Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|White band syndrome pathogens identified|
|Written by Tim Wijgerde|
Scientists finally discovered which pathogens may be responsible for the demise of many corals in the wild. The well-known genus Vibrio contains many harmful bacteria, which have been found on bleached and necrotic coral tissue. This phenomenon is often referred to as white band syndrome.
For their research, scientists collected tissue samples of diseased corals from several reefs; Nikko Bay in the Republic of Palau, Nelly Bay in the central Great Barrier Reef (GBR) and Majuro Atoll, Republic of the Marshall Islands. For analysis, they employed DNA sequencing to determine bacteria type. It was found that pathogens could be classified in the γ-proteobacteria family Vibrionacae, each with greater than 98% DNA sequence identity to the well-known coral bleaching bacterium Vibrio coralliilyticus.
The scientists tested samples from the species Pachyseris speciosa, Montipora aequituberculata and Acropora cytheria. They crushed the tissue samples and made cultures of the bacteria growing on them. The figure below shows that diseased coral tissue yielded 20 to 200 times more bacteria!
Left: Bacterial isolates cultured on feeding medium from Pachyseris speciosa crushed tissue. The amount of bacteria are expressed in CFU’s (colony forming units) per ml per gram of coral tissue. The black bars represent bacteria cultured on TBCS agar, a growth medium preferred by Vibrio bacteria. The white bars represent general growth medium. The diseased tissue yielded a lot more bacteria compared to healthy isolated tissue. Note that the scale is logarithmic! This means that for every step on the Y-axis, the amount of bacteria increases with a factor of 10 (Sussman et al, PLoS ONE, 2008).
After having identified the culprits behind the disease, scientists took healthy fragments from all three species and exposed (inoculated) them to six putative pathogens; strains P1 to P6. They used a concentration of 1 million bacteria/ml. After exposure to the strains, the corals developed white band syndrome in the laboratory, which was similar to the disease observed in the field. All pathogens caused infections in the corals, with various infection rates.
Bottom: Montipora aequituberculata coral fragments exposed to the P1 strain. After 150 hours the inoculated fragments showed signs of disease, whereas the negative control showed nothing (Sussman et al, PLoS ONE, 2008).
Loss of zooxanthellae and coral digestion
They also found that the corals lost zooxanthellae after exposure. For Acropora hyacinthus, the first disease phase was bleaching, 9-12 hours after exposure. This was followed by tissue necrosis. P. speciosa fragments which were exposed to strain P3, paling and loss of Symbiodinium algae commenced in coenosarc tissue (tissue between polyps) starting 12 h after exposure. Next to this, the Vibrio bacteria showed proteolytic activity, which means they produced enzymes able to digest the coral tissue. The bacteria were also found to home in on the corals; bacteria which were co-incubated with coral fragments decreased in free-floating numbers. This did not occur without the coral fragments.
Finally, the researchers determined the LT50, which is the time needed to cause 50% mortality (death) of the exposed fragments. This was 60 hours for P. speciosa (strains P3-4-5-6), 180 hours for Montipora aequituberculata (strain P1) and only 18 hours for Acropora hyacinthus (strain P2)! This underscores the phenomena which unfortunate aquarists observe at home; the rapid death of Acroporids. These results show how relentless bacteria can be, and that they can cause the demise of whole reef sections. Global warming and pollution are putting more and more stress on the reefs, making it more difficult for them to fend off these killer bacteria. For instance, it is now known that the bleaching of the coral Oculina patagonica in the Mediterranean Sea is caused by the bacterium Vibrio shiloi, which has now become a seasonal event.
This research is a collaboration between the ARC Centre of Excellence for Coral Reef Studies at the James Cook University, Australia, the Australian Institute of Marine Science (AIMS) and the Palau International Coral Reef Center.
Meir Sussman, Bette L. Willis, Steven Victor, David G. Bourne, Coral Pathogens Identified for White Syndrome (WS) Epizootics in the Indo-Pacific, PloS ONE, 2008, 3(6)