Enumeration of cell concentrations in suspensions of colonial freshwater microalgae, with particular reference to Microcystis aeruginosa

Abstract

Three published methods for the reduction of colonies of Microcystis aeruginosa Kütz. emend. Elenkin to single cells were investigated using a laboratory culture. Alkaline hydrolysis in 0·01, 0·1 or 1·0 m KOH was ineffective at room temperature; hydrolysis at 80°C using 0·01 m KOH reduced the colonies to single cells, but the use of higher KOH molarities resulted in cell loss. Sonication (20 kHz, c. 50 W) did not always completely reduce the colonies to single cells and it was found that cell losses occurred on some occasions. Heating at 80°C for 15 min, followed by 30 s vortex-mixing, was found to completely reduce the colonies to a suspension of single cells. Published results and experimental evidence suggest that the ability of each method to reduce colonies of M. aeruginosa to single cells without cell loss appears to depend on the particular strain used. The heating method was preferred for reasons of simplicity and the absence of associated cell losses; the technique was characterized with respect to temperature, duration of heating, and length of vortex-mixing period. Application of the heating method to 20 species of colonial freshwater microalgae (mainly planktonic) showed that the technique was successful with laboratory cultures of Dictyosphaerium ehrenbergianum Näg., D. pulchellum Wood, D. tetrachotomum Printz, Merismopedia glauca (Ehrenb.) Näg. f. insignis (Schkorb.) Geitler, and Pleodorina illinoisensis Kofoid.