intertidal with <i>C. officinalis</i> and <i>C. caespitosa</i>. Latitudinal comparisons were performed across the full extent of <i>C. officinalis</i>’ range in the NE Atlantic (Iceland–northern Spain), and for <i>C. caespitosa</i> in northern Spain. <i>Ex situ</i> RLCs with dark recovery were further employed to assess the optimal, as compared with actual, photophysiology across seasons and latitudes. <i>Corallina</i> species were shown to photoacclimate at seasonal timescales to changing irradiance, increasing light-harvesting during low-light autumn/winter periods and protecting photosystems during high-light summer conditions. Seasonal photoacclimation was achieved through alteration in the number of photosystem (PS) units (PSII and light harvesting antennae) over time. Non-photochemical quenching (NPQ) served as an important photoregulation mechanism utilized by <i>Corallina</i> to prevent or minimize photoinhibition over shorter time scales (seconds–hours), though the efficiency of NPQ was dependent on the seasonal-acclimated state. With increasing latitude the efficiency of photoregulation decreased, representing potential differential photoadaptation of <i>Corallina</i> across species ranges in the NE Atlantic. In contrast, highly conserved inter-specific patterns in photophysiological responses to irradiance were apparent. This study demonstrates the photophysiological mechanisms allowing <i>Corallina</i> to optimize use of the variable irradiance conditions apparent in rock pool environments, when and how they are employed, and their limitations.