Disease and nutrient enrichment as potential stressors on the Caribbean sponge Aplysina cauliformis and its bacterial symbionts

Sponge diseases have recently emerged as potential forces structuring coral reefs. The increasing prevalence of disease on reefs may be due to changes in the virulence of pathogens and/or to decreases in host resistance as a result of changing environmental conditions. Coral reef ecosystems typically thrive in oligotrophic waters; however, runoff of fertilizers or sewage that contains elevated concentrations of nutrients can lead to eutrophic conditions. Aplysina cauliformis is a dominant member of the Caribbean sponge community, and is susceptible to Aplysina Red Band Syndrome (ARBS), a disease that causes reduced sponge growth and survival. We assessed the independent and interacting effects of nutrient enrichment and disease on A. cauliformis, using a factorial field experiment in which healthy and diseased sponges were exposed to nutrient-enriched or non-enriched treatments. Impacts on ARBS virulence (rate of lesion growth) and host response (both sponge and cyanobacterial symbiont growth and physiology) were assessed. ARBS lesions increased rapidly regardless of nutrient treatment, and disease had a significantly greater detrimental impact on sponges than did nutrient enrichment, as evidenced by a decline in sponge mass and reduced total protein content. The sponge-cyano bacterial symbiont relationship was less impacted by disease, although the sponge-associated bacterial community was significantly affected by sponge condition, with healthy sponges and diseased tissue hosting significantly different bacterial assemblages. In contrast, nutrient enrichment had no effects on sponge or symbiont physiology. Disease is a much greater stressor than eutrophication on the growth and physiology of A. cauliformis and its cyanobacterial symbionts.