Petrosia heatwave

Global climate change exacerbates the incidence of marine heatwaves (MHWs), which increased in intensity and frequency over the past years, causing severe impacts on marine coastal ecosystems. MHWs have already caused massive coral bleaching events in the tropics and mass mortalities of habitat-forming species, including sponges and gorgonians, in temperate and polar seas. In the Mediterranean, these abrupt enhancements of temperature have shown to cause tissue necrosis, accompanied by microbial dysbiosis in several temperate sponges, facilitating the entrance of opportunistic microorganisms. During the summers of 2022 and 2023, populations of the sponge Petrosia ficiformis (Poiret, 1789) were conspicuously observed with signs of thermal stress linked to MHWs around the Gulf of Naples, Tyrrhenian Sea (Italy). These included depigmentation spots (bleaching) and tissue texture alterations, which often evolve in necrotic processes and eventual death. In the peak of both MHWs, though, apparently thermoresistant sponges co-occurred with sensitive unhealthy specimens. In order to explore potential microbial drivers correlated with these divergent thermal stress tolerances, healthy and affected individuals were sampled along the coast of the Island of Ischia in September 2022.