A Comparison of Bacterial Symbionts within Corals

Photosynthetic corals are known for their symbiotic relationships with zooxanthellae that live within them, supplementing their energy demand with the products of photosynthesis. They also harbour bacterial symbionts that also play a role in the corals life.

In the paper by La Rivière et al. (2015) they aim to try and find out if sympatric gorgonian corals harbour specific bacterial assemblages and share common associates, and also to see if these assemblages differed from summer to winter. Three species of gorgonian coral (Eunicella singularis, Eunicella cavolini, and Corallium rubrum) were used, apical branches were collected from the specimens and tissue was then extracted from them. The symbionts DNA is extracted from this tissue and put through PCR using universal primers to amplify the bacterial 16S rDNA. They were then sequenced using an ABI 3130 Genetic Analyzer, and the variation within the communities was detected using terminal restriction fragment length polymorphism (T-RFLP). Some of the products from the PCR process were used to create clones of the symbionts and the genetic sequences of the 16S rDNA were then used to phylogenetically list the symbionts to the lowest possible taxonomic rank and assemble them into clone libraries.

They found that there was no major differences between the assemblages in each species sampled in the winter compared to the summer, but analysis of the T-RFLP profiles indicated that each species of coral harboured a distinct community of symbionts. The symbionts found were assembled into 25 operational taxonomic units, into which two of these are unknown, and 11 of them cannot be identified lower than their Class (Alphaproteobacteria or Gammaproteobacteria). C. rubrum was found to have a distinct group of symbionts to the Eunicella species having a much wider range of Gammaproteobacteria (nine identified) and is the only coral to have Bacteria that could not be identified inside it. That compared to E. singularis having only Hahellaceae sp. 1 of the Gammaproteobacteria found inside it and E. cavolini having four different types. The Gammaproteobacteria have a clear dominance inside these corals, but that being said all three species of coral share specific groups of Bacteria.

They mention the possibility of one of the Hahellaceae ribotypes found in this study to be from a new undescribed bacterial genus which I would say is a pretty bold statement given the main basis is that it the similarity from this particular analysis doesn’t reach the cut off for it to be classified, more work needs to be carried out to confirm this.

They decided to use two species from one genus and one species from another, I think the results would have proved vastly different if they chose species that were from the same genus or from completely separate genera. This would have given either a greater contrast in results or proved some uniformity, as can be seen in the species from the same genus.

In conclusion this paper is the first to compare bacterial communities in sympatric coral species, which is useful information to have as it can give assumptions about the bacterial communities of other coral species.

Reviewed paper: La Rivière M., Garrabou J. and Bally M., (2015). Evidence for host specificity among dominant bacterial symbionts in temperate gorgonian corals. Coral Reefs. 34(4). pp.1087-1098.