Alkalinity-induced aggregation in Chlorella vulgaris II. Changes in the cell wall during the cell cycle

In a Chlorella culture growing synchronously at pH 6.3 under a 12 hr light-12 hr dark regime, autospores are released during the dark period of the cycle by mechanical rupture of the cell wall. When the culture is made in an alkaline medium (pH 9.5) during the first 8 hr of the light period, the release of autospores is prevented, and at the end of the cycle, the cells are enlarged. It was found that the diurnal fluctuations in cellulase specific activity and in polysaccharide content, which normally take place at pH 6.3, do not occur at alkaline pH. The concentration of polysaccharides, as measured by the periodic acid-Schiff reagent, and cellulase activity are almost doubled. Peak levels are reached at the 8th hour of the cell cycle rather than the usual 4th hour and persist during the second part of the cell cycle, whereas they normally decrease. It is suggested that under alkaline conditions, the diurnal increase in cellulase activity is amplified and prolonged, resulting in increased flexibility of the mother cell wall. As a result, the cell wall is stretched, rather than ruptured, during the development of daughter autospores, which are thus prevented from being released.