A review of: Reshef L et al (2006) The Coral Probiotic Hypothesis. Environmental Microbiology, 8: 2068-2073.
Coral biologists predict that most of the planet’s coral reefs will be destroyed by 2050 based on the assumption that corals won’t be able to adapt fast enough to environmental changes and emerging diseases. Bleaching events have been studied extensively in coral species such as Oculina patagonica. Vibrio shiloi was determined to be the causative agent of in O. patagonica, although after a few years it was discovered that V. shiloi can no longer be found or infect O. patagonica, leading to the proposal of the coral probiotic hypothesis.
Probiotics are living microorganisms which confer health benefits to the host. The concept is that corals live in symbiosis with diverse metabolically active populations of bacteria and archaea. When environmental conditions change the relative abundance of microbial species adjusts allowing the coral holobiont to adapt. Moreover, the diverse bacterial species impart a significant genetic potential to adapt rapidly to the changing environment.
Corals do not have an adaptive immune system and cannot fight off infection in theory. The authors described several studies on coral bacterial communities to establish the validity of the hypothesis. For example, similar bacterial populations were found in same coral species that were geographically separated, supporting the idea of specific bacteria-coral associations. Studies reveal differences in bacterial communities between healthy and diseased corals, particularly Vibrio populations which increased during bleaching but returned to previous levels during recovery. Similar dynamics were also observed in white plague diseased Favia favus. In addition, O. patagonica bacterial communities changed with the season indicating a complex and highly adaptive association.
An experiment conducted in 2001 exhibit V. shiloi adherence and penetration of coral exoderm where it multiplied to 108 cells /cm2 after 25 days. However, in 2006 V. shiloi was shown to adhere and penetrate coral tissue but was undetected after 4 days inferring an unknown mechanism of bacterial lysis by the coral holobiont.
Corals have a high surface area to volume ratio with thin exoderms to maximize nutrient uptake, but this makes them susceptible to infection. Despite their supposed vulnerability, corals do not suffer higher morbidity compared to organisms with adaptive immune systems. Corals thrive in oligotrophic waters and the mutualistic associations appear to extend beyond photosynthetic dinoflagellates providing nutrients while benefiting from host exudates. Bacteria and archaea may impart other advantages such as defence from free radicals, bacteriocins which protect from pathogenic colonization, provide nutrients from microbial anaerobic metabolism at night or from bacterial degradation of complex polymers, nitrogen fixation, etc. It is evident that corals must be considered a symbiotic organism comprising the coral animal, the endosymbiotic zooxanthellae and the diverse intracellular and mucosal colonies of archaea and bacteria.
8 comments:
Hi Mario
Good post mate.
The work I did with Emma over the summer showed how complex these probiotic communities are. She isolated all the bacterial species from a coral called Eunicella, 36 in total. As you saw, it took alot of agars to get through each and every interaction. The interactions we did at higher temperatures showed alot of antagonism and restriction of growth with some species. As sea temperatures rise I imagine this will have some really negative effects on the corals.
Hey Lee. Thanks. Nice avatar! I've seen him before. Is he the evil zooplankton in sponge bob?
It would be interesting to see Emma's final paper once it is published. I imagine antagonism on agar/MSA is indicative of what takes place in the natural environment but is not quite the whole story judging by how complex these interactions are in the coral holobiont.
An experiment was mentioned in the paper I reviewed, conducted by Bourne and Munn (2005) which highlighted differences in P. damicornis bacterial populations in tissues (mostly alpha-protebacteria) and mucus (mostly gamma-proteobacteria). Are there interactions between the two populations in the natural environment? Were bacteria isolated from the corals via homogenisation which would have mixed the populations up thus making it less representative of in situ populations?
Hi Mario
Great review, I wonder if coral biologists have changed their predictions about coral reefs being destroyed by 2050, given that this paper suggests corals can adapt more rapidly than previously thought, thanks to these interactions.
Hi Mario.
Ha ha yes his name is plankton, the evil genius who runs a restaurant called the chum bucket. He is always trying to steal the secret recipe to the krabby patty off Mr Krab. Its great having kids!
I am not completely sure about how she isolated them. I think she took samples off the corals and sub-cultured the bacteria down until she was left with single species. Some of the species were a number of vibrios isolated from the water column, really just to see how the symbiotic species coped with them. Another thing of course is that we only compared one on one but in nature there would be multiple interactions going on.
Look forward to reading her thesis!
Hey Helen,
Thanks! I like corals. The authors of the paper suggest that coral reefs generate $375 billion dollars per year in resources and services (e.g. tourism, fishing, etc). That is a lot of dough; certainly enough for politicians to start caring. :)
It is estimated that 120 million people from 6 countries directly rely on the coral triangle (6 million sq miles of reefs) in South East Asia for survival. Their disappearance will certainly be felt economically! Good thing they have some 'bacterial defence'.
Hey Lee, I wonder what Mr Krab's secret sauce is :). You are right about the multiple interactions. I also forgot to mention a lot of the bacteria can be non-culturable which makes it difficult to get a broader picture on the interactions.
Cheers for the comments. Mario
We will look at this area of research of research in some detail. Without wishing to dampen Helen's optimism too much, one of the major threats to corals is the stimulation of microbial activity that causes imbalance in the normal microbial community structure. The little book 'Corals in Microbial Seas' by Forrest Rohwer describes this really well. Rosenberg 's Coral Probiotic Hypothesis and the subsequent Hologenome Theory have received some fierce criticism (see Research Focus Box 11.1).
Hi Mario,
nice review!
I recently read a paper about V.shiloi and coral bleaching,perhaps the same you refer in your article. In that study it was found that V. shiloi was the causative agent of bleaching the coral O. patagonica in Mediterranean sea. Authors made an experiment to investigate the mechanism through which V. shiloi penetrate coral tissue and they discovered that endosymbiothic zooxanthellae play a key role in adhesion of V. shiloi to the coral by secreting an important compound of mucus on the coral surface.
I found this paper interesting,although is relatively recent (2001);i will soon post a review.
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