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

Abstract

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, cadmium, nickel, zinc, lead) in vivo is an important damage mechanism in stressed plants. This substitution prevents photosynthetic light-harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intensity. 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.

Differential cadmium stress tolerance in five Indian mustard (Brassica junceaL.) cultivars

“…29 Substitution of Mg 2+ ion of the Chl molecule by Cd, Cu, Zn, Pb, Ni, Zn or Hg may be reason of arrested photosynthesis. 30 Yield characteristics were also found to be reduced by Cd in the soil in all the cultivars. Maximum reduction in yield characteristics was noted in RH30 and lesser in Alankar.…”

Photosynthetic activity in relation to chlorophylls, carbohydrates, phenolics and growth of a hybrid Salix purpurea × triandra × viminalis 2 at various Zn concentrations

“…This might be connected with formation of reactive oxygen species (ROS), which can affect photosynthetic activity as well as further plant growth (Cuypers et al 2001). Photosynthesis is affected by damage of photosynthetic apparatus, inhibition of electron transport, carbon fixation capacity and photophosphorylation potential (Küpper et al 1996). It was found that especially photosystem II (PS II) is affected by an excessive level of Zn (Vernay et al 2007;Paiva et al 2009;Janik et al 2010;Dhir et al 2011), while on the other hand an increase of PS I activity supported the build-up of a transthylakoid proton gradient, which subsequently helped maintain the photophosphorylation potential (Dhir et al 2011).…”

The effects of heavy metal concentration on bio-accumulation, productivity and pigment content of two species of marine macro algae

“…Some heavy metals, such as Hg 2+ , Cu 2+ , Cd 2+ , Zn2 + and Ni 2+ , are known to substitute the central Mg 2+ atom in the chlorophyll molecule, a process that lowers the fluorescence quantum yield and results in a shift in the fluorescence spectrum (Küpper et al 1996). As heavy metal toxicity is generally considered to be dose-dependent, it could be assumed that exposure to higher concentrations or over longer periods should result in higher toxicity as stated by Baumann et al (2009).…”