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[PhD Thesis] Surface microbial communities of reef-building corals
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Dr Reia Guppy
Division of Biology
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This study investigates the natural bacterial community structure on the surface mucus layer (SML) of two Caribbean scleractinian corals, the star coral Montastraea faveolata and brain coral Diploria strigosa, using 16S rRNA gene molecular techniques. Using a biogeographical approach, strong site and host differences in bacterial communities determined that the SML bacterial distribution is deterministic and not random. Influence of geographic distance on the SML bacterial community differed between hosts. Spatial scale was an important consideration for coral SML bacterial biogeography, where holistically neither host nor region could adequately explain the clustering of sites. Denaturing gradient gel electrophoresis (DGGE) fingerprint patterns showed that the bacterial community in the biofilm (on glass slides) and SML shared a higher degree of similarity to each other than to the water column. The extent to which the SML community represented a settler community was therefore investigated using amplified ribosomal DNA restriction analysis (ARDRA) and sequencing techniques. ARDRA patterns showed lower similarity between samples than the DGGE fingerprints, with both the biofilm and SML containing Actinobacteria, α-proteobacteria and cyanobacteria. The differences in SML bacterial communities could not be clearly explained using the water quality parameters tested. There were no obvious environmental gradients in the water quality, possibly due to the fact that all sites showed evidence of a eutrophic state. However, given the strong differences in water quality between the wet and dry months, a comparison of DGGE operational taxonomic unit (OTU) diversity from coral SML across the wet and dry months indicated that the influence of time on the bacterial community also differed with host. Overall, M. faveolata SML bacterial communities represent better candidates as potential bioindicators of environmental change than D. strigosa, but further investigations on the influencing factor(s) for the changes observed needs to be elucidated.
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