Coral and their bacteria: a complex relationship

Bacteria associated with coral are proving to be increasingly important in coral health. It is already known that successful establishment and survival of a coral reef is made possible by symbiosis of dinoflagellates belonging to the genus Symbiodinium, however, it is not known just how extensive the bacterial interactions or what benefit each type of bacteria confers to the coral or is conferred upon the bacteria by the coral. This study points out that this is mainly due to a lack of replicability in previous studies on the natural variation of the bacterial community within the coral holobiont.

This study used a variety of techniques in order to assess the diversity of bacteria associated with two different types of coral Acropora hyacinthus and Stylophora pistillata, including Denaturing gradient gel electrophoresis (DGGE) and Principle component analysis (PCA). This allows the study more power when it comes to drawing any sort of conclusion. The samples that they collected were from three different sites across the Great Barrier Reef around Heron Island, Tenements, Harry’s Bommie and Wistari reef. They took 69 coral samples and 12 samples of the water. The water samples taken by the authors allows them to run the same tests upon the water around the reef, which in turn allows them to rule out bacterial contamination from the surrounding water when drawing conclusions about the natural diversity of coral associated bacteria.    

This study found that the bacterial communities which were identified in both A. hyacinthus and S. pistillata were not only significantly different from each other but differed significantly between the sampling sites. This can lead to the conclusion that different corals have different requirements which results in a diverse bacterial community across the different coral species. The differing communities between sites is an interesting find though as this could lead to the speculation that the bacterial community can also be influenced by the environment as well. The authors of this paper have certainly entertained the thought particularly in the case of coral pathogens as the authors found several Vibrio spp. on apparently healthy corals as well. They also state that ‘local small-scale oceanographic processes may affect the bacterial communities associated with the coral colonies’.  In addition to the main findings the authors also found that a group of γ-proteobacteria were present across the sites. There wasn’t enough data available to the authors to be able to properly classify the bacteria into genera so they were instead placed under the term of Type A associates. The paper goes on to state that even though these Type A associates are relatively common on reef corals their precise function has not yet been determined, perhaps extra research into these associates would be prudent in order to more fully understand the coral-bacteria associations that are present.

Kvennefors ECE., Sampayo E., Ridgway T., Barnes AC., Hoegh-Guldberg O.,2010, Bacterial Communities of Two Ubiquitous Great Barrier Reef Corals Reveals Both Sites- and Species-Specificity of Common Bacterial Associates, PLoS ONE 5(4)