Corals as Underexplored Habitats

Although fairly short, this paper nicely highlights the importance of coral reefs as microbial habitats. Recent advances in sequencing have led to the discovery of many taxonomically diverse microbes existing at low abundances in marine environments, but little is known about this so-called ‘rare seawater biosphere’. Several questions remain about the ecological niches occupied by these microbes and their response to changes in environmental conditions, which could theoretically see them becoming more abundant, and perhaps even dominant.

The authors investigated the role of corals as specialized habitats for microbes including rare taxa. They analysed over 350,000 16S rRNA sequence tags and 1960 nearly full-length 16S rRNA gene sequences from samples of seven Caribbean coral species and a reef water sample.

The results showed that corals provide specialized habitats for rare microbes - many taxa rarely detected in reef water were abundant or even dominant in the coral samples. However, it is difficult to discern whether this is due to the diluting of bacteria in reef waters, or due to coral hosts providing suitable environments for rare microbes from the water column to replicate. Over 8500 OTUs were discovered in total, with mound-shaped coral colonies exhibiting higher diversities than branch-forming and gorgonian corals, suggesting that morphology has an important role in shaping the diversity of coral microbiota.

It was also found that closely related corals had a similar composition of bacterial communities, but the results at higher taxonomic levels were inconsistent. It is suggested that environmental factors are more important for shaping bacterial community composition, and coral-microbial associations may not be stable over time.

Many bacteria were found to be host-specific, associated with only one coral species, although the authors did not sample different individuals of the same species so it was not possible to determine variation between individuals. Evidence also suggested that corals harbor species yet to be classified as well as potentially novel genera.

Two of the corals sampled in this study (Acropora palmata and A. cervicornis) are listed as threatened under the U.S. Endangered Species Act. This study points out that corals are underexplored and makes many recommendations for further investigations so that more can be understood about these microbial communities. This paper emphasises the importance of coral conservation and the potential loss of biodiversity that would occur if corals were not preserved.

A review of: Sunagawa, S., Woodley, C.M. and Medina, M. (2010) Threatened Corals Provide Underexplored Microbial Habitats. PLoS ONE, 5(3)