Colorimetry, nephelometry, and chlorophyll assays of cultures of Chlamydomonas reinhardtii Dangeard, Chlorella pyrenoidosa Chick., and Scenedesmus quadricauda (Turp.) Bréb. were used as criteria for measuring algae growth. Among the four s-triazines studied, cyprazine appeared to be the most inhibitory to algae growth, followed by atrazine > simazine > cyanazine. Stimulatory effects of cyanazine, simazine, and cyprazine on chlorophyll content of C. reinhardtii and C. pyrenoidosa were also observed. Cyanazine was most stimulatory on both species but simazine also showed stimulation. Cyprazine was stimulatory only to C. reinhardtii at 0.22 μM concentration but apparently was inhibitory to C. pyrenoidosa at the same concentration. In one experiment, atrazine caused a slight stimulation in C. reinhardtii. Averaged over two consecutive experiments, however, atrazine was inhibitory. Chlorophyll content of the algae was found to be the best criterion for determining effect of the chemicals on algae growth. Continuous exposure of C. reinhardtii culture to the highest concentration of each of the four herbicides resulted in selection of algal lines having increased triazine tolerance.

In the course of the cell cycles of synchronous cultures of the chlorococcal alga Scenedesmus quadricauda, the following were monitored: total protein and RNA accumulation as a measure of growth processes, the timing of the commitment points at which the cells trigger the sequence of reproductive processes (DNA replication, nuclear and cellular division) and the course of the reproductive processes. The synchronous cultures were grown either under various lighting regimes, or in the temporary presence of specific inhibitors of either proteosynthesis (cycloheximide) or DNA replication (5-fluorodeoxyuridine). By adjusting the length of the light period, the cell cycle could be manipulated. Cell cycle patterns could be altered to give different numbers of sequences of reproductive processes. The extent of their mutual overlap could be influenced and the number of daughter cells produced could be altered. Schematic illustrations of various cell cycle patterns and comparisons with those of higher plants and other algal species are presented.