Donadio, R.; Pica, ML.; Vitale, E.; Costanzo, G.; Colletti, A.; Fabrizzi, E.; Fraschetti, S.; Arena, C. Assessing the resilience of the coralline macroalgae Ellisolandia elongata to high irradiances and desiccation in natural habitat. 2nd Congress on Marine Evolution. r-Evolutionary Routes in the Sea. 14-17 November 2023

Aim Calcified algae are less common or absent in intertidal regions because they are particularly vulnerable to desiccation. Among corallinaceae, Ellisolandia elongata (J. Ellis & Solander) inhabits intertidal rock pools where large fluctuations in physic-chemical variables occur daily and represent a suitable model to study the capability of this species to cope with environmental constraints. To pursue this aim, individuals of E. elongata were sampled in the Marine Protected Area of the Gaiola Underwater Park (Gulf of Naples, Italy) during a prolonged low tide event occurring in early February to assess the photosynthetic efficiency and the occurrence of stress-defense ecological strategies in both submerged and exposed thalli. Methods Photosynthetic-light curves (LRC) were performed in vivo in exposed semi-bleached and submerged thalli utilizing a pulse amplitude-modulated fluorometer (Junior-PAM, Walz Gmbh, Effeltrich, Germany). On samples, the expression of the D1 protein of photosystem II, photosynthetic pigments, and antioxidant activity were also evaluated to study the adaptive response of E. elongata photosynthetic apparatus. Results At over-saturating irradiances, the prolonged low tide event determined, surprisingly, an increase of photosynthetic efficiency and a decline of non-photochemical thermal dissipation processes in exposed compared to submerged thalli together with a reduction of maximum photochemical efficiency, D1 protein expression, photosynthetic pigment content, and antioxidant activity. Main conclusion The prolonged exposure of submerged thalli to multiple stresses (i.e., high irradiance and desiccation) may have triggered a regulation strategy by which photosystem II is down-regulated to avoid photoinhibition of photosynthetic apparatus. The decreased antioxidant activity confirmed scavenger molecules' crucial role in lowering oxidative stress at the cellular level, contributing to photoprotection in E. elongata.