Corals and bacterial acquisition: a process still unknown!

Since corals are a consortium of different organisms belonging to all domains of life, they are considered holobionts. All of these organisms play a key role insight the consortium. Many studies focused on the role and acquisition of zooxanthellae, but there is still little known about the symbiotic relationship between corals and other organisms, such as bacteria, archaea and fungi. Bacterial communities have been proved to play different roles in coral health, according to the species involved. The acquisition of bacteria is a key process in a better understanding of the meaning of symbiotic association.

The aim of this study was to investigate whether this symbiosis occurs via a vertical transmission or an horizontal acquisition of bacteria by animals. To this purpose, presence of bacteria has been investigated in gametes, larvae and juvenile polyp of several hermaphroditic coral species.

Briefly, eggs and sperms were collected using a non-invasive nylon mesh tents collection attached to propylene collection jars. After fertilization, the resulting embryos developed into larvae, called planulae, that upon a variable time of swimming, selected and attached to suitable substrata (microscopy glass slides) and underwent metamorphosis into a juvenile polyp. All of the stages (gametes, planulae and juvenile polyp) were sampled at several times and fixed for FISH and microscopy analysis. Authors used 3 general eubacterial probes to detect The presence of bacterial cells in each stage, and a negative probe for the control. No bacteria were detected in eggs and sperm of six different coral species. No bacterial cells were detected in planulae stages, even if the water in which they swam as well as the substrata they selected to metamorphose contained bacteria. Interestingly, bacteria appeared only after the metamorphosis into a juvenile polyp, at a particular time dependant on single coral species investigated.

These results confirm that there is no vertical transmission of bacteria via gametes, hence the acquisition occurs from the environment, but authors were not able to provide any explanation of which factors are involved in the process.

It would have been suitable if authors looked at the gene expression of each stage, especially the expression of genes involved in metamorphoses process, as the change in tissues and the related shift in the gene expression can play a key role in the selection and in the subsequently uptake of bacteria, perhaps in a process which is very similar to the well described acquisition of V. fisheri by E. scolopes.

Reference: Sharp KH, Ritchie KB, Schupp PJ, Ritson-Williams R, Paul VJ (2010) Bacterial Acquisition in Juveniles of Several Broadcast Spawning Coral Species. PLoS ONE 5(5): e10898. doi:10.1371/journal.pone.0010898