Toxicity and removal of heavy metals (cadmium, copper, and zinc) by Lemna gibba.

Effects of cadmium, copper, and zinc on the aquatic plant Lemna gibba were determined under controlled conditions; in parallel their removal from the growth medium was followed. The results showed that the three heavy metals affected growth, a physiological stress index defined as the ratio of Chlorophyll to phaeophytin (D665/D665a), and the contents of proline. After 4 days, 10(-3)-10(-1)mg/L Cd inhibited growth by 25-100%, reduced D665/D665a by 35-89%, and increased proline content by 44-567%. Under the same conditions, 10(-4)-10(-1) microg/L Cu inhibited growth by 36-75%, reduced D665/D665a by 19-81%, and increased proline content by 67-288%. Comparable concentrations of Zn had little effect. However, higher concentrations (4, 30, and 50mg/L) inhibited growth by 50-79%. Also, 0.1 and 30 mg/L induced a small reduction of D665/D665a (-3.8% and -22%) and an increase in proline contents (+144% and +177%). When it was observed, proline accumulation was always transient and the maximum was reached after 4 days. Monitoring metal concentration in the medium showed that L. gibba was able to remove metals from the medium. Zn and Cu removal was biphasic, it was rapid during the first 2 days (> 60% reduction) and slow (10-20%) during the following 8 days. For Cd, removal was linear and depended on the initial concentration. It reached approximately 90% after 6 or 8 days for initial concentrations of 10(-1) and 10(-3)mg/L, respectively.