Cadmium is a toxic heavy metal released from smelting and absorbed and bioaccumulated by organisms along the food chain. Cadmium binds to organic molecules by forming bonds with sulfur and nitrogen, thereby inactivating proteins, and is one of the three contaminants which, according to the EPA, pose the most threat to the environment. The objective of this study was to determine the long-term effect of various concentrations of cadmium chloride (CdCl2) on the growth of cultures of Chlamydomonas reinhardtii, a unicellular green algae found in fresh water, by measuring optical density and chlorophyll concentration. This species serves as a good indicator of contamination because it is a simple, sensitive, and ubiquitous organism. Thus, it can provide important information on the toxic effects of pollutants on general metabolic processes and aquatic ecosystems' responses to heavy metal contamination. Although the concentration of cadmium in soil and water can be determined chemically, these methods do not provide information about the effects on growth and metabolism.

For this long-term growth study, 100mL solutions of algal 3M media containing a vitamin mix and various concentrations of CdCl2 (0 (control), 10, 25, 50, 75 and 100 mg/L) were used. Measurement of growth was made using a spectrophotometer at 750, 645 and 663 nm to measure optical density, chlorophyll a and chlorophyll b, respectively. This study was done for a two-week period in quadruplicate. Importantly, our results indicate that the viability and chlorophyll content of the cells decreases with a significant increase in the concentration of cadmium chloride. However, sub-lethal concentrations (25 mg/L CdCl2 and less) seemed to only inhibit the onset of exponential growth of the algal cells, while 10 mg/L CdCl2 showed results similar to the control (0mg/L). The growth rate for the concentration of 25 mg/L CdCl2 was inhibited severely, but an extreme drop in optical density was observed for 50 mg/L, 75mg/L and 100 mg/L of CdCl2. The results from the chlorophyll studies were very similar to those obtained for optical density. At the conclusion of the study, pH levels were also measured. Results showed that when growth was inhibited by the high concentrations of CdCl2, the pH levels decreased from 8.92 to a range between 6.68 and 7.04. When growth was observed, the pH increased to a pH level between 8 and 9. These results correlated well with the growth curve and the chlorophyll study.