HEAVY METAL‐INDUCED INHIBITION OF PHOTOSYNTHESIS: TARGETS OF <i>IN VIVO</i> HEAVY METAL CHLOROPHYLL FORMATION¹

TLDR

The study examined how Cu²⁺ and Zn²⁺ target the photosynthetic machinery of algae across multiple phyla. In Scenedesmus quadricauda, low light caused heavy‑metal substitution of Mg²⁺ in LHC II chlorophylls (shade reaction), whereas high light led to damage of the PSII reaction center (sun reaction); algae lacking LHC II did not show this dual response, and red and brown algae displayed reversed irradiance thresholds, indicating distinct mechanisms for sun reaction and photoinhibition.

Abstract

The targets of heavy metal (here Cu 2 + and Zn 2 + ) attack on the photosynthetic apparatus of algae belonging to different phyla were investigated. Experiments with the green alga Scenedesmus quadricauda confirmed previous findings that according to the irradiance level two different phenomena occur, which were further characterized by specific changes in several photosynthetic parameters. The reaction occurring under low irradiance ( shade reaction ) is characterized by heavy metal substitution of Mg 2 + in chl molecules bound predominantly in the light harvesting complex II of Chlorophyta (LHC II). Under high irradiance ( sun reaction ) the LHC II chls are inaccessible to substitution and the damage occurs in the PSII reaction center instead. Algae with antenna proteins other than the LHC II did not show the two types of heavy metal attack at different irradiances. In red algae ( Antithamnion plumula) , low Cu 2 + concentrations induced the sun reaction even at very low irradiance. In brown algae ( Ectocarpus siliculosus ) the shade reaction occurred even in saturating irradiance. These results also indicate that despite some similarity in their features, the primary step of the sun reaction and photoinhibition is different.