Inhibition of photosynthetic electron transport in Nostoc muscorum by Ni2+ and Ag+.

Abstract

This paper presents the inhibition of photosynthetic electron transport chain of Nostoc muscorum by divalent Ni2+ and monovalent Ag+. PS I (DCPIP/ASC-MV) and PS II (H20-*PBQ) activities were markedly inhibited by Ni2+ and Ag+ in a concentration-dependent fashion but PS II was more susceptible than PS I to Ni2+. Ago was more toxic to PS I than PS II. The greater sensitivity of PS II to Ni2+ was further confirmed by the inhibition of DCPIP photoreduction and Chl a fluorescence. Restoration of Agtinduced inhibition of DCPIP photoreduction and Chl a fluorescence by artificial electron donors (DPC, NH2OH, MnCl2) and their failure to restore Ni2t induced inhibition suggests that Ni24 inactivates PS II by causing alteration and destruction of photosynthetic membranes, but Ag+ inhibits the electron flow at the oxidizing side of PS II. Nevertheless, the suppression of the fluorescence intensity at low concentrations of both metal cations points to the involvement of phycobilisomes in the inhibition of PS II activity.Though a considerable amount of work has been done on the metabolic processes of algae (12,22), compared to higher plants, the photosynthetic system of algae and cyanobacteria as affected by heavy metals are least explored. Interest in work on metal toxicity in cyanobacteria stems from: (i) their prokaryotic nature but higher plant type photosynthesis, (ii) their role in the nitrogen economy of the biosphere, and (iii) the ease of experimentally manipulating them.The effects of Hg2+, Cd2+, Zn2+ and Ni2+ on the electron transport of isolated chloroplasts to ascertain the modes of action of these cations on the photosynthesis have been documented (3,7,18,19). These studies indicate that the metal ions specifically affect the carriers and reaction centers thereby interrupting the flow of

Gene Expression, Protein Function and Pathways of Arabidopsis thaliana Responding to Silver Nanoparticles in Comparison to Silver Ions, Cold, Salt, Drought, and Heat

“…If Ag + could penetrate plasma membrane fast, then cytotoxicity would be severe. Thus, it is hypothesized that fast penetration of Ag + across plasma membrane could affect photosynthetic electron transport and slow down primary metabolic pathways sooner [ 112 , 113 ]. Once primary metabolic pathways were slowed down, affected genes would be up- and down-regulated to allow plants to adapt into their Ag + stress.…”

EARLY TOXIC EFFECTS OF ZINC ON PSII OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE)1

“…Despite their high degree of stress tolerance, it has been shown that cyanobacteria are highly sensitive to heavy metal pollution (Reed and Gadd 1990, De Filippis and Pallaghy 1992). Early effects of metals, such as Hg 2+ , Ni 2+ , Cu 2+ , Cr 2+ , Pb 2+ , and Ag 1+ , on the photosynthetic performance of cyanobacteria have been attributed to inhibition of the photosynthetic electron transport, mainly at the PSII reaction center (Mallick and Rai 1989, Murthy et al 1989, Singh et al 1989, 1991, Prasad et al 1991, Rai et al 1995, Lu et al 2000). In addition, it seems that a decrease in energy transfer from phycobilisomes to PSII could also contribute to inhibition of PSII activity (Murthy et al 1989, Murthy and Mohanty 1991, 1993, Prasad et al 1991, Rai et al 1995).…”