The role of microorganisms in coral bleaching

Coral bleaching is the disruption of the symbiosis between the coral host and its endosymbiotic algae. The prevalence and severity of the disease have been correlated with high seawater

temperature. During the last decade, the major hypothesis to explain coral bleaching is that high water temperatures cause irreversible damage to the symbiotic algae resulting in loss of

pigment and/or algae from the holobiont. Here, we discuss the evidence for an alternative but not mutually exclusive concept, the microbial hypothesis of coral bleaching.

Coral reefs develop through the massive and long-term build-up of calcareous skeleta of scleractinian corals. These corals are modular cnidarians that secrete a CaCO3 exoskeleton on and in

which other organisms may grow. Scleractinian corals are made up of one or more polyps composed of an external protective body layer (ectoderm), an inner gastrodermal layer that carries out

most of the coral's digestive and reproductive functions and between them an acellular mesoglea (Figure 1). The ectodermal layer that is apposed to the substrata produces CaCO3 as a support

for the coral tissue layers. In colonial forms, there is a tissue and gastrovascular confluence between polyps so that nutrients and signaling can be transferred between different parts of

the colony. Corals are conduits to numerous microorganisms, including the intracellular zooxanthellae, other protists such as thraustochytrids, prokaryotes (bacteria and archaea) and viruses

(Rohwer et al., 2002; Wegley et al., 2004; Kramarsky-Winter et al., 2006; Davy and Patten, 2007; Rosenberg et al., 2007; Harel et al., 2008; Marhaver et al., 2008). Many of these organisms

may be considered symbionts and play an important role in coral health.

Anatomy of a polyp of a scleractinian coral (after Galloway et al., 2007).

Bleaching is defined as the disruption of the symbiosis between the coral host and its endosymbiotic zooxanthellae (of the genus Symbiodinium). This can be the result of the loss of the

algal symbiont and/or of the algal pigments. The coral tissues then become transparent making the underlying white calcium carbonate skeleton visible. Other signs of bleaching include

thinning of host tissue, reduction in mucus and often inhibition of sexual reproduction. If bleaching is not reversed, corals will die.

Various studies carried out during the last few years have led to the view that coral bleaching is a disease that is affected by biotic and environmental factors. A disease is defined as a

process resulting in tissue damage or alteration of physiological function, producing visible symptoms (Stedman, 2005). Accordingly, coral bleaching is clearly a disease and should be

referred to as such. Bleaching has been shown to be induced by a variety of factors, including high temperature and irradiance (reviewed in Jokiel, 2004), low salinity (Goreau, 1964),

sediments (Peters, 1984), exposure to cyanide (Cervino et al., 2003), decreased seawater temperature (Muscatine et al., 1991) and bacterial infection (Kushmaro et al., 1996, 1997; Ben-Haim

and Rosenberg, 2002). All of these stressors caused bleaching both in the laboratory and in the sea (Brown, 1997).

During the last two decades, there have been reports of bleaching being caused by a variety of microbial pathogens. The assertion that these biotic agents were indeed the causative agent of

this disease was demonstrated by applying Koch's postulates. Unfortunately, despite the growing evidence for the role of bacteria in coral bleaching and other diseases, there are still some

authors who discount the direct effect that microorganisms play in the disease processes (for example, Ainsworth et al., 2008).

Extensive bleaching of the coral Oculina patagonica in the eastern Mediterranean Sea occurs every summer (Kushmaro et al., 1996). Kushmaro et al. (1996, 1997) reported that the bleaching of

O. patagonica was the result of an infection by Vibrio shiloi (Figure 2). The demonstration that V. shiloi was the causative agent of the disease was established by rigorously satisfying all

of Koch's postulates, including the fact that bleached coral in the sea contained the bacterium (Kushmaro et al., 1996, 1997), whereas it was absent from healthy corals (see Figure 3a).

Furthermore, Kushmaro et al. (1998) showed that the infection and subsequent bleaching occurred only at temperatures above 25 °C. Thus, for bleaching to occur, both elevated temperature and

the causative agent must be present.

Photograph of bleached and unbleached colonies of Oculina patagonica collected from the Mediterranean coast of Israel.

Transmission electron micrographs of the coral Oculina patagonica from 1996. (a) Naturally bleached colony collected on 29 July1996 from Sdot Yam, Mediterranean coast, Israel at a water

temperature of 29 °C. (b–d) Colony 18 h after experimental infection with Vibrio shiloi 106 cells incubated at 29 °C in the lab. (b) Ectoderm with visible bacteria (b) and gastrodermis

showing vacuolization in tissue surrounding zooxanthellae. (c) Enlargement of (b) showing the Vibrio shiloi in ectoderm adjacent to the necrotic tissue. (d) Breakdown of the zooxanthellae in

the infected coral and their digestion by the host.

The specific steps in the infection of O. patagonica by V. shiloi have been studied extensively (Rosenberg and Falkovitz, 2004). The bacteria are chemotactic to the coral mucus, adhere to a

β-galactoside-containing receptor on the coral surface, penetrate into the epidermal layer (Figure 3) and multiply intracellularly, reaching 108–109 cells per cm3. The intracellular V.

shiloi produces an extracellular peptide toxin (PYPVYPPPVVP) that inhibits algal photosynthesis. Another important factor for the virulence of V. shiloi is the expression of superoxide

dismutase. Adhesion, production of the toxin and expression of superoxide dismutase are all temperature-dependent reactions, occurring at summer (25–30 °C) but not at winter (16–20 °C)

temperatures. Thus, V. shiloi cannot infect, multiply or survive in the coral during the winter. Sussman et al. (2003) demonstrated that the marine fireworm Hermodice carunculata is a winter

reservoir and spring–summer vector for V. shiloi.

One of the stress factors reported to induce bleaching on coral reefs is high solar radiation (Lesser et al., 1990). Gleason and Wellington (1993) concluded that it was the high ultraviolet

radiation that affected the corals inducing bleaching in Montastrea annularis. The generality of this conclusion was challenged when it was shown that colonies of O. patagonica in shallow

water (0–80 cm) tidal pools (high ultraviolet radiation) showed negligible bleaching, even though summer temperatures there in shallow water were 2–4 °C warmer than in the open water (Fine

et al., 2002). In this study, fragments transplanted from 4 m depth to shallow reef flats (>30 cm depth) in May showed no bleaching, whereas fragments transplanted from the shallow reef to 4

 m depth underwent extensive bleaching. Moreover, when O. patagonica was incubated with V. shiloi in aquaria exposed to bright sunlight, the bacteria were rapidly killed and no bleaching

occurred. However, when the corals were protected from ultraviolet light with a Pexiglass filter, V. shiloi multiplied and bleaching was induced. Thus in the case of bacterial bleaching of

O. patagonica, ultraviolet radiation actually inhibits bleaching occurring in very shallow water (