Increased toxic effect of methylmercury on <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> under high light and cold stress conditions

The toxic effect of methylmercury on the photosynthetic activity of Chlorella vulgaris was shown to increase under high illumination and unfavorable low temperature. Increased toxic action of methylmercury resulted from the decreased capacity of photosystem II (PS II) for reparation. It was proposed that mild stress conditions might be used to enhance the detection limit of toxicants by microalgae used as test objects.     

Effect of silver nanoparticles on the parameters of chlorophyll fluorescence and P700 reaction in the green alga Chlamydomonas reinhardtii

Acute toxicity of silver nanoparticle (AgNP) for photosynthesis in Chlamydomonas reinhardtii was studied using an M-PEA2 fluorimeter. Analysis of the fluorescence induction curves in the presence of AgNP at low concentrations revealed inhibited electron transport on the PS2 photosystem and increased content of Q_B-nonreducing centers. No direct effect of AgNP on the reactions of P₇₀₀ oxidation in PS1 was found, while the energization of the photosynthetic membranes was affected. Investigation of the parameters of the prompt and delayed fluorescence is proposed as a method for early detection of AgNP in the environment.     

Effect of ATP on the Content of Inactive PSII Complexes in Chlorella

A prolonged (20 h) dark incubation of Chlorella pyrenoidosa algae at 37°C resulted in a twofold rise of the slowly rising phase (10–15 min), sF _v, in the kinetics of variable chlorophyll fluorescence, F _v (F _v = F _m – F ₀) in diuron-treated cells. This effect suggests the accumulation of inactive photosystem II (PSII) complexes with low efficiency of primary quinone acceptor of electron of PSII (Q_A) reduction. The presence of methylamine (MA), a thylakoid membrane uncoupler, or N, N-dicyclohexylcarbodiimide, an inhibitor of ATPase, precluded the accumulation of inactive PSII complexes. When salicylhydroxamate promoted the reduction of the plastoquinone (PQ) pool, exogenous ATP accelerated the accumulation of inactive complexes. Dark PQ oxidation in the presence of nonmetabolized glucose analog, 2-deoxy-D-glucose, lowered the content of inactive PSII complexes, and NaF, an inhibitor of chloroplast phosphatases, retarded this process. These data are considered as evidence for a mechanism regulating the content of inactive PSII complexes in the process of redox-dependent phosphorylation of D1- and/or D2-proteins of PSII.     

Oxygen-Evolving System of Inactive Photosystem II Complexes in Chlorella

Incubation of green alga Chlorella pyrenoidosa Chick in darkness at 37–38°C for 10–30 h resulted in inactivation of the oxygen-evolving complex (OEC): the maximum yield of oxygen evolution during a series of short light flashes shifted from the third to the fifth flash; the transition of S₂- and S₃-states of OEC to a stable S₁-state was markedly accelerated. This inactivation of OEC was accompanied by the accumulation of inactive complexes of photosystem II (PSII), in which the reduction of primary quinone acceptor and the conversion into the closed state occurred with a low efficiency, even in the presence of 5 M DCMU. The treatment of light-grown algal cells with hydroxylamine impaired OEC functioning, in similarity to the effect of dark incubation, but caused no accumulation of inactive PSII complexes. We conclude that the inactivation of OEC is not the cause of the inactivation of PSII complex. The decline in the efficiency of electron-transport reactions, both on the donor and acceptor sides of PSII may be related to modification of major proteins in the PS II reaction center.     

Stabilization of the electron in the quinone acceptor part of the <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis> reaction centers

The evolution of the light-induced absorption difference spectrum (380–500 nm) of the reaction centers from photosynthetic purple bacteria Rhodobacter sphaeroides has been examined over 200 μs. The observed changes are interpreted as the effects of proton movement along the H-bond between the primary quinone acceptor and its protein surroundings. A theoretical analysis of the spectral evolution, considering the proton tunneling kinetics, corroborates this interpretation. The electronic state of the primary quinone is stabilized within tens of microseconds; the process is retarded upon deuteration of the reaction center as well as in 90% glycerol, and accelerated upon nondestructive heating to 40°C.     

Effects of oxygen on the dark recombination between photoreduced secondary quinone and oxidized bacteriochlorophyll in Rhodobacter sphaeroides reaction centers

The influence of duration of exposure to actinic light (from 1 sec to 10 min) and temperature (from 3 to 35^°C) on the temporary stabilization of the photomobilized electron in the secondary quinone acceptor (Q_B) locus of Rhodobacter sphaeroides reaction centers (RC) was studied under aerobic or anaerobic conditions. Optical spectrophotometry and ESR methods were used. The stabilization time increased significantly upon increasing the exposure duration under aerobic conditions. The stabilization time decreased under anaerobic conditions, its dependence on light exposure duration being significantly less pronounced. Generation of superoxide radical in photoactivated aerobic samples was revealed by the ESR method. Possible interpretation of the effects is suggested in terms of interaction between the semiquinone Q_B with oxygen, the interaction efficiency being determined by the conformational transitions in the structure of RC triggered by actinic light on and off.