Effect of Heavy Metals on the Structure and Function of Photosystem II: Potential and Prospects for Use as Bioindicator Jirí Masojídek1, Jan Malý1,2, Sabrina Alessandrelli3, Michal Koblížek 1,4, Marco Rizzuto3, Bettina Geiken3, Jirí Kopecký1, Josef Komenda1, Ondrej Prašil1 and Maria Teresa Giardi3 1 Institute of Microbiology, Academy of Sciences, 37981 Trebon, Czech Republic2 Dept. of Biology, University of J.E. Purkyne, 400 96 Ústí n. Labem, Czech Republic3 IBEV-CNR, Via Salaria Km 29.3, 00016 Monterotondo Scalo, Italy4 Dept. of Plant Physiology, University of South Bohemia, 370 05 Ceské Budejovice, Czech Republic
Heavy metals (HMs) exert multiple inhibitory effects on photosynthesis at different structural and metabolic levels. Interaction of HMs with the functional SH-groups was generally proposed as the mechanism for several inhibitory reactions. It has been suggested that the Calvin cycle reactions are more likely to be the primary targets of the toxic effects of HMs. Subsequently, the reduced demand for ATP and NADPH upon the Calvin cycle causes a down-regulation of both photosystem II photochemistry and of linear electron transport. A strong influence of cadmium at the first stage of plant treatment on D1 protein turnover has been observed. Pulse-chase experiments in vivo followed by the separation of thylakoids into grana and stroma exposed regions indicated that the synthesis, degradation and assembly of the D1 protein can be greatly affected by HMs. Modulation of D1 turnover under stress is a commonly occurring process (for recent review see Giardi et al. 1997). Monitoring the effect of HMs (Cd, Cu, Zn) and As in algal cells by more advanced biochemical and biophysical techniques (fluorescence microscopy, fluorescence induction using double-modulation fluorometry, thermoluminescence) was done in order to obtain a more detailed information on the mechanism how HMs affect the photosynthetic apparatus. The complex scheme of the possible inhibition mechanism of the photosynthetic apparatus under the HM stress was proposed by Malý (1998). The effect of HMs on the function of PSII was employed in design and construction of biosensors based on isolated thylakoids and PSII particles as well as intact cell of algae. The use of biosensors open a possibility for monitoring environmental pollution of aquatic and terrestrial ecosystem by HMs. When compared with classical analytical methods (HPLC, ELISA etc.), the biosensors can provide more precious information about real biological effect of pollutants since phytotoxicity is determined from the measurement of electron transport activity, photocurrent or photosynthetic oxygen evolution.
Keywords: D1 turnover; Biosensor; Heavy metal; Photosystem II; Fluorescence; Thermoluminescence Abbreviations: HM, heavy metal; PSII, Photosystem II |