Macro algae decrease growth and alter microbial community structure of the reef-building coral,Porites astreoides

 

Corals host species-specific communities of bacteria that perform a wide diversity of roles on reefs therefore its likely that coral associated microbes are critical for the maintenance of coral survival. Anthropogenic impacts are the main cause of loss to coral biodiversity and studies have shown that their trails are most severe in Caribbeancorals which have declined by up to 80% in recent decades. Increasing macro algae (MA) are thought to increase the current coral departure by various factors such as facilitating the spread of coral diseases. Despite evidence showing that increased MA abundance has negative effects on corals, we understand little about the mechanisms by which MA competition may impact the coral-microbial symbiosis. Increases in MA abundance may alter the normal microbial communities on corals as these shifts are likely to influence the taxonomic and metabolic diversity of coral-associated bacteria. In this study Thurber et al tested whether coral-MA interactions affect the microbial community on the coral, Porites astreoides, and whether such alterations impact coral health. This study tested this question in two ways; first growth was measured in different treatments with four species of macro algae to provide competition. The second was to examine for differences in the microbial taxonomic diversity in these treatments.

 

MA competition decreased coral growth rates yet, there were interspecific differences in how MA affected coral growth. Dictyota menstrualis, one of the species of algae used in a treatment, did not significantly suppress growth in P. astreoides. The other algae species all lowered coral growth rates relative to controls. Microbial abundance differed across MA species and among the coral-algal competition experiment. Community analysis also revealed that corals in competition with certain MA hosted microbial communities significantly different from those on control corals. These findings have shown that direct competition with some MA species increases the taxonomic variability of microbes on P. astreoides. However there are some problems used with the analysis employed in this investigation, the TRFLP analysis is not sensitive at detecting rare members of the community so it cannot be ruled out that rare members of the coral and MA microbiomes were not detected using this technique. Because of this it is possible that some microbial taxa, which appeared to be present on MA-treated corals but not on controls, were in fact also present on control corals but below the detection threshold of Thurber et als TRFLP analysis.

 

I believe this study is among the first to quantitatively analyse shifts in coral-associated microbial communities resulting from competition with MA and provides a good basis for further study. Thurber et al have shown that the presence of certain MA species reduces the growth rate of the coral P. astreoides, these reduced coral growth rates occurred with changes in microbial community composition. Furthermore this investigation has also shown that contact with MA can change coral-microbial specificity, allowing microbial taxa that are not normally associated with a host to become established. However one drawback of this experiment is that Thurber et al did not include an algal mimic treatment: (e.g., a piece of plastic placed in physical contact with coral fragments). Such a treatment would have indicated if the effect of live MA on microbial communities and coral growth was a result of multiple factors or solely physical abrasion or shading. Some results of this study conflict with past investigations, therefore i do not believe this study largely increases the scope of our understanding in this area as more work needs to carried out in order to clarify these conflictions.

 

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0044246