Selenium Requirement of a Bloom-Forming Planktonic Alga from Softwater and Acidified Lakes

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Abstract
Lakewide odor episodes have been associated with blooms of the planktonic alga Chrysochromulina breviturrita Nicholls (Prymnesiophyceae) in lakes influenced by acidic precipitation. We demonstrate an absolute requirement in this flagellate for selenium (Se) in axenic culture. The alga is capable of utilizing several forms of Se: selenite, selenate, dimethylselenide (DMSe), and selenomethionine. This is believed to be the first demonstration of the utilization of DMSe as a Se source by any organism. As the microflora of aquatic sediments recycle Se as volatile DMSe, this may have significance in its biogeochemistry in acidified waters. Bioassays of lake waters from the Muskoka–Haliburton region of Ontario show that in one lake (Dickie) a simulated Se spike resulted in a significant increase (70%, p < 0.05) in algal growth. Growth in Cinder Lake water, which had supported a population of C. breviturrita, was marginally stimulated (24%, p < 0.05). Distant coal-fired power plants, which have been implicated in the episodic deposition of Se, may account in part for the unpredictable blooms of the alga. Chrysochromulina breviturrita cannot be maintained in culture without this micronutrient, and several Se sources may be important triggering factors for the initiation of blooms.