Environmental variability shapes productivity and thermal responses of a free-living coralline alga

Free-living coralline algae are key foundation species of worldwide distributed coralline algal beds, habitats recognized as biodiversity hotspots and significant sites of carbonate production. Among them, Phymatolithon lusitanicum forms extensive beds along the southern coast of Portugal, where frequent upwelling-downwelling alternations create a highly dynamic environment. These oceanographic shifts drive rapid changes in seawater temperature and light, with downwelling associated with higher values and upwelling with lower ones. This variability is likely to influence algal physiological performance and resilience to marine heatwaves (MHWs), which in this region often coincide with intense downwelling. Here, we examined the physiological responses of P. lusitanicum to simulated fluctuations in temperature and irradiance, with and without the superimposition of a MHW event, through a mesocosm experiment that realistically simulated natural fluctuations in temperature and light. Physiological responses (photosynthesis, respiration and calcification) were measured during upwelling, downwelling, and transitional periods. Phymatholithon lusitanicum showed considerable phenotypic plasticity, with physiological responses closely tracking environmental variability. Metabolic activity increased during downwelling, in terms of both primary and carbonate productivity, whereas upwelling periods led to negative net primary productivity and declines in calcification. Superimposing a MHW on a downwelling period caused no significant adverse effects, indicating high thermal tolerance. These results emphasize the dual role of environmental variability in regulating algal productivity and fostering resilience to extreme warming events, while highlighting the potential for strong negative impacts of cold spells, which represent the extreme end of upwelling regimes and occur as frequently as MHWs in the study region.