David Dewez

Silver nanoparticles toxicity effect on photosystem II photochemistry of the green alga Chlamydomonas reinhardtii treated in light and dark conditions

The inhibitory effect of silver nanoparticles (AgNPs) on photochemical reactions of photosynthesis was investigated using the green alga model Chlamydomonas reinhardtii. Algal cells were exposed to 1, 5, and 10 µmol L−1 of AgNPs under both light and dark conditions during 6 h. The rapid rise of chlorophyll a fluorescence and the fluorescence imaging system were employed to investigate the alteration of photosystem II (PSII) photochemical reactions and the associated electron transport activity. When algal cells were exposed to 5 and 10 µmol L−1 of AgNPs, our results showed the evidence of a structural deterioration of PSII reaction center, the alteration of the oxygen evolving complex and the inhibition of electron transport activity, which was stronger for AgNPs treatment under light exposure. Under these conditions, there was no activation of regulated photoprotective mechanisms against excess absorbed light-energy by the antenna system of the PSII complex. The highest deteriorating effect on the structural and functional integrity of PSII was observed for algal cells exposed 6 h in light condition to 10 µmol L−1 of AgNPs. Therefore, we provide valuable data in this study permitting to use photosynthetic-based fluorescence parameters for aquatic toxicological risk investigation of polluted water that may contain AgNP suspension.

Alteration of photosystem II activity by atrazine on Chlamydomonas reinhardtii synchronized and asynchronized cell cycle cultures

Inhibition of photosystem II (PSII) activity by atrazine was investigated in the green alga Chlamydomonas reinhardtii during different states of the cell cycle. The algal cultures were maintained under continuous light or under light/dark cycle (16/8 h) to obtain homogenized cell cycle distribution. The cycle state of algal population was determined by the DNA content using flow cytometry and defined as newly divided cells before the initiation of DNA replication (G0/G1) and cells at the end of the replication cycle with fully duplicated DNA content (G2/M). Under different synchronized states of the cell population, the photosynthetic activity was investigated after treatment at 10, 100, and 1000 µmol L−1 atrazine exposed for 24 h by using fluorescence parameters related to PSII activity measured with a plant efficiency analyzer and pulse-amplitude modulated methods. In this study, we found that the atrazine effect was different depending on cell cycle phases and the period of illumination. Algal cells under light–dark cycle showed inhibition of the PSII electron transport leading to an increase of heat energy dissipation by the PSII reaction center. Algal cells grown under continuous light was shown to be more resistant to atrazine than the cells grown under light–dark cycle.

Effect of the anthocyanic epidermal layer on Photosystem II and I energy dissipation processes in Tradescantia pallida (Rose) Hunt

The effect of anthocyanic cells of the epidermal layer was investigated on photosynthetic activity of the higher plant Tradescantia pallida. To determine the possible indirect role of anthocyanin in photosynthesis, analysis was done on intact leaves and leaves where anthocyanic epidermal layer was removed. Energy dissipation processes related to Photosystem II (PSII) and Photosystem I (PSI) activity was done using simultaneously Chlorophyll a (Chl a) fluorescence and P700 transmittance signals change. In anthocyanic epidermal-less leaves, PSII photochemical activity was more decreased in dependence to increasing light irradiance exposure. We found that photoinhibition of PSII decreased PSI activity by reducing the electron flow toward PSI, especially under high light intensities. Under those conditions, it resulted in the accumulation of oxidized PSI reaction centers, which was stronger in leaves where the anthocyanic epidermal layer was removed. In conclusion, our results showed that the anthocyanic epidermal layer had a photoprotective effect only on the PSII and not on the PSI of T. pallida leaves, supporting the role of anthocyanin pigments in the regulation of photosynthesis for excess absorbed light irradiance.

Cadmium accumulation and toxicity affect the extracytoplasmic polyphosphate level in Chlamydomonas reinhardtii

Cadmium is a non-essential metal and highly toxic for biological functions. Depending on the dose of Cd2+, the biochemical response will differ. In this study, we investigated the level of extracytoplasmic polyphosphate (polyP) when Chlamydomonas reinhardtii was exposed to the effect of CdCl2. When compared to control cells, Cd2+-treated cells to 400-600 µM showed a decrease in the growth rate from 0.78 to 0.38 d-1 for the strain CC-125, and a decrease from 0.81 to 0.32-0.35 d-1 for CC-503. This indicated a strong toxicity effect on the population growth of cells during 72 h. In addition, the results demonstrated the decrease in the synthesis and/or the degradation of polyP that was correlated with the accumulation of Cd2+ in both algal strains. Furthermore, the level of polyP decreased in relation to the decrease of FV/FM value. The toxicity of Cd2+ induced a high level of cell necrosis for CC-503, and the level of polyP could not be recovered at 72 h. In response to the toxic effects of Cd2+, the observed formation of palmelloid colonies by CC-125 cells was correlated with the recovery of the polyP level. Nevertheless, both algal strains were able to accumulate significant amount of Cd2+ in their biomass. Therefore, our study demonstrated a distinct impact of Cd2+ on the metabolism of polyP (synthesis and/or degradation), which was dependent on the concentration of CdCl2 and the Chlamydomonas strain. Based on this study, the level of polyP can be used as a biomarker of Cd2+ toxicity at 24-48 h, even with the cell wall-deficient strain CC-503.