Kushmaro, A.; Y. Loyola; M. Fine and E. Rosenberg. 1996. Bacterial
infection and coral bleaching. Nature 380(4): 396. 

	Bleaching in stony-corals is the result of disruption of 
symbioses between the coral hosts and photosynthetic microalgal 
endosymbionts (zooxanthellae). Coral bleaching events of unprecedented 
frequency and global extent were reported in the 1980s and early 
1990s1-3. Coral bleaching may be evoked by a variety of environmental 
stimuli, including increased seawater temperature4 and ultraviolet 
radiation5. There has been speculation that large scale bleaching 
episodes are linked to global warming6,7. The data presented here 
demonstrate for the first time that coral bleaching, in this case, 
bleaching of Oculina patagonica, is caused by a bacterial infection. 

	We first observed bleaching of O. patagonica in the summer of 
1993. Bleached patches of polyps appeared throughout the affected 
colonies (Fig 1A). Although the tentacular rim of the polyp may retain 
its pigmentation, the extratentacular coenosarc of the tissue was 
bleached showing a total loss of pigmentation (Fig.1B). Histological 
sections of bleached tissues showed a 70%-90% reduction of  algal 
densities. The initial observation that led us to consider that coral 
bleaching may be due to bacterial infection was the presence of large 
aggregates of rod-shaped bacteria on the border of the bleached zone at 
the tentacular rim (Fig. 1C). In unbleached tissues, no bacterial 
aggregates were observed. Bleached and unbleached corals were collected 
from the Mediterranean coast of Israel and samples streaked onto Marine 
Agar. Characteristic cream colored bacterial colonies, referred to as 
strain AK-1, appeared on all eight samples that had been taken from 
bleached corals and were absent from all 14 unbleached corals. Strain 
AK-1 has been classified as a Vibrio by standard microbiological tests8. 

	Pure cultures of Vibrio AK-1 inoculated to healthy O. patagonica 
caused bleaching. Two types of experiments were performed: In the first 
set, 5 x 106 bacteria were placed on each of five healthy corals and the 
corals then put back in an aquarium maintained at 25 C. All of the corals 
showed bleaching after 6-8 days. In a control experiment, in which 
sterile medium was used in place of the bacteria, none of the corals 
showed any signs of bleaching. Bleaching was due to the bacterial cells 
themselves and not extracellular products, because ultrafiltration led to 
an inactive cell-free supernatant fluid. The washed cells were as active 
as the original culture.

	The remaining experiments were carried out at 26 C without 
removing the corals from two-liter aerated aquaria. In the first test, 
two aquaria containing O. patagonica were inoculated with 105 Vibrio AK-1 
per ml. Antibiotics were added to one of these infected aquaria. A third 
control aquarium was treated in exactly the same manner except that it 
was inoculated with sterile medium in place of bacteria. After 44 days, 
90% of the coral surface in the experimental aquarium was bleached, 
whereas no bleaching was observed in the control and the 
antibiotic-treated aquarium (Fig. 1D). In a second test, three aquaria 
were infected with 5 x 106 Vibrio AK-1 per ml and two additional aquaria 
served as controls. In all three experimental aquaria there was extensive 
bleaching after 42 days and tissue retraction after 52 days. The two 
control colonies remained healthy. Attempts to infect corals with 105 
Vibrio AK-1 per ml at 16 C were unsuccessful.

	Our findings demonstrate that the causative agent of bleaching of 
Oculina patagonica is Vibrio AK-1, which was (a) present in all bleached 
O. patagonica examined, (b) obtained in pure culture and (c) shown to 
cause bleaching when inoculated onto healthy (unbleached) corals. Sea 
water temperature is a contributing factor; an increase in temperature 
may influence the outcome of the infection by lowering the resistance of 
the coral and increasing the virulence of the bacterium.

A. Kushmaro, Y. Loya, M. Fine, Department of Zoology, Tel Aviv 
University, Ramat Aviv, Israel 69978.

E. Rosenberg, Department of Molecular Microbiology and Biotechnology, Tel 
Aviv University, Ramat Aviv, Israel 69978.

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