Are cold water corals picky like their tropical counterparts?

Cold water corals are hotspots for biodiversity within the abyssal depths. The reef building scleractinian corals provide a safe haven for commercially important juvenile fish and provide habitats for other organisms. There are no symbiotic zooxanthellae, unlike their more extensively researched tropical counter-parts. But there has been distinct bacterial assemblages found, similar to warm water corals. Previous research has left gaps in the knowledge of the microbiomes of the dominant scleractinian cold water corals: Lophelia pertusa and Madrepora oculata. These knowledge gaps led Meistertzheim et al (2016) to resolve if these two species of coral have species-specific assemblages and temporal variation, similar to tropical corals.

Samples were taken from the upper limits of cold water coral distribution, from the submarine canyon Lacaze-Duthiers, in the Gulf of Lion. Two sets of samples were taken in November and May to assess temporal variation. Microsatellite genotyping was used to verify the species and if clonal polyps at the base and apical points of branches were present. 16s rRNA sequencing was used to identify the bacteria found within the microbiome of each sample.

Meistertzheim et al (2016) found that the two species do have significantly different bacterial assemblages, suggesting host specific associations, similar to other studies. But contrary to other studies, there was no temporal variation in M. oculata. Also the scleractinian had 60% similarity of bacteria associated between samples, suggesting a stable community. This disparity in results may be due to previous work sampling from mucus or scrapings of the coral, which may only represent opportunistic bacteria. In accordance with previous research, L. pertusa had very different microbial associations between different sampling times as well as different colonies (similarities as low as 4%) and even large variations within the same branch. For M. oculata the most dominant clade was Gammaproteobacteria, whereas L. pertusa did not have just one and included Alphaproteobacteria, Fusobacteria and Bacteroidetes, which were all weakly associated to one another. Also opposing previous research was the idea that L. pertusa is less stable than M. oculata. The less varied range in M. oculata microbiome may be associated with a narrow diet. Whereas the wider assemblage association from L. pertusa may correspond to a more opportunistic method of feeding.

I feel this paper is well structured and important because the corals are vulnerable to anthropogenic activities and climate change. The paper also does well to highlight marine microbes in an ecological context. However, I feel it does not fully answer the questions posed by previous research. Instead I think more work needs to be done to clarify why there are disparities between results. As well as this, further research into how the microbiome influences diet should be done, as I believe this study does not provide convincing and conclusive evidence. I also wonder whether the location of this study is applicable to other environments as it was about 8 degrees C warmer than previously studied sites. One major issue with the methods is that they selected to use ‘healthy looking’ corals. I cannot help but question how they defined something so subjective, especially when diseases in cold water corals are poorly understood. Although, they did find the presence of Endozoicomonas, a genus of bacteria associated with healthy tropical corals, the authors did little other than speculate this may be the cause of a stable community in  M. oculata.

Meistertzheim, A. L., Lartaud, F., Arnaud-Haond, S.,Kalenitchenko, D., Bessalam, M., Le Bris, N., & Galand, P. E. (2016).Patterns of bacteria-host associations suggest different ecological strategiesbetween two reef building cold-water coral species. DeepSea Research Part I: Oceanographic Research Papers, 114, 12-22.