Environmental relevance of heavy metal-substituted chlorophylls using the example of water plants

Following experiments which studied the substitution of the central ion of isolated chlorophylls by heavy metal ions in vitro, in vivo experiments with submersed water plants were carried out. It was discovered that the substitution of the central atom of chlorophyll, magnesium, by heavy metals (mercury, copper, cad- mium, nickel, zinc, lead) in vivo is an important damage mechanism in stressed plants. This substitution pre- vents photosynthetic light-harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intens- ity. In low light irradiance all the central atoms of the chlorophylls are accessible to heavy metals, with heavy metal chlorophylls being formed, some of which are much more stable towards irradiance than Mg-chlorophyll. Consequently, plants remain green even when they are dead. In high light, however, almost all chlorophyll decays, showing that under such conditions most of the chlorophylls are inaccessible to heavy metal ions.