The Microbiome of Corals

Corals are reef building animals that are inhabited by bacteria, fungi and archeae. The microbes can live within the coral in their tissue or the skeleton or on their surfaces within mucus. This association allows that corals colonized different habitats. Corals are exposed to changes e.g. in temperature and have to deal with diseases like the white band or the white plague disease. They are caused by microbes but it is hard to identify the causal agents. This study by Chu & Vollmer (2016) aims to indentify bacterial species that are associated with healthy corals and how specific these relationships are. They studied the changes in coral microbiomes over time and space by analysing 16S rRNA from coral-associated bacterial communities. Chu & Vollmer used 100 coral colonies near Bocas del Toro, Panama. The samples were taken from Acropora cervicornis, A. palmata, Diploira labyrinthiformis, D. Strigosa, Porites astreoides and P. furcata in December, April and October 2012-2013.

The results show that the bacterial community composition within the corals depends mostly on coral host species while site and time are less important. Furthermore, the phylogeny of the corals is reflected by the similarity of the composition of their associated bacteria.

Chu & Vollmer found that Acropora spp. incorporate high abundances of Campylobacteraceae which are associated with the white band and white plague disease in Monastrea spp. That leads to the conclusion that healthy corals of one species can host potentially pathogenic microbes.

The core microbiomes seem to be shared between the species. Campylobacteraceae and Endozoicimonaceae were found as the core microbiomes in Acropora. Porites were dominated by Endozoicimonaceae and Gammaproteobacteria while Dilpoira showed high abundances of Alteromonadales. The dominant phylum in the OTU’s are Proteobacteria with high abundances of the order Flavobacteriales, Alteromonadales, Oceanospirillaler and Burkholderiales. Thus, the core microbiome ‘includes shared and host-specific bacterial phylotypes’ (Chu & Vollmer, 2016).

All in all this study shows the complexity of corals and their symbionts and provides information about potential coral pathogens that are incorporated in healthy species which do not seem to be effected by the pathogens. Furthermore the colonizing bacteria seem to have different niches and nutrient demands and are therefore found in different coral species and are host specific.

In my opinion this study is a base for further research on identifying the potential pathogens for a better understanding of coral diseases and their mechanisms. The methodology part is missing and at some points it is not clear what the authors aim to state. Additionally, it might be easier to split up the results and discussion part and not to put all together like Chu & Vollmer did. It would be interesting to see how the bacteria found in the corals effect their hosts and to know more about their ecological function. Last but not least I would like to see how global warming effects the pathogens in the healthy coral hosts and if they can become pathogenic for their hosts, too.

Reviewed paper:

Chu, N. D., & Vollmer, S. V. (2016). Caribbean corals house shared and host-specific microbial symbionts over time and space. Environmental microbiology reports.