Photobiology of natural populations of zooxanthellae from the sea anemone Aiptasia pallida: assessment of the host's role in protection against ultraviolet radiation.

Natural populations of the sea anemone Aiptasia pallida containing endosymbiotic dinoflagellates were acclimated to different irradiance regimes, with and without ultraviolet radiation (UV). They showed a compensatory response in the amount of chlorophyll and the activities of enzymes responsible for detoxifying active species of oxygen that are produced by the interaction between visible or ultraviolet radiation and photosynthetically produced oxygen. Protection from active species of oxygen is essential to prevent the photooxidation of chlorophyll and the concomitant loss of productivity. Bulk analyses of chlorophyll showed differences between the populations exposed to varying irradiance regimes, but revealed no significant independent effect of UV. However, analysis by flow cytometry of the individual cells from treated populations did detect statistically significant differences in cell size and the amount of chlorophyll fluorescence per cell, which could be attributed to treatment with ultraviolet radiation. With flow cytometry we are able to detect the population variability that is undetectable by bulk measurements which is important in assessing the effects of environmental parameters in both symbiotic and free-living microalgae. Research using simultaneous measurements by flow cytometry could add considerable insight into the population dynamics of both zooxanthellae and host cells.