Natural Variability in Coral Microbiotas: Has it Undermined our Understanding of Coral Health and Disease?

Investigations into the diversity of coral-associated microbes and inferences as to their roles within the coral holobiont (i.e. the coral host plus mutualistic Symbiodinium, bacteria, archaea, and fungi), have been based around the premise that coral microbiotas are important to the health and function of the host, and that alterations in this community are linked to coral disease.  However, without an understanding of the natural variability of coral-microbial communities and a realistic baseline description of healthy coral microbiotas, conclusions as to the causes and effects of coral disease are unlikely to be reliable.  Coral disease is a major threat to the future of reef-building tropical corals; with corals in chronic global decline, resolution of this issue could not be more urgent or relevant.

Recently, Kvennefors et al (2010) reported on the variability of the microbial communities associated with both healthy and diseased coral colonies: culture independent methods such as DGGE (of the V3 region of 16S rDNA) and FISH were employed to investigate the diversity and community structure of microbes associated with two coral species, Acropora hyacinthus and Stylophora pistillata.  Replicate samples were collected from three different sites around Heron Island (southern GBR, Australia), from both apparently healthy coral colonies, and those suffering from White Syndrome (WS).  The authors observed both species-specificity and site-specificity in the microbial communities of coral colonies, plus pronounced community shift in those affected by WS.  However, variability between replicates was also noted: a set of discrete coral-community profiles were described.  Species accumulation curves indicated that the minimum number of replicate samples needed to adequately investigate bacterial diversity was six (each collected from a different colony); Kvennefors et al (2010) concluded that at least six replicates were therefore needed to infer any influence of host species, location or disease.

The importance of this finding is illustrated by an investigation undertaken by Littman et al (2009): diversity of coral-associated microbes in three acroporid coral species was reported as varying between two GBR sites, but not between the host species, in healthy colonies.  Evidence of coral species specificity in associated microbial communities has been reported by many (e.g. Frias-Lopez et al., 2002; Bourne & Munn, 2005); however, it was suggested by the authors that lack of species-specificity in the microbiotas of these corals was due to the three host species surveyed being very closely related (in the same genus).   Littman et al (2009) collected three replicates of each coral species (in each site): Is the reported similarity in the microbiotas of closely related host species genuine, or would more extensive sampling suggest otherwise?  The evidence from Kvennefors et al (2010) indicates that without more adequate sampling, this question remains.

Possibly due to the time and money required to undertake the various molecular methods employed in such studies, work continues to be reported as having been undertaken using considerably less than the minimum number of replicates suggested by Kvennefors et al (2010).  This year (2012), Ceh et al published an investigation into the potential changes to coral-microbiota diversity and structure before and after a coral mass spawning event: two colonies per coral species were sampled.  It appears that under-replication may have inhibited our ability to understand the health (and thus disease) of the coral holobiont in the past; if so, it may continue to hinder us for some time to come.

 

Kvennefors, E.C.E. et al., (2010). Bacterial Communities of Two Ubiquitous Great Barrier Reef Corals Reveals Both Site- and Species-Specificity of Common Bacterial Associates. PLoS ONE 5, p.14

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0010401

Littman, R.A. et al., 2009. Diversities of coral-associated bacteria differ with location, but not species, for three acroporid corals on the Great Barrier Reef. Fems Micriobiology Ecology 68, p.152-163

http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6941.2009.00666.x/abstract

Ceh, J. et al., 2012. Coral-bacterial communities before and after a coral mass spawning event on iingaloo Reef. PLoS ONE, 7(5), p.e36920

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3353996/

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