The increasing prevalence of coral diseases (6, 8, 28, 30, 31, 32, 52, 58) and thermal stress (14, 15, 19, 21, 22, 23, 25, 26, 27, 67, 69) have severely impacted reef-building corals over the past 20 years. The decline of coral reefs is significant, because coral reefs maintain high levels of biodiversity, provide habitats for coastal fisheries (69), may contain potential pharmacological compounds, and protect shorelines from erosion (25).
There is a lack of knowledge about the causative agents of many diseases affecting corals (30, 32, 49, 52) and their symbiotic algae (zooxanthellae). However, microbial pathogens are being increasingly identified with coral diseases, as in black band disease (51). Microbial pathogens affecting corals include the fungi Aspergillus sydowii (47, 60) and Aurantimonas coralicida, which affect sea fans and other gorgonians and caused white plague type II in the Florida Keys in 1995 (12). At least 18 species of corals were reported to be susceptible to this disease, including massive branching and plating species. The bacterium Serratia marcescens was reported to be the pathogenic agent causing white-pox disease (48, 50). This disease reduced populations of a significant coastal reef-building coral, Acropora palmata. S. marcescens is a common human intestinal bacterium and is found in sewage and contaminated soils. Human sewage has been linked to the disease, marking the first time a gut bacterium has been found to harm a marine invertebrate (48). Bacteria associated with black band disease of corals also included common human gut microbiota, but these were not found in healthy coral tissue (18).
Members of the genus Vibrio are common marine bacteria. Various species have been isolated from almost all marine niches, including some associated with toxic-shellfish poisoning (11, 33). Vibrio communities associated with corals were shown to increase in bleached corals (A. Kushmaro, personal communication). Kushmaro et al. (43) suggested that bacterial bleaching is caused by a temperature-influenced infection by Vibrio shilonii (2). Vibrio coralliilyticus was also shown to cause bleaching and tissue lysis in the Indo-Pacific coral Pocillopora damicornis (3). Marine vibrios tend to favor temperatures between 17 and 35°C (56).
Any impairment of the host-symbiont relationship can affect coral growth (17, 19, 40, 62, 64). Yellow blotch/band disease (YBD), which affects the major reef-building corals Montastraea spp., has been documented throughout the Caribbean and can spread across a coral colony at 0.5 to 1.0 cm month–1 (5). During its earliest stages, YBD develops as a pale-yellow blotch on the coral tissue, which eventually expands in size into a band as the disease progresses (5, 59) (Fig. 1). This paleness represents a decrease in chlorophyll concentration and a lack of symbiotic zooxanthellae (5).
Corals harbor diverse bacterial communities and seem to exhibit some specificity within coral species (13, 55). Bacteria present on the coral surface utilize the rich mucopolysaccharide layer (13, 54). Rohwer at al. (55) recently described bacteria associated with Montastraea spp. and found a variety of gram-positive and gram-negative bacteria on the surfaces of these reef-building species. Zooxanthellae play a role in host mucus biosynthesis by synthesizing and relesing precursor biochemicals (1, 4, 65), thereby affecting the composition of mucus and the bacteria on it.
Thermal coral reef bleaching is the expulsion of whole symbiotic zooxanthellae from host endoderm tissue (7, 16, 19, 39, 46, 62, 64, 67, 70). Corals expel their zooxanthellae during the warmest months, when temperatures reach 1 to 2°C above their normal environmental threshold (26, 29). Corals can also bleach, or expel their symbiotic zooxanthellae, as a result of temperature stress coupled with high light levels, resulting in a loss of pigments and damage to photosystems (20, 34, 38, 40, 42, 63, 66, 67), or in response to changes in salinity (24), chemical exposure from cyanide fishing (9), or sedimentation (49).
Here, we show the effects of temperature and inoculation with exogenous Vibrio strains on the division rates of zooxanthella symbionts; we also show how this results in low symbiont densities and decreased chlorophyll concentrations in the corals.