Macroalgal competitors cause shifts in coral-associated bacteria

Corals harbour diverse eukaryotic and prokaryotic microorganisms that form dynamic associations with their coral host. The ecological roles of coral associated microbes are not, at present, fully understood but it is clear that they are of significant importance to coral health as shifts in these microbiota have been shown to cause detrimental effects on coral health. As a result of the plight facing corals and the potential significance of their associated microbes, many scientists have begun studying the shifts in microbiota in relation to anthropogenic change, such as increased sea temperatures and lowered pH levels. However, anthropogenic change is also causing shifts in the dominant species in coral reefs themselves. Raised nutrient levels have led to an influx of macroalgae to colonise and compete with reef-building corals, macroalgae are known to produce a wide range of metabolites to defend against bacterial colonisation, either by inhibiting behaviour or causing bacterial lysis, such metabolites are also likely to indirectly affect coral-associated bacteria.

Morrow et al. (2012) aimed to study what effect macroalgae had on coral-associated bacteria by applying crude extracts from three common Caribbean macroalgae directly on to two species of reef-building coral found in Florida and Belize. Denaturing gel electrophoresis (DGGE) was used to examine changes in the bacteria assemblages associated with the surface mucus layer (SML) of both coral species.

Results showed that bacterial communities within the SML of both coral species were significantly altered from their initial communities by some macroalgal extracts on both Florida and Belize reefs. The extent to which extracts shifted the bacterial assemblages depended on both the coral host and type of macroalgae tested. Furthermore, there was also a significant increase in glutathione S-transferase activity (an essential enzyme for detoxification) within coral tissues exposed to macroalgal species, suggesting a heightened level of stress.

The findings from this study are an important first step toward examining the impact of macroalgal compounds on coral-associated bacteria and could be another potential stressor effecting corals, especially in areas with low herbivore activity. Although authors show the presence of shifts in the coral-associated bacteria, they fail to isolate the associated bacteria, and so, the bacteria which are affected remain unknown. Furthermore, the structure of the coral host provides a heterogenous habitat, with distinct and diverse communities but the authors of this study only look at one (SML), this could of hidden further shifts in coral-associated bacterial assemblages. Future research should include the identification of the coral-associated bacteria and also study the communities associated with the coral tissue surface and SML.

It seems obvious to me that anthropogenic shifts may favour coral competitors causing stress to corals through competition for light, nutrients and space. However, I had completely missed the potential effect of metabolite defences produced by competitors on coral-associated bacteria, a consideration which I believe many scientists studying anthropogenic change on corals have also overlooked, and so, is my reason for reviewing this paper.

I welcome any questions.

Morrow, K.M., Ritson-Williams, R., Ross, C.,  Liles, M.R. and Paul, V.J. (2012) Macroalgal Extracts Induce Bacterial Assemblage Shifts and Sublethal Tissue Stress in Caribbean Corals. PLOS one 7, e44859. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0044859