What was done
At monthly intervals between June and December 2000, the authors recorded the percent bleaching of 40 tagged colonies of Oculina patagonica on a reef flat just off the coast of Sdot-Yam, Israel, as well as the percent bleaching of 40 colonies in tide pools and 200 at depths of 1 to 6 meters.  In addition, cores of several reef flat colonies were transplanted to a depth of 4 meters, while cores of colonies at 4 meters depth were transplanted to the reef flat.  At the same time, similar cores from each depth were retained at their sites of origin to serve as transplant controls.  Laboratory experiments were also conducted to test the hypothesis that solar radiation, or some wavelength-dependent portion of it, may protect corals from bacterial-induced bleaching.

What was learned
The causative agent of bleaching in Oculina patagonica in the Mediterranean Sea is the bacterium Vibrio shiloi, whose virulence increases with increasing seawater temperature (Kushmaro et al., 1998).  Fine et al., however, observed what appeared to be just the opposite in terms of expected consequences: "colonies in shallow water (0 to 80 cm depth) showed negligible bleaching, despite being exposed to higher temperatures (ca 2°C) than the deeper water corals."  Likewise, they report that "fragments transplanted from 4 m depth to a shallow reef flat (less than 30 cm depth) in May showed no bleaching during the summer months, whereas intact colonies at 4 m and fragments transplanted to that depth from the reef flat underwent extensive bleaching."

Searching for an explanation for this conundrum, the Israeli scientists found that "the concentration of V. shiloi was very high (greater than 2x10⁴ cm^-2) in bleached fragments at 4 m, whereas in non-bleached fragments from the reef flat, V. shiloi could not be detected."   Last of all, they observed that "when O. patagonica was infected with V. shiloi in laboratory aquaria and subsequently exposed to sunlight, the intracellular bacteria were rapidly killed, aborting the infection and preventing bleaching," but that "when the infected corals were protected from ultraviolet (UV) light, the intracellular V. shiloi multiplied and the coral bleached."

What it means
Fitting all of these pieces of information together, Fine et al. came to the conclusion that "in the case of O. patagonica, UVR is lethal to the causative agent of bleaching."  In addition, they describe why, noting that "Toren et al. (1998) demonstrated that bleaching of Oculina patagonica is temperature regulated via production of the adhesin required for adhesion of Vibrio shiloi to its host coral," and that "in addition to adhesion, intracellular growth of the bacteria and toxin production are also temperature dependent (Banin et al., 2000a,b)."

References
Banin, E., Ben-Haim, Y., Israely, T., Loya, Y. and Rosenberg, E.  2000a.  Effect of the environment on the bacterial bleaching of corals.  Water, Air and Soil Pollution 123: 337-352.

Banin, E., Israely, T., Kushmaro, A., Loya, Y., Orr, E. and Rosenberg, E.  2000b.  Penetration of the coral-bleaching bacterium Vibrio shiloi into Oculina patagonica.  Applied Environmental Microbiology 66: 3031-3036.

Toren, A., Landau, L., Kushmaro, A., Loya, Y. and Rosenberg, E.  1998.  Effect of temperature on adhesion of Vibrio strain AK-1 to Oculina patagonica and on coral bleaching.  Applied Environmental Microbiology 64: 1379-1384.