Photosynthetic and growth acclimation strategies of Ulva lacinulata to eutrophic Thessaloniki Bay (Greece): a multilevel, integrated analysis

Ulva lacinulata constitutes a critical yet often overlooked component of Mediterranean eutrophicated coasts. Investigating its physiological responses to weather events may enable environmental management and aquaculture applications. Field sampled U. lacinulata, a subtropical to water-temperate species, from the eutrophic Thessaloniki Bay, Greece, and studied it under laboratory conditions. Field seawater variables were correlated with biochemical (photopigments), biophysical (JIP-test), physiological (Rapid Light Curves-RLC), and organismal (Relative Growth Rate-RGR) responses using multivariate analysis (RDA) combined with Monte Carlo permutation tests. Temperature, salinity, and nutrients, were the main drivers of photosynthesis and growth (p = 0.001), in that order, enabling U. lacinulata to acclimate and survive year-round. Photosynthetic acclimation, decoupled from growth, and primarily influenced by extreme heat, rainfall, and cold/warm thermal cycles, represented the dominant strategy for survival. Overall, the maximum quantum yield of PSII photochemistry (Fv/Fm) was sustained near 0.7, indicating a potentially efficient photosynthetic apparatus. Despite moderate irradiance and relatively low temperatures in early spring 2021, plants achieved their highest RGR. This was attributed to high nitrogen availability, especially ammonium, and an acclimation mechanism in which total irradiance capture was enhanced via increased chlorophyll α and b, and β-carotene synthesis. Such a result explains the unimodal growth pattern of U. lacinulata, in which biomass accumulation begins in winter or early spring as water temperatures rise and light availability increases, culminating at a peak usually in late spring or early summer. This study demonstrated U. lacinulata’s high potential for photosynthesis and growth acclimation, positioning it as a promising candidate for biomass management and cultivation in Mediterranean eutrophic waters.