Assessing the resilience of the coralline macroalga Ellisolandia elongata in response to a prolonged low tide

Ellisolandia elongata, thriving in the low rocky shore intertidal zone, is an excellent model for investigating how coralline species handle multiple stressors. This study explores the adaptive mechanisms of an E. elongata population subjected to an unusual low tide event lasting about eleven days. We analyze structural and functional traits in submerged (S), exposed (E), and re-submerged (R) thalli after a prolonged low tide to identify the resilience strategy of this species. The extended low tide resulted in bleaching in 74 % of the local exposed population. Compared to S and R thalli, photosynthetic efficiency (quantum yield of PSII electron transport and maximum PSII photochemical efficiency) decreased in the E thalli, as well as the PSII-D1 protein and the photosynthetic pigment content. Conversely no difference in non-photochemical quenching was observed. The E thalli also exhibited lower concentrations of carbohydrates and antioxidant compounds (polyphenols, flavonoids, tannins, and overall antioxidant activity), suggesting the occurrence of oxidative stress that likely required the consumption of antioxidants for free radical scavenging. In the R thalli, photosynthetic activity, antioxidant levels, and pigment content recovered, indicating that E. elongata maintains its physiological integrity and photosystem functionality during desiccation by down-regulation of PSII photochemical efficiency and reduction of D1 protein and pigment synthesis. To face multiple stressors, antioxidant compounds are activated, enabling photosynthesis recovery after disturbance. Our results highlight the species' adaptability to tidal excursions and can help predict future changes in its distribution patterns, particularly under global warming scenarios.