Sunscreen-derived nano-TiO2 undermines viability and recruitment of the Mediterranean foundation alga Ericaria amentacea

Ericaria amentacea is a brown alga endemic to the Mediterranean Sea and is highly threatened by human activities. A growing concern in coastal water is the increasing detection of titanium dioxide nanoparticles (nano-TiO2), a widespread component in commercial sunscreens, whose effect are still largely unexplored. Here, we combined photophysiological, metabolic, biochemical, and ultrastructural analyses to assess the responses of both germlings and adult thalli of E. amentacea after 10 and 20 days of exposure to four nano-TiO2 concentrations (0, 5, 50, and 500 μg L−1) under controlled laboratory conditions. In adult thalli, nano-TiO2 exposure induced moderate but detectable alterations in photosynthetic performance and primary productivity, characterized by transient stimulation at low concentrations followed by efficiency decline under higher doses and prolonged exposure. These shifts were accompanied by chlorophyll a depletion and activation of antioxidant defences, indicative of oxidative stress. In contrast, germlings displayed pronounced sensitivity across all exposure levels, with significant reductions in PSII efficiency, pigment loss, and enhanced oxidative stress even at the lowest tested concentration. SEM–EDS analyses confirmed the penetration and accumulation of TiO2 particles inside algal tissues, particularly in germlings, underscoring their limited capacity to prevent nanoparticle adhesion and uptake. Collectively, our results provide the first mechanistic evidence that sunscreen-derived nanoparticles impair key physiological processes in a Mediterranean foundation species, with early life stages being highly vulnerable. This sensitivity is of particular concern because germling recruitment coincides with the peak tourist season, when nanoparticle release from sunscreen use is maximal, creating a seasonal bottleneck for population renewal. Our findings highlight the negative effect of nano-TiO2 on this endemic Mediterranean fucalean species, underscoring the urgent need to investigate these emerging pollutants and their impacts on key ecosystem-services providers.