Not all coral dinoflagellates are symbiotic

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A recent study suggests that the coral-dinoflagellate relationship may be more one-sided than previously believed.

The current school of thought dictates a symbiotic relationship between reef-building corals and the dinoflagellate algae they harbour " the dinoflagellates exchange inorganic waste metabolites from the coral for organic nutrients it fixes via photosynthesis.

However, in a paper published in a recent issue of the Proceedings of the National Academy of Sciences, Michael Stat, Emily Morris, and Ruth Gates show that not all dinoflagellates are truly beneficial to their coral hosts.

The authors study the dinoflagellate genus Symbiodinium, which is long known to be associated with coral hosts.

They first examined the phylogenetic relationships of Symbiodinium using nuclear (rDNA) markers and identified eight lineages (referred to as clades A"H, with clade C being the dominant symbiont found in corals).

The authors then challenge the assumption that clade A and C Symbiodinium form equally mutualistic symbioses with corals by evaluating the health state of corals that harbour clade A and C Symbiodinium, and investigating in vitro carbon fixation and release rates by Symbiodinium clades A and C.

They found that corals hosting clade A symbionts showing a significantly higher incidence of disease or abnormal phenotypes than those hosting clade C, and that clade A symbionts fixed and released significantly lower amounts of carbon.

According to the authors Taken together, we suggest that Symbiodinium clade A is less beneficial to corals than other Symbiodinium lineages and may be more representative of a parasitic than a mutualistic symbiont, whereby the animal host does not receive sufficient nutritional input from the dinoflagellate symbiont, a circumstance that ultimately renders the coral more susceptible to disease and mortality.

For more information, see the paper: Stat, M., E Morris and RD Gates (2008) Functional diversity in coral dinoflagellate symbiosis. Proceedings of the National Academy of Sciences 105, pp. 9256"9261.