Different coral species share a small core bacterial community within highly diverse communities

The diverse and complex communities of coral-associated bacteria that form part of the coral holobiont play an important role in coral health, disease and the tolerance of abiotic stress. Currently there is much contradiction and inconsistency between studies regarding the specificity of coral-associated assemblages. Some studies report that bacterial communities are specific to the species of coral; others describe bacterial communities that were influenced more by location than coral species. Other studies report that both coral species and location are equally important. Clearly, further investigation is warranted to investigate the specificity of coral-associated bacterial assemblages globally.

This study by Li et al. (2013) investigated the bacterial communities associated with three dominant coral species in the South China Sea, with the aim of characterising bacteria communities that were specific to particular coral species and those that were common to all three coral species. The study looked at coral species Porites lutea, Galaxea fascicularis and Acropora millepora as these were dominant in the South China Sea. Coral reefs within the South China Sea occupy a similar area to the Great Barrier Reef and have both a comparable latitudinal range and biodiversity.

Samples of the three coral species along with seawater were collected from the Luhuitou fringing reef, China at a depth of 3-5 m using a punch and hammer. Samples were then filtered, DNA extracted and the 16S rRNA gene was targeted for PCR amplification. Quantified sequences were grouped into operational taxonomic units (OTUs) with a minimum of 97% similarity. The analysis of pyrosequencing libraries combined with barcoded PCR primers highlighted that bacterial assemblages associated with the three coral species were more diverse than previously thought and three new bacterial phyla were discovered. Bacterial communities associated with A. millepora differed from those of P. lutea and G. fascicular, which had more similar bacterial communities. In addition, all three coral community types differed from those found in seawater.

Between the three coral species there were only 22 97% OTUs that were shared and these were found in the following groups: Alphaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, Chloroflexi, Actinobacteria, Acidobacteria plus an uncategorized bacterial group. The proportions of these phyla differed between the three coral species. Additionally, the study detected potential nitrogen-fixing bacteria and bacteria involved in the degradation of DMSP and DMS. This has important implications in oligotrophic waters where many corals reside. There was also an abundance of Actinobacteria in coral samples compared to seawater. This phylum is known to produce a range of antibacterial compounds which may offer protection against pathogens.

Bacteria associated with the corals examined in this study were highly diverse and different from seawater bacterial communities, yet a small core bacterial community may be shared by different coral species. Coral morphology may have a role in microbial diversity as previous reports showed that mound-forming corals had higher coral-associated bacterial diversity than branch-forming corals such as A. millepora, as supported by this study. Furthermore, bacterial communities in the South China Sea associated with A. millepora and P. lutea were different from those found in the Great Barrier Reef and Indo-Pacific reefs. The authors report that these differences may be due to the different environmental conditions at separate locations rather than species-specific differences.

I feel that this paper has downplayed the implications of the study’s discovery in terms of the importance of characterising the bacterial communities associated with corals and that the overall message was slightly unclear when considering the arguments of other similar studies. This study has however helped support Rohwer’s theory that corals may harbour specific and conserved bacterial populations. This has implications for the health and stability of corals, particularly with the threat of declining coral reef ecosystems globally.

Reference:

Li, J., Chen, Q., Zhang, S., Huang, H., Yang, J. Tian, X.P. and Long, L.J. (2013) Highly heterogeneous bacterial communities associated with the South China Sea reef corals Porites lutea, Galaxea fascicularis and Acropora millepora, PLOS One, 8, (8), 1-8.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0071301