Withstanding the heat: Resilience of free-living coralline algae to marine heatwaves

Marine heatwaves (MHWs) can severely impact benthic ecosystems, driving major ecological shifts. As they become longer, more frequent, and intense, MHWs are emerging as a dominant threat to marine biodiversity. Yet, their potential effects on coralline algal beds (CABs, including rhodolith beds), habitats of critical ecological importance, remain largely unknown. This study investigated the physiological responses of three rhodolith species (Lithothamnion cf. valens, Lithothamnion sp., Lithophyllum incrustans), which dominate the community of a Mediterranean CAB, to an experimentally simulated ecologically relevant scenario of two consecutive summer MHWs. Species responses in key physiological processes, photosynthesis, respiration, and calcification, were assessed before, during, and after the heatwave events. Overall, temperature increases had minor effects on photosynthesis and respiration, while calcification responses to MHWs were more pronounced and species-specific. Significant declines were observed in light calcification of L. incrustans during the first MHW, and in dark calcification of all three species, with L. incrustans and L. cf. valens responding during the first and second MHWs, respectively. Lithothamnion sp. exhibited some minor effects on light calcification and a significant shift toward carbonate dissolution in darkness during the first MHW, which reversed over time. Despite these effects, all species fully recovered by the end of the experiment, and the stronger impact of the first MHW suggests a potential priming effect that may enhance tolerance to subsequent heat events. These findings demonstrate rhodoliths' overall capacity to withstand MHWs, while highlighting the role of species-specific responses in shaping the net carbonate balance of CABs under future climate scenarios.