On the functional significance of molecular variation in Symbiodinium, the symbiotic algae of Cnidaria: photosynthetic response to irradiance

The photosynthetic response to irradiance (PI response) of dinoflagellate algae of the genus Symbiodinium was quantified immediately after isolation from symbiosis with 9 species of corals and allied taxa on Bermuda. Significant variation in PI parameters was identified among the algal isolates, but no consistent differences between representatives of different Symbiodinium phylotypes were obtained. In a parallel analysis of Montastraea franksi, a dominant Bermudian coral species, colonies from 4 to 16 m (all bearing Symbiodinium of Phylotype B) were acclimated to high light conditions. The photoacclimatory response was slight. It included an increase in the dark respiration rate by algae from all collection depths, increased light use efficiency and decreased saturating irradiance of algae from 4 m, and increased maximal photosynthetic rate per unit chlorophyll (chl), but not per cell, for algae from 8 and 16 m. We conclude that generalisations about differences between the photosynthetic traits between Symbiodinium phylotypes are not valid, and that the wide depth distribution of M. franksi cannot be attributed to either depth-dependent association with multiple Symbiodinium phylotypes with different photosynthetic properties or strong photoacclimatory capabilities of its complement of Symbiodinium. These data suggest that Symbiodinium phylotype is not generally an important determinant of the abundance and distribution of symbioses on coral reefs.