Coral Mucus Is a Hot Spot for Viral Infections

There is increasing suspicion that viral communities play a pivotal role in maintaining coral health, yet their main ecological traits still remain poorly characterized. In this study, we examined the seasonal distribution and reproduction pathways of viruses inhabiting the mucus of the scleractinians Fungia repanda and Acropora formosa collected in Nha Trang Bay (Vietnam) during an 11-month survey. The strong coupling between epibiotic viral and bacterial abundance suggested that phages are dominant among coral-associated viral communities. Mucosal viruses also exhibited significant differences in their main features between the two coral species and were also remarkably contrasted with their planktonic counterparts. For example, their abundance (inferred from epifluorescence counts), lytic production rates (KCN incubations), and the proportion of lysogenic cells (mitomycin C inductions) were, respectively, 2.6-, 9.5-, and 2.2-fold higher in mucus than in the surrounding water. Both lytic and lysogenic indicators were tightly coupled with temperature and salinity, suggesting that the life strategy of viral epibionts is strongly dependent upon environmental circumstances. Finally, our results suggest that coral mucus may represent a highly favorable habitat for viral proliferation, promoting the development of both temperate and virulent phages. Here, we discuss how such an optimized viral arsenal could be crucial for coral viability by presumably forging complex links with both symbiotic and adjacent nonsymbiotic microorganisms.