Photo-oxidants and algal community composition (Anabaena flos-aquae, Selenastrum capricornutum, Scendesmus quadricauda)

Algal blooms in eutrophic freshwaters are typically dominated by large, colonial cyanobacteria. The euphotic zones in such systems are typically restricted to a shallow surface layer; hence, algae must be able to withstand high levels of irradiance and photo-oxidants. This study examines whether cyanobacteria are more tolerant of high UV levels and associated photo-oxidants than eucaryotic competitors. Anabaena flos-aquae grew in batch culture under 14-hour per day exposures to UV confirming that colonial cyanobacteria are quite resistant to UV. The green algae, Selenastrum capricornutum and Scendesmus quadricauda were unable tolerate exposures of 3 to 6 hours of UV per day, respectively. Extracellular production of photo-oxidants had no effect on algal survival in batch culture whereas adding ascorbate, a known anti-oxidant, to media provided S. capricornutum some protection from UV. The results show that the harmful effects of UV are likely caused by intracellular, not extracellular, absorption of photons and that the cyanobacterium A. flos-aquae is highly tolerant of UV. Measuring strong photo-oxidants such as superoxide and hydroxyl radicals can prove difficult due to the transient nature of these reactive species. This study also tested a novel and simple method for assaying the relative photo-oxidation potential (POP) of natural and synthetic waters. The assay measures loss of absorbance of the blue dye, erioglaucine, by photo-oxidants in irradiated water samples. The dye is stable in the dark, is not sensitive to UV, hydrogen peroxide or superoxide but is sensitive to hydroxyl radicals. There was no relationship between POP and the onset of a cyanobacterial bloom in eutrophic Heart Lake near Toronto, Ontario, a finding consistent with the batch culture studies which showed that algal survival is not affected by extracellular photo-oxidants.