High occurrence of viruses in the mucus layer of scleractinian corals Hanh Nguyen-Kim, 1,2 Thierry Bouvier, 1 Corinne Bouvier, 1 Hai Doan-Nhu, 2 Lam Nguyen-Ngoc, 2 Emma Rochelle-Newall, 3 Anne-Claire Baudoux, 4 Christelle Desnues, 5 Stéphanie Reynaud, 6 Christine Ferrier-Pages 6 and Yvan Bettarel 1 * 1 Institute of Research for Development (IRD), CNRS, ECOSYM, Montpellier, France. 2 Institute of Oceanography of Nha Trang, VAST, Viet Nam. 3 IRD, BIOEMCO, Paris, France. 4 CNRS, AD2M, Roscoff, France. 5 CNRS, URMITE, Marseille, France. 6 Scientific Center of Monaco (CSM), Monaco. Summary Viruses attract increasing interest from environmen- tal microbiologists seeking to understand their func- tion and role in coral health. However, little is known about their main ecological traits within the coral holobiont. In this study, a quantitative and qualitative characterization of viral and bacterial communities was conducted on the mucus of seven different coral species of the Van Phong Bay (Vietnam). On average, the concentrations of viruses and bacteria were, respectively, 17- and twofold higher in the mucus than in the surrounding water. The examination of bacterial community composition also showed remarkable differences between mucus and water samples. The percentage of active respiring cells was nearly threefold higher in mucus (m = 24.8%) than in water (m = 8.6%). Interestingly, a positive and highly significant correlation was observed between the proportion of active cells and viral abundance in the mucus, suggesting that the metabolism of the bacte- rial associates is probably a strong determinant of the distribution of viruses within the coral holobiont. Overall, coral mucus, given its unique physicochemical characteristics and sticking proper- ties, can be regarded as a highly selective biotope for abundant, diversified and specialized symbiotic microbial and viral organisms. Introduction Prokaryotes play a pivotal role in the health of reef eco- systems because they provide scleractinian corals with crucial ecological services (Ceh et al., 2013), including protection from pathogen colonization (Krediet et al., 2013) and nutrition (Rosenberg et al., 2009). However, these prokaryote associates have been shown to be highly susceptible to environmental perturbations (increased temperature, chemical pollutants, changes in nutrients, salinity, pH, etc.), and changes in their compo- sition and/or physiology may directly or indirectly affect scleractinian metabolism, as postulated in the probiotic theory of coral diseases (Reshef et al., 2006). Recently, other (biological) sources of prokaryotic mortality have been identified within the coral holobiont. For example, the discovery of virus occurrence in coral tissues (Bettarel et al., 2012), mucus (Davy and Patten, 2007) and skel- eton (Vega Thurber et al., 2008) has raised a number of questions regarding their effective roles in coral health, which could be ambivalent. Indeed, viruses could either cause profound changes in the commensal prokaryotic and eukaryotic communities, which may lead to coral disfunctioning or diseases (van Oppen et al., 2009), or conversely act as a natural phage therapy for diseased scleractinians by removing bacterial pathogens (Atad et al., 2012; Cohen et al., 2013). Despite their potentially decisive role in coral health, viruses remain surprisingly understudied (Garren and Azam, 2012). Viral functions and ecological traits in wild corals are still poorly understood, and so are their specific interactions with the various members of the coral holobiont. Prior to evaluating their potential role, there is an urgent need for basic ecological information, such as their in situ abundance in corals, their variability among the large diversity of scleractinian species, their colonization processes and the ecological links with their hosts. In this study, the specific objectives were to (i) deter- mine the concentrations of coral-associated viruses in the superficial mucus layer of seven different scleractinian species (Favia truncatus, Acropora aculeus, Platygyra pini, Montastrea colemani, Acropora muricata, Favites Received 13 February, 2014; accepted 26 March, 2014. *For corre- spondence. E-mail yvan.bettarel@ird.fr; Tel. +84 1 695 437 450; Fax +84 4 373 467 14. Environmental Microbiology Reports (2014) doi:10.1111/1758-2229.12185 © 2014 Society for Applied Microbiology and John Wiley & Sons Ltd