Combined Salinity and Nano-TiO2 Stress in Posidonia oceanica and Caulerpa prolifera: Ecophysiological Responses and Recovery

This study investigated the combined effects of climate change-related salinity extremes and nanoparticle pollution on the seagrass Posidonia oceanica and the macroalga Caulerpa prolifera. Both species were exposed, individually and in co-occurrence, to different salinity regimes (34; 38 and 42 g kg−1) and to the emerging contaminant nano-TiO2 (0.7 mg L−1, environmentally relevant concentration, and 5.0 mg L−1, high-stress exposure). Biochemical and physiological responses were assessed at baseline (T0) and after 3, 6, and 12 days of exposure, followed by a 12-day recovery phase to evaluate post-stress resilience. This multifactorial design enabled the evaluation of interactive and cumulative effects of salinity shifts associated with climate change and nanoparticle contamination. Results showed that P. oceanica was particularly sensitive to nano-TiO2 at a concentration of42 g kg−1. Reduced photosynthetic performance was associated with enhanced oxidative stress and limited recovery capacity, suggesting potential long-term impacts on meadow persistence and ecosystem functioning. In contrast, C. prolifera exhibited higher tolerance and recovery efficiency, potentially gaining a competitive advantage under climate-induced environmental variability and increasing the risk of seagrass decline and community shifts in coastal ecosystems. These biochemical markers of early stress do not necessarily reflect direct population effects, particularly in long-lived foundation species such as Posidonia oceanica.