Toxic effects of cobalt and zinc to Chlorella pyrenoidosa (26) in soft and hard water.

Different concentrations (0.83, 3.33, 10 ppm) of cobalt (cobalt sulphate) and zinc (zinc chloride) were added to bristol media prepared using soft water (addition of 20 mg/l calcium carbonate) and hard water (addition of 400 mg/l calcium carbonate). A known amount of Chlorella pyrenoidosa (26) was inoculated into various media, and the number of Chlorella cells was counted every other day for a period of 8 days. Cobalt and zinc were more toxic in media prepared using soft water than hard water as revealed by the growth rate of C. pyrenoidosa. These results agree with studies elsewhere on other aquatic organisms.