InChlorella vulgariscell aggregation, the clustering of single cells into groups is induced by an alkaline pH (9.5). The process of alkalinity-induced aggregation may be divided into two stages: the first stage (0–24 hr after exposure to the alkaline pH) is characterized by enhanced precipitation of cells from the medium, as well as by a seven fold increase in cell volume. The second stage (24–120 hr) is associated with a further increase in the extent of cell precipitation in the culture, which seems to result from the aggregation of clusters of enlarged cells. Electron micrographs reveal the existence, at this phase, of a number of autospores in the cells within a modified multi-layered mother cell wall. The pectin content of cells at this stage is twice that of control cells grown at pH 6.3. In addition, the relative content of the different pectin fractions is modified as a result of the exposure to alkalinity. It is suggested that the aggregates result from the repeated failure of the cells to detach from their original mother cell walls, thus forming clusters which represent several generations of ce
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