Physiological response of the endemic Mediterranean seagrass, Posidonia oceanica to multiple stresses

The endemic Mediterranean marine angiosperm, Posidonia oceanica , stands out for being a biostructuring seagrass species with a great importance in trophic relations and coastal erosion protection. This study aimed to understand the physiological response of this emblematic seagrass to multiple stresses, with an emphasis on evaluating the acclimatory capacity of juvenile leaves to low temperatures during winter in three sites with a contrasting exposure to heavy metals of anthropogenic origin in the Mediterranean coast near Cadaqués (NE Spain). Heavy metal concentrations together with multiple physiological stress markers, including the capacity of photo- and antioxidant protection and the extent of lipid peroxidation, were evaluated. Shoots from the three studied sites accumulated foliar concentrations of heavy metals that were higher than reference threshold values, especially for Cu, which showed a decreasing concentration gradient from the coast to the open sea. The maximum photochemical efficiency of Photosystem II (PSII) ( F v / F m ratio) of juvenile leaves was sensitive to low temperatures, despite values attained did not indicate damage to PSII, which was confirmed with studies of recovery of the F v / F m ratio and the absence of symptoms of photo-oxidative stress. However, the presence of heavy metals seemed to negatively influence PSII photochemistry, as sites with the highest Cu concentrations also showed the lowest F v / F m ratios during spring, even though water temperature had already warmed. F v / F m values attained were not recoverable after an extended dark acclimation period, thus indicating damage to PSII. Heavy metals did not seem to exert a negative synergistic effect with low temperatures but rather affected the physiological performance of newly produced juvenile leaves during spring. Despite the great capacity of this species to adapt to multiple stresses, results underscore the need to urgently reduce the current human footprint on seagrass meadows in the Mediterranean basin. • Seagrass meadows in NE Spain contained Cu, Zn, and Mo at higher levels than reference threshold values. • The high concentrations of heavy metals coincided with photoinhibition of the photosynthetic apparatus. • Despite the great resilience of P. oceanica to abiotic stress, heavy metals negatively influence performance. • Contaminants have negative consequences to seagrass meadows in Mediterranean-type ecosystems.