Tolerance of coralline algae to ocean warming and marine heatwaves

Ocean warming (OW) and marine heatwaves (MHWs) rapidly transform marine ecosystems, especially when they impact keystone or foundation species. Foundation species such as kelps, fucoids and corals are highly sensitive to heat stress, which threatens the future of temperate seaweed forests and tropical reefs. However, functioning and resilience of these systems also rely on the less conspicuous coralline algae, whose thermal tolerances have gone largely untested. Here, we examined the sensitivity of four temperate coralline algal morphotypes from three different species to four realistic present-day and future OW and MHW scenarios (ambient [16°C constant]; ambient+MHW [16°C baseline + a symmetric two-week heatwave with a peak intensity of 18.7°C]; future [18.7°C constant]; future+MHW [18.7°C baseline + a symmetric two-week heatwave with a peak intensity of 21.4°C]). Photo-physiology (e.g., Fv/Fm) and calcification physiology (e.g., proxies for calcifying fluid saturation state Ω CF ) were generally unaffected by the treatments, implying a high thermo-tolerance of our study species compared to other important marine foundation species. We ascribe this mainly to their photosynthetic apparatus that, unlike in other photoautotrophs, continued to function under heat stress. Experimental evidence presented here and elsewhere implies that coralline algae are likely to continue to play their crucial ecological roles in a warming ocean. Yet, such predictions are fraught with uncertainty due to the substantial gaps in our knowledge. We attempt to highlight some of these gaps and aim to present potential physiological underpinnings of their thermo-tolerance.