Hg ++ - a DCMU independent electron acceptor of photosystem II

Mercury effects on Thalassiosira weissflogii: applications of two-photon excitation chlorophyll fluorescence lifetime imaging and flow cytometry

The toxic effects of inorganic mercury [Hg(II)] and methylmercury (MeHg) on the photosynthesis and population growth in a marine diatom Thalassiosira weissflogii were investigated using two methods: two-photon excitation fluorescence lifetime imaging (FLIM) and flow cytometry (FCM). For photosynthesis, Hg(II) exposure increased the average chlorophyll fluorescence lifetime, whereas such increment was not found under MeHg stress. This may be caused by the inhibitory effect of Hg(II) instead of MeHg on the electron transport chain. For population growth, modeled specific growth rate data showed that the reduction in population growth by Hg(II) mainly resulted from an increased number of injured cells, while the live cells divided at the normal rates. However, MeHg inhibitory effects on population growth were contributed by the reduced division rates of all cells. Furthermore, the cell images and the FCM data reflected the morphological changes of diatom cells under Hg(II)/MeHg exposure vividly and quantitatively. Our results demonstrated that the toxigenicity mechanisms between Hg(II) and MeHg were different in the algal cells.

Mercury inhibition at the donor side of photosystem II is reversed by chloride

Mercury is an environmental contaminant that strongly inhibits photosynthetic electron transport, photosystem II being the most sensitive target. We investigated in greater detail the effect of mercury using photosystem II submembrane fractions of higher plants. Oxygen evolution was strongly inhibited and variable chlorophyll fluorescence was severely quenched by mercury. Chloride, an inorganic cofactor known to be essential for the optimal function of photosystem II, significantly reversed the inhibitory effect of mercury. However, calcium, another essential cofactor, showed no reversal capacity. It is concluded that on the donor side of PSII, mercury exerts its action by perturbing chloride binding and/or function. Considering the exceptional affinity of mercury for sulfhydryl groups of proteins, the results suggest the implication of cystein residue(s) in maintaining structural and functional integrity of photosystem II.

Inhibition of photosynthesis by heavy metals

Inhibition of photosynthesis by heavy metals is well documented. In this review the results are compared between in vitro experiments on isolated systems (chloroplasts, enzymes -.), experiments on excised leaves and intact plants and algae in vivo. In vitro experiments suggest potential sites of heavy metal interaction with photosynthesis at several levels of organisation, which are not necessarily confirmed in vivo. Analytical data on subcellular heavy metal level are generally missing to discuss their mechanism of action in the intact organism. In the field factors such as soil characteristics and air pollution have to be taken into account for assessing the mechanism of action of heavy metals on photosynthesis in plants, growing in a polluted erea.

Dichlorophenylurea-resistant oxygen evolution in Chlorella after cerulenin treatment

Fluorescence spectra at 77 K, oxygen evolution at 30 degrees C and delayed fluorescence at 25 degrees C were measured in Chlorella pyrenoidosa cultures with and without cerulenin and subsequent 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU) treatment, respectively. In pure algal cultures the oxygen evolution was inhibited by DCMU and the long-time component of fluorescence was highly influenced by DCMU, as expected. In contrast, both oxygen evolution and delayed fluorescence became DCMU-resistant in cerulenin-treated cultures. The DCMU-resistance is correlated with a change in the fatty acid distribution of the thylakoid membrane, which also leads to changes in the prompt fluorescence. Cerulenin appears to be a promising new tool of diagnostics for the hitherto unsatisfactorily understood processes of oxygen evolution in photosynthesizing organisms.

A partial reaction in photosystem II: reduction of silicomolybdate prior to the site of dichlorophenyldimethylurea inhibition

Silicomolybdate functions as an electron acceptor in a Photosystem II water oxidation (measured as O2 evolution) partial reaction that is 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) insensitive, that is, reduction os silicomolybdate occurs at or before the level of Q, the primary electron acceptor for Photosystem II. This report characterizes the partial reaction with the principal findings being as follows: 1. Electron transport to silicomolybdate significantly decreased room temperature Photosystem I side of the DCMU had no effect on the fluorescence level, consistent with silicomolybdate accepting electrons at or before Q. In the absence of DCMU, silicomolybdate is also reduced at a site on the Photosystem I side of the DCMU block, prior to or at plastoquinone, since the plastoquinone antagonist dibromothymoquinone (DBMIB) did not affect the electron transport rate. 3. Electron transport from water to silicomolybdate (+ DCMU) is not coupled to ATP formation, nor is there a measurable accumulation of protons within the membrane (measured by amine uptake). Silicomolybdate is not inhibitory to phosphorylation per se since neither cyclic nor post-illumination (XE) phosphorylation were inhibited. 4. Uncouplers stimulated electron transport from water to silicomolybdate in the pH range of 6 to 7, but inhibited at pH values near 8. These data are consistent with the view that when electron flow is through the abbreviated sequence of water to Photosystem II to silicomolybdate (+ DCMU), conditions are not established for the water protons to be deposited within the membrane. Experiments reported elsewhere (Fiaquinta, R.T., Dilley, R.A. and Horton, P.(19741 J. Bioenerg. 6, 167-177) and these data, are consistent with the hypothesis that electron transport between Q and plastoquinone energizes a membrane conformational change that is required to interact with the water oxication system so as to result in the deposition of water protons either within the membrane itself or within the inner oxmotic space.