Scleractinian corals form an intracellular association with dinoflagellates. Due to this association, scleractinian corals are capable of higher growth rates than other coral species; allowing them to build large structures that are the framework of coral reefs.
Recent studies show that reef building corals may be associated with several species of symbiotic dinoflagellates. Whilst the genetic relationships between different clades of symbiotic dinoflagellates are being rapidly generated, little is known about the physiological or ecological ramifications of the diversity of symbionts reported to occur among symbiotic invertebrates.
Elevated temperatures and high solar irradiance can cause the expulsion of symbiotic dinoflagellates from coral tissues and/or the loss of photosynthetic pigments, giving the coral a pale or white appearance. This phenomenon, called 'coral bleaching', is a major concern because it is the cause of significant loss of live coral in many parts of the world. There has been growing interest in understanding the acquisition, maintenance and dynamics of symbiotic dinoflagellates by coral species as the symbiotic dinoflagellates are thought to be the primary site at which coral bleaching begins, and genetic variability may offer some hope in providing variability in tolerance among hermatypic corals faced with rapidly rising sea temperatures.
This project focused on the coral Acropora longicyathus. Adult colonies of this coral may harbour one of two clades of symbiotic dinoflagellates (clade A or clade C) or a mix of these two clades. It explored how the recruits of spawning corals, especially in the case of where transmission is horizontaL select the symbiotic dinoflagellates. Also explored how the coral host and the symbiotic dinoflagellates are affected by environmental fuctors that varied seasonally (water temperature, irradiance and photoperiod), especially in the case where different colonies of the same coral species may harbour different symbionts. Sampling was carried out over a period of two years on the reef surrounding One Tree Island in the Capricorn-Bunker Group, southern Great Barrier Reef.
My results suggest that the acquisition of symbionts by the coral recruit does not relate to the symbionts associated with the parental colonies. Samples taken 5-10 days after settling showed that recruits took principally symbionts of the clade A regardless of the parental symbiont. The same populations of recruits analyzed 3 month later harboured at least 3 different genotypes of symbiotic dinoflagellates including the parental symbionts (clades A and C). Remarkably the other genotype found was a dinoflagellate common in tropical environments (clade D). Different models are discussed as possible mechanisms of colonization of the coral recruits by symbionts. Survivorship of juvenile colonies is also discussed as a function of the symbionts the recruits acquire.
The temporal study of the coral-dinoflagellate association shows that biomass characteristics of the symbionts presented a cyclic pattern in density, mitotic index and chlorophylls a and c2 per area of coral. Also the biomass characteristics of the coral presented a cyclical pattern congruent with the one presented by the symbionts studied (protein and lipid content in the tissue, and coral growth). Water temperature was perhaps the most influential variable in some of the physiological characteristics studied followed by solar irradiance.
There were significant differences in the biomass characteristics of symbionts belonging to the different clades as well as in the biomass characteristics of the corals associated with the different symbionts. Clade C and corals associated with this clade presented the highest values for most of the characteristics. Symbionts clade A and the mix of clades and corals associated with these symbionts usually presented similar values for most of the biomass characteristics.
The findings that coral recruits may acquire a different symbiotic dinoflagellate than the ones observed in the adult colonies and that the dinoflagellates studied presented distinct physiological characteristics over the sampling period is relevant to the current debate on coral bleaching and the acclimatory and/or adaptive capacity of the coral.
Further research, involving more replication over a single year as well as more years and (if possible) more replicates per symbiont clade, should be perform in order to allow a generalization.