[Heavy metal]-chlorophylls formed in vivo during heavy metal stress and degradation products formed during digestion, extraction and storage of plant material

Abstract

This chapter discusses the occurrence, properties and relevance of chlorophyll (Chl) degradation products that are formed either in vivo in heavy metal-stressed plants or by digestion of algae in marine invertebrates, or that are formed during extraction or processing of dead plant material. The in vivo substitution of the central Mg2+ ion of chlorophyll by heavy metals constitutes an important part of the damage occurring in metalstressed plants. In Chlorophyta, this reaction varies with light intensity. In low irradiance combined with a dark phase, the light-harvesting complex II (LHC II) is the main target, while in high irradiance the LHC II is inaccessible to substitution of Mg2+. Instead, an insertion of heavy metals into the pheophytin (Phe) of the Photosystem II reaction center (PS II RC) has been proposed. In algae with different light harvesting proteins, this light-dependent difference is absent. In brown algae, Chl a in the Chl a/c LHC is always accessible to substitution of Mg2+ by heavy metals. While heavy metal chlorophylls are not functional in higher plants and algae,[Zn]-bacteriochlorophyll ([Zn]-BChl) was found to function as a photosynthetic pigment in aerobic photosynthetic bacteria of the genus Acidiphilium. Since these bacteria live in an acidic environment extremely rich in heavy metals, the advantage of much higher resistance against substitution by metals other than Zn seems to more than compensate for the disadvantages of [Zn]-BChl complex compared to [Mg]-BChl as a photosynthetic pigment. The questionable beneficial in vivo formation of rare-earth chlorophylls ([RE]-Chls) in plants growing on soils rich in rare earths is discussed in this chapter. Further, new knowledge of allomerization and photodegradation of chlorophyll solutions is discussed in relation to the processes of chlorophyll degradation during storage and extraction of plant material for analysis or human consumption. The partial digestion of chlorophyll and subsequent use of the transmetalated product, tunichlorin, in marine invertebrates is also briefly discussed.