Reef-building corals are comprised of close associations between the coral animal, symbiotic zooxanthellae, and a diversity of associated microbes (including Bacteria, Archaea and Fungi). Together, these comprise the coral holobiont - a paradigm that emphasizes the potential contributions of each component to the overall function and health of the coral. Little is known about the ecology of the coral-associated microbial community and its hypothesized role in coral health. We explored bacteria-bacteria antagonism among 67 bacterial isolates from the scleractinian coral Montastrea annularis at two temperatures using Burkholder agar diffusion assays. A majority of isolates exhibited inhibitory activity (69.6% of isolates at 25 degrees C, 52.2% at 31 degrees C), with members of the gamma-proteobacteria (Vibrionales and Alteromonadales) being especially antagonistic. Elevated temperatures generally reduced levels of antagonism, although the effects were complex. Several potential pathogens were observed in the microbial community of apparently healthy corals, and 11.6% of isolates were able to inhibit the growth of the coral pathogen Vibrio shiloi at 25 degrees C. Overall, this study demonstrates that antagonism could be a structuring force in coral-associated microbial communities and may contribute to pathogenesis as well as disease resistance.

The population structure of benthic marine organisms is of central relevance to the conservation and management of these often threatened species, as well as to the accurate understanding of their ecological and evolutionary dynamics. A growing body of evidence suggests that marine populations can be structured over short distances despite theoretically high dispersal potential. Yet the proposed mechanisms governing this structure vary, and existing empirical population genetic evidence is of insufficient taxonomic and geographic scope to allow for strong general inferences. Here, we describe the range-wide population genetic structure of an ecologically important Caribbean octocoral, Gorgonia ventalina. Genetic differentiation was positively correlated with geographic distance and negatively correlated with oceanographically modelled dispersal probability throughout the range. Although we observed admixture across hundreds of kilometres, estimated dispersal was low, and populations were differentiated across distances <2 km. These results suggest that populations of G. ventalina may be evolutionarily coupled via gene flow but are largely demographically independent. Observed patterns of differentiation corroborate biogeographic breaks found in other taxa (e.g. an east/west divide near Puerto Rico), and also identify population divides not discussed in previous studies (e.g. the Yucatan Channel). High genotypic diversity and absence of clonemates indicate that sex is the primary reproductive mode for G. ventalina. A comparative analysis of the population structure of G. ventalina and its dinoflagellate symbiont, Symbiodinium, indicates that the dispersal of these symbiotic partners is not coupled, and symbiont transmission occurs horizontally. IntroductionLarval dispersal is the primary means of gene flow among populations of most benthic and demersal marine organisms (Hedgecock 1986). Many species have long-lived, pelagic propagules that can be entrained by ocean currents and disperse across great distances, and the amount of time larvae spend in the water column is generally correlated with dispersal distance (Bohonak 1999). Although examples of substantial long-distance gene flow and panmixia do exist (Shulman & Bermingham 1995;Lessios et al. 2001;Neethling et al. 2008), exceptions are not uncommon, and it has become increasingly clear that even marine species with theoretically high dispersal potential can exhibit strong population differentiation over relatively small spatial scales (Quesada et al. 1995;Barber et al. 2000;Taylor & Hellberg 2003a). A number of mechanisms have been proposed to explain such population structure, including spatially divergent selection (Bongaerts et al. 2010), physical oceanographic barriers (Cowen et al. 2000) or larval behaviour/mortality that results in local retention (Burton & Feldman 1982). The relative importance of each of these explanations is a matter of ongoing debate (Colin 2003;Taylor & Hellberg 2003b;Warner & Palumbi 2003), and additional empirical studi...

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Recent outbreaks of new diseases in many ecosystems are caused by novel pathogens, impaired host immunity, or changing environmental conditions. Identifying the source of emergent pathogens is critical for mitigating the impacts of diseases, and understanding the cause of their recent appearances. One ecosystem suffering outbreaks of disease in the past decades is coral reefs, where pathogens such as the fungus Aspergillus sydowii have caused catastrophic population declines in their hosts. Aspergillosis is one of the best-characterized coral diseases, yet the origin of this typically terrestrial fungus in marine systems remains unknown. We examined the genetic structure of a global sample of A. sydowii, including isolates from diseased corals, diseased humans, and environmental sources. Twelve microsatellite markers reveal a pattern of global panmixia among the fungal isolates. A single origin of the pathogen into marine systems seems unlikely given the lack of isolation by distance and lack of evidence for a recent bottleneck. A neighbour-joining phylogeny shows that sea fan isolates are interspersed with environmental isolates, suggesting there have been multiple introductions from land into the ocean. Overall, our results underscore that A. sydowii is a true opportunist, with a diversity of nonrelated isolates able to cause disease in corals. This study highlights the challenge in distinguishing between the role of environment in allowing opportunistic pathogens to increase and actual introductions of new pathogenic microorganisms for coral diseases.