Coral holobiont analysis to try and identify pathogens?

Bacterial profiling of White Plague Disease in a comparative coral species framework

Corals are now understood to be so much more than simple cnidarians. The coral holobiont is now widely accepted to incorporate all of the micro-organisms associated with corals as well as the polyps themselves. This makes characterization of disease very hard especially in discerning between a single responsible pathogen and a group of contributing micro-organisms. Diseases are usually grouped together by the displayed response. However we don't know if similar responses are caused by the same pathogen or if the same pathogens cause similar symptoms in different coral species. To try and answer some of these questions Roder et al., (2014) set out to compare the microbial community patterns of two white plague disease (WPD) infected coral species, Porites lutea and Pavona duerdeni, with their healthy counterparts. All samples were collected from the same reef in Thailand.

Using both Phylochip microarray and 16S rRNA sequence analysis they found 14,213 operational taxonomic units (OTU) across the coral species. Diseased coral samples had roughly 30% more OTUs then their healthy counterparts in both species indicating that a less diverse more stable community is beneficial. Between species there were 1003 OTUs that differed in abundance, which emphasizes the difficulty of isolating a single pathogen as there are many species contributing to the holobiont. Even within one geographic location these coral species have differences in bacterial community structure. There we 629 OTUs that differed in abundance between healthy and diseased corals of which two thirds were more abundant in the diseased individuals. Healthy coral had higher abundances of Comamonadaceae, Enterobacteriaceae and Streptococcaceae while Colwelliaceae, Pseudomonadaceae, Rhizobiaceae, Oceanospirillaceae, Vibrionaceae and Rhodobacteraceae were more abundant in the diseased corals. This implies some relationship between these families and the disease however there is no evidence for this and it is equally likely that these are opportunistic microbes that make use of the corals weakened state for colonization and growth.

These results differ from a previous study investigating WPD in the Caribbean though differences should be expected as the species investigated were different as well as the reefs being in different oceans. This might indicate that the diseased phenotype is actually caused by many different pathogens or pathogenic communities depending on coral species or location though many further studies will have to be carried out before any responsible microbe(s) can be identified. Aurantimonas coralicida a previously proposed pathogen was not identified in this study. This was thought to be responsible for WPD in the Caribbean though this could not be confirmed and it's absence here at least implies multiple geographic variants of the disease's pathogen if not providing evidence against Aurantimonas coralicida's pathogenicity. In future this Phylochip technique could prove useful in categorizing diseases and, as the genetic databases grow, will provide a more and more comprehensive view of the microbes that constitute the coral holobiont. I think it would be particularly interesting to see how microbial communities differ within the same coral species but at different geographic locations as we may get some insights into how much the coral influences it's own community make up compared to the influence of the surrounding water column.

(Roder et al., 2014)

Roder, C., Arif, C., Bayer, T., Aranda, M., Daniels, C., Shibl, A., … Voolstra, C. R. (2014). Bacterial profiling of White Plague Disease in a comparative coral species framework. The ISME journal, 8(1), 31–9.