Viral communities associated with healthy and bleaching corals
Kristen L. Marhaver1,*, Robert A. Edwards2, Forest Rohwer3Article first published online: 9 MAY 2008
http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2008.01652.x/full
The coral holobiont is the integrated assemblage of the coral animal, its symbiotic algae, protists, fungi and a diverse consortium of Bacteria and Archaea. Corals are a model system for the study of symbiosis, the breakdown of which can result in disease and mortality. Little is known, however, about viruses that infect corals and their symbionts. Here we present metagenomic analyses of the viral communities associated with healthy and partially bleached specimens of the Caribbean reef-building coral Diploria strigosa. Surprisingly, herpes-like sequences accounted for 4–8% of the total sequences in each metagenome; this abundance of herpes-like sequences is unprecedented in other marine viral metagenomes. Viruses similar to those that infect algae and plants were also present in the coral viral assemblage. Among the phage identified, cyanophages were abundant in both healthy and bleaching corals and vibriophages were also present. Therefore, coral-associated viruses could potentially infect all components of the holobiont – coral, algal and microbial. Thus, we expect viruses to figure prominently in the preservation and breakdown of coral health.
Within a coral's skeleton, tissue and mucus, there exists a diverse assemblage of Bacteria, Archaea, algae, fungi and protists (Knowlton and Rohwer, 2003). Endosymbiotic algae, called zooxanthellae, and some Bacteria form relatively stable and species-specific associations with corals (Rohwer et al., 2002; Goulet, 2006). It has been hypothesized that the coral animal can adapt to differing ecological niches by ‘switching’ its algal and microbial associates. In the case of corals and zooxanthellae, this so-called adaptive bleaching may allow the coral animal to adjust to changing water temperatures (Buddemeier et al., 2004). Coral-associated Bacteria can serve as a food source for corals (Sorokin, 1973; Bak et al., 1998) and provide beneficial metabolic capabilities such as nitrogen fixation in at least one coral species (Lesser et al., 2004; 2007). It has been hypothesized that changes in microbe–coral associations will facilitate the survival of corals under future environmental changes (Reshef et al., 2006).
The least-studied constituents in the coral holobiont are the viruses. No cnidarian viruses have been isolated to sufficient purity to be identified genetically prior to this study, although viruses have been observed visually in association with corals and other cnidarians. An observation of virus-like particles (VLPs) in the zooxanthellae of anemones first implicated viruses in coral bleaching (Chapman, 1974; Wilson and Chapman, 2001). VLPs were later observed in the tissues of heat-shocked and control specimens of the scleractinian coral Pavona danai (Wilson et al., 2005) and in the tissue and zooxanthellae of three coral species and one species of zoanthid, all under thermal stress (Davy et al., 2006). The origin of these VLPs was not known. A recent study demonstrated that UV stress induced one type of latent virus in cultures of coral zooxanthellae (Lohr et al., 2007). In sum, observations of VLPs in corals have generally been made under the impression that their presence is an indicator of coral stress or disease (Wilson et al., 2005; Davy et al., 2006). However, given the abundance and diversity of coral-associated microbes, it is expected that these virus populations will consist of abundant and diverse bacteriophages in addition to viruses suspected to target eukaryotic cells, and that viruses will consistently be found in association with corals.
Viral genetic diversity is difficult to characterize because viruses share no single conserved sequence that can be used in a manner analogous to the sequencing of ribosomal RNA from cellular organisms (Rohwer and Edwards, 2002). Individual viruses contain extremely small amounts of DNA (Steward et al., 2000) and often use modified bases, making cloning difficult (Warren, 1980). Viruses also carry genes toxic to bacterial cloning hosts (Wang et al., 2000). Thus, in order to characterize an entire community of coral-associated viruses genetically, the viruses must be physically isolated from bacterial, archaeal, algal and host cells, as well as free DNA, prior to DNA extraction and cloning (Rohwer et al., 2001a). Here, a homogenization and centrifugation technique was developed to purify viruses from the tissues of healthy and partially bleached specimens of the Caribbean coral Diploria strigosa. Shotgun sequencing and metagenomic analyses were then used to determine the genetic content and diversity of these two viral communities. Our results show that coral-associated viruses are extraordinarily diverse and potentially infect all members of the coral holobiont.....................................
................Here we have described the complexity of an under-studied facet of the coral holobiont. Herpes-like viruses occur in both healthy and bleaching corals. This should be a focus for future research on coral holobiont complexity, symbiosis and immunology. The largest identified functional group of coral-associated viruses, cyanophages, may affect the population structure of symbiotic cyanobacteria and endolitic algae, while vibriophages present in coral tissue may affect the pathogenesis of coral-associated Vibrio spp. While these are important structuring forces for the coral holobiont, the prediction that up to 28 600 viral types occur in a healthy coral's viral community indicates that there are myriad functions and interactions still unidentified in this viral assemblage. When compared in the framework of a phage phylogenetic tree, coral-associated phage communities from bleaching and healthy corals are not significantly different from each other, but the coral holobiont as a phage environment is distinct from that of coral reef and oceanic waters. Thus, it appears that a diverse community of viruses continuously occupies coral tissues. With the potential to target animal, algal and microbial cells, viruses are likely to be crucial in maintaining the overall function of the coral holobiont.
