Coral bleaching is the loss of the pigmented zooxanthellae causing the coral turn white.
Vibrio shiloi has been proved to be the causative agent of bleaching the coral Ocrulina patagonica, which exibited massive bleaching in the eastern
- adhesion of V.shiloi to coral surface
- penetration into coral epidermal layer
- differentiation into viable but not culturable state
- intracellular multiplication
- production of extracellular toxins that inhibit photosynthesis,bleach and lyse zooxanthellae
The adhesion is the critical step (non adhering mutants are not virulent) and is mediated by a beta-D-galactopyranoside-containing receptor on the coral surface.
Authors investigated V. shiloi infectious process in 3 forms of O. patagonica: white cave colonies,bleached and normal healthy colonies. The results indicated that healthy corals produced large amount of mucus compared to cave and bleached colonies and V. shiloi adhered just to normal healthy coral, thus zooxanthellae are required for this process.
In adhesion experiments mucus was removed from the healthy pigmented coral and fixed. Overnight cultures of V. shiloi pregrown at different temperature were added to the mucus,and the chemotaxis to O. patagonica was detected through a tube capillary assay. Bacteria pregrown at 25°C showed a great higher chemotaxis compared to bacteria pregrown at 16 °C, and both of them were higher than the control. The same data were obtained with mucus removed from corals mantaining at 16° and 25°C, indicating the temperature dependance of the infectious process.
Methyl-beta-D-galactopyranoside blocked the adhesion,indicating that the beta-D-galactoside-containing receptor is found in the coral mucus.
In order to investigate the role of zooxanthellae in adhesion process, several pieces of coral were treated to remove mucus and incubated with DCMU to inhibit photosynthesis of zooxanthellae during adhesion. Results showed that mucus-depleted corals treated with photosynthesis inhibitor did not produce fresh mucus and no adhesion occurred, thus zooxanthellae must be present and photosynthetically active for synthesis or secretion of the receptor. These results were confirmed by other adhesion experiments using 5 different coral species showing that V. shiloi was found only in the three coral species possessing zooxanthellae.
These results are consistent with other data indicating that bacteria are against algae, in fact toxins produced does not damage coral tissue: coral bleaching is due to the loss of zooxanthellae,not directly to V. shiloi infection, and it occurs only during summer;when seawater temperature drops in the winter,V.shiloi is no longer present in the coral,and the symbiosis with zooxanthellae is restablished.
The presence of arabinose (a sugar not commonly found in animals) in the mucus suggestes that the zooxanthellae may contribute carbohydrates directly to the mucus.
Although this paper is relatively recent,I find it very interesting and useful because it contibuted to our knowledge of coral bleaching process focusing on the key interaction between bacteria and zooxanthellae.
A review of: Banin, E., Israely, T., Fine, M., Loya, Y. and Rosenberg, E. (2001), Role of endosymbiotic zooxanthellae and coral mucus in the adhesion of the coral-bleaching pathogen Vibrio shiloi to its host. FEMS Microbiology Letters, 199: 33–37. doi: 10.1111/j.1574-6968.2001.tb10647.x
4 comments:
it would be useful to understand why V.shiloi produces toxins against zooxanthellae and not directly against corals, leading to coral bleaching which will damage coral anyway? Further, why the virulence is so temperature dependent?
It seems that increasing temperature obliges bacteria to penetrate in coral tissue(perhaps to find better conditions!?!?) and that "cohabitation" with coral and zooxanthellae at the same time is not convenient for V.shiloi (this could explain the toxic effect against zooxanthellae). But why?How could we investigate this aspect?
Valentina - the various aspects of the V.shiloi - Oculina interactions made a very neat and elegant model of infection. It is a real shame that no other group repeated the work by Rosenberg's group before V. shiloi apparently stopped infecting coral in 2002 (see posts on the Coral Probiotic Hypothesis). however, the papers by Sussma et al. on V. coralliilyticus infections of corals on the GBR do confirm that there is a very specific effect on the zooxanthellae in this system. Here, however, it is due to a metalloprotease rather than the low MW peptide/ammonia toxicity suggested for V. shiloi. I discuss this in RF Box 11.1 in the book and we'll be looking at it in the lectures.
Hi Valentina,
Very interesting paper and good review! Did the authors describe what processes are involved regarding symbiont photosynthesis and the production of mucus and receptor for V. shiloi attachment? Natasha, on a previous post, brings up a good point about signalling molecules like hydrogen peroxide and other ROS which is generated from photosynthesis. I wonder if these molecules are involved in some way.
Colin and Mario- Authors state that mucus is foundamental for V.shiloi adhesion and this assumption is supported by the observation that corals without mucus are not infected. The presence and photosynthetic activity of symbionts seem to be necessary for mucus production but any particular process has been described. Authors found that one of the most important mucus compound is arabinose,which is a sugar not commonly found in animals,likely producted and released by zooxanthellae.
Reading this paper, I came to a consideration: I suppose that the strongly dependant relation between zooxanthellae (present and photosynthetically active) and infection is due to the vital importance of symbiosis itself; we must consider that corals are holobionts and the symbiotic relation is vital both for animal and algae. If this symbiosis will not exist,holobiont will die. So, if zooxanthellae do not espicate their symbiotic function, coral will not be able to produce anything, mucus neither! This would explain why there is no mucus ( and subusequently infection ) in corals without algae. According to this consideration, I really think that further investigation on this aspect should be conducted, to a better knowledge of symbiotic relations.
Mario- any information about ROS and other molecules involved in infection has been provided,unfortunately. I think it would be interesting to delve into this topic, we can look for on web.
PS: which is the review by natasha you are reffering to??I do not find on the blog -
too much reviews, Colin will be satisfied! ;)
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