Bacterial Associations with Developing Coral Embryos

Many studies have focused on the bacteria associated with adult corals, but this paper investigates the initiation and  importance of these relationships to the survival of the coral in early development. Embryos of the coral Pocillopora meandrina were collected from Kaneohe Bay, Oahu. A sub sample of 4 hour old embryos were prepared and frozen for immediate analysis while the majority were put into individual petri dishes containing raw seawater, sterile seawater or individual bacterial strains. Once the coral planulae reached an age of 170 hours, terminal restriction fragment length polymorphism (T-RFLP) analysis was used to determine the associated bacterial communities. Fluorescence in situ hybridization (FISH) was used to demonstrate the density of bacteria associated with coral planulae while electron microscopy located the position within the planulae.

 From the experiment, it was concluded that the composition of bacteria in the surrounding environment influences the density and variety of bacterial associations formed with coral planulae. The planulae formed prominent associations with bacteria within raw seawater, Roseobacter clade strain HIMB1 and Pseudoalteromonas strain HIMB1276 but didn't with other isolates. This suggests that the planula, bacteria or both play an active role in the relationship. Proteins within the planulae are thought to facilitate in the recognition of certain bacterial cells in order for them to cross the epithelium therefore bacteria which aren't recognised cannot form an association.

 Roseobacter clade strain HIMB1 formed the closest internal association, although this is not surprising as Roseobacters commonly form relationships with adult corals, performing a variety of beneficial functions for the coral. As bacteria was not externally or internally associated with 4 hour embryos, it is suggested that the bacteria form associations with the coral surface and are later incorporated into the ectodermal tissues, during development.

 The majority of the Pseudoalteromonas strain HIMB1276 was found on the very edge of the ectoderm, possibly playing a role in the settlement or adhesion of the coral planulae, as well as chemical signals produced by microbial biofilms. The other, less prominent, isolates used in the experiment were considered not suitable in a host environment.

 Viruses were also found to be associated with the coral planulae but further research is needed to understand the beneficial effects and threats to the coral and its holobiont. Further investigation is also required to establish the importance of bacterial communities for the successful development of coral embryos into adult colonies.

 Apprill, A., Marlow, H.Q., Martindale, M.Q. and Rappe, M.S. (2012) Specificity of Associations between Bacteria and the Coral Pocillopora meandrina during Early Development. Applied and Environmental Microbiology. 78: 7467-7475