PHOTOSYNTHESIS OF ALGAL CULTURES AND PHYTOPLANKTON FOLLOWING AN ACID pH SHOCK1

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Abstract
In acidifying lakes, pH decreases abruptly in response to acid precipitation events. We tested the hypothesis that, in comparison to a circumneutral lake, phytoplankton photosynthesis in an acidifying lake is less sensitive to a rapid decrease in pH (acid pH shock).Phytoplankton in Plastic Lake, which is undergoing acidification, was characterized by a predominance of Pyrrophyta, and phytoplankton photosynthesis decreased to a lesser extent in response to an acid pH shock than the photosynthesis of populations from St. Nora Lake, a circumneutral lake located nearby, in which Pyrrophyta were not abundant. Rates of phytoplankton photosynthesis in acid pH shock experiments were significantly correlated with hydrogen ion but not with dissolved inorganic carbon (DIC) concentrations.Depression of photosynthesis following an acid pH shock occurred in axenic cultures of Chlorella pyrenoidosa Chick but was not observed in axenic cultures of the acidophilic alga Chlorella saccharophila (Krug.) Nadson or in three species isolated from Plastic Lake. However, the three isolates were not acidophilic during growth. We conclude that phytoplankton in acidifying lakes consists predominantly of species which are tolerant to acid pH for short periods (hours) but cannot grow at these pHs.

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