Scleractinian (or hard corals) are an integral part of the worlds’ coral reefs, and are thought to be formed as a result of the interactions between these corals and their symbiotic zooxanthellae and other microorganisms which are numerous but their function is yet to be confirmed. It has been suggested that microorganisms residing on the surface of the coral may provide protection against pathogens through inter-specific competition or by the secretion of antibiotic substances. They may also be providing the coral with nutrients that are not being supplied by the symbiotic zooxanthellae, such as nitrogen and phosphorous.
A mucous-rich micro-layer extends a few millimetres above the surface of Scleractinian corals where nutrient transfer occurs. This layer is also thought to provide a growth medium for these microorganisms, and white patches have been recorded covering the surface of some of the larger species. These patches are caused by unicellular heterotrophs belonging to the stramenopile group.
Favia Corals from the gulf of Eilat and the red sea were sampled in 2005-2006 from water 1-6 metres deep. The mircobiota from these samples were cultured, cloned and their DNA amplified using PCR. The DNA was then sequenced to determine the species of microbes present in the white patches.
DNA profiling revealed that 90% of the microbes present in these patches were stramenopiles of the Thraustochytriidae family, while several other families shared the remaining 10%. From the 90% majority, 50% were unable to be identified to genus level, while the other 50% were found to be one of 3; Aplanochytrium (23%), Thraustochytrium (17%) and Labyrinthuloides (10%).
In contrast to previous studies that found negative coral-protist associations, the results of this study suggests that these white patches do not cause any visible damage to the coral hosts. The microorganism family Thraustochytriidae are known to degrade a wide variety of organic substrates. It is possible that this process aids symbiosis with the corals by providing them with smaller, more easily absorbed organic compounds.
The authors of this paper consider the relationship between the Favia coral and the microorganisms that cause the white patches to be mutualistic, as the microbes are thought to live off the coral mucus, and in turn provide the coral with pre-processed small organic compounds. The relationship is however poorly understood at present, and is in need of considerable further study.
A review of N. Siboni, D. Rasoulouniriana, E. Ben-Dov, E. Kramarsky-Winter, A. Sivan, Y. Loya, O. Hoegh-Guldberg, A. Kushmaro (2010) Stramenopile Microorganisms Associated with the Massive Coral Favia sp., The journal or Eukaryotic Microbiology, 57 236-244
2 comments:
These thastrochytrids are a very intranet group because of their production of omega 3 poly unsaturated fatty acids, which are very important in larval nutrition. it seems a bit curious to me that the authors suggest (if I have interpreted you correctly) that the coral absorbs small organic ompounds. The protists must be big enough for the coral polyps to ingest them and obtain nutrition that way. Wouldn't this make more sense?
Hi Colin
The paper does seem to suggest that coral absorb the nutrients rather than ingesting the protists. I found a nice looking paper that might be able to clarify this, though unfortunately I can't gain full access to it!
Arotsker L., Kramarsky-Winter E., Kushmaro A. (2011) Coral-Associated Heterotrophic Protists, Beneficial Microorganisms in Multicellular Life Forms, 151-161
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