Can synergistic effects of marine heatwaves and Vibrio proliferation act as potential triggers of widespread demosponge disease? A case study in the Mediterranean Sea

Marine heatwaves associated with climate change are increasingly creating favourable conditions for the proliferation of pathogenic bacteria, leading to disease outbreaks and mass mortality events particularly in sessile suspension-feeding invertebrates, such as sponges. In summer 2024, a disease outbreak affecting two demosponge species, Petrosia ficiformis and Agelas oroides , was documented along the Northern Ionian coast of the Mediterranean Sea (Italy). Diseased sponges exhibited extensive surface necrosis or multiple lesions distributed across the body. Necrotic areas appeared whitish and were frequently coated with a thin mucous layer composed of bacterial aggregates. Microbiological culture analyses revealed elevated bacterial densities on sponge surfaces, with total culturable bacteria reaching 4.3 ± 0.2 × 10⁵ CFU/g and culturable vibrios 7.0 ± 0.4 × 10⁴ CFU/g in P. ficiformis , and 3.6 ± 0.3 × 10⁵ CFU/g and 6.6 ± 0.2 × 10⁴ CFU/g, respectively, in A. oroides . All Vibrio isolates obtained from the conspicuous whitish lesions on diseased sponges were identified as Vibrio alginolyticus , based on concordant phylogenetic, morphological, cultural, and biochemical analyses. Vibrio alginolyticus is a well-established pathogen of numerous aquatic organisms, suggesting that marine heatwaves may enhance Vibrio abundance and increase infection frequency during summer periods. Nevertheless, the specific factors initiating the epidemic in the examined sponge populations remain unresolved. We hypothesize that disease onset involves a synergistic mechanism in which environmental stressors, such as elevated temperature, increased nutrient availability, and reduced water circulation, promote the transition of sponge-associated bacteria, particularly V. alginolyticus , toward virulence. Under these conditions, host physiological defences may be compromised, allowing uncontrolled bacterial proliferation. Overall, our findings indicate that the interaction between thermal stress and pathogenic vibrios represents a plausible trigger for sponge disease outbreaks. Further investigations are required to elucidate the etiological pathways involved and to identify the mechanisms underlying sponge recovery following epidemic events that result in mass mortality. A substantial decline in key sponge species may have serious consequences for ecosystem functioning, given their critical role as filter feeders in marine bioremediation and habitat health.