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.

Effect of mercury on the polyphosphate level of alga Chlamydomonas reinhardtii.

In this study, the accumulation and toxicity effect of 1-7 µM of Hg was determined during 24-72 h on two strains of Chlamydomonas reinhardtii, CC-125 and CC-503 as a cell wall-deficient mutant, by monitoring the growth rate and the maximum quantum yield of Photosystem II. In addition, the level of extracytoplasmic polyphosphates (polyP related to the cell wall) was determined to understand the polyP physiological role in Hg-treated algal cells. The results showed that the polyP level was higher in the strain CC-125 compared to CC-503. When algal cells were exposed to 1 and 3 µM of Hg, the accumulation of Hg was correlated with the degradation of polyP for both strains. These results suggested that the degradation of polyP participated in the sequestration of Hg. In fact, this mechanism might explain at 72 h the recovery of the polyP level, the efficiency of maximum PSII quantum yield, the low inhibition of growth rate, and the low accumulated Hg in algal biomass. Under the effect of 5 and 7 µM of Hg, the degradation of polyP was complete and could not be recovered, which was caused by a high accumulation and toxicity of Hg already at 24 h. Our results demonstrated that the change of polyP level was correlated with the accumulation and effect of Hg on algal cells during 24-72 h, which can be used as a biomarker of Hg toxicity. Therefore, this study suggested that extracytoplasmic polyP in C. reinhardtii contributed to the cellular tolerance for Hg.

Effect of cadmium accumulation on green algae Chlamydomonas reinhardtii and acid-tolerant Chlamydomonas CPCC 121.

Cadmium is one of the most dangerous metals found in wastewater since exposure to low concentrations are highly toxic for cellular functions. In this study, the effect of cadmium accumulation on Chlamydomonas reinhardtii and acid-tolerant strain CPCC 121 was investigated during 48 h under 100-600 µM of Cd and two pH conditions (4 and 7). The toxicity of accumulated Cd was determined by the change of cellular and photosynthetic parameters. Obtained results showed that the maximum capacity of Cd accumulation in algal biomass was reached for both strains at 24 h of exposure to 600 µM of Cd. Under this condition, C. reinhardtii showed a higher uptake of Cd compared to the strain CPCC 121, inducing a stronger cellular toxic impact. Chlamydomonas CPCC 121 showed a tolerance for Cd due to the exclusion of Cd at the cell wall surface, which was higher at pH 4 than pH 7. TEM images and EDX spectrum of Cd distribution within the cell confirmed the role of the cell wall as a barrier for Cd uptake. Although Cd2+ concentration was the highest in the medium, CPCC 121 was the most tolerant at pH 4, but was not enough efficient to be considered for the phycoremediation of Cd. At neutral pH, the efficiency of C. reinhardtii for the removal of Cd was limited by its toxicity, which was dependent to the concentration of Cd in the medium and the time of exposure.

Toxicity of Nickel Oxide Nanoparticles on a Freshwater Green Algal Strain of Chlorella vulgaris.

A freshwater microalga strain of Chlorella vulgaris was used to investigate toxic effects induced by nickel oxide nanoparticles (NiO-NPs) in suspension. Algal cells were exposed during 96 h to 0-100 mg L-1 of NiO-NPs and analyzed by flow cytometry. Physicochemical characterization of nanoparticles in tested media showed a soluble fraction (free Ni2+) of only 6.42% for 100 mg L-1 of NiO-NPs, indicating the low solubility capacity of these NPs. Toxicity analysis showed cellular alterations which were related to NiO-NPs concentration, such as inhibition in cell division (relative cell size and granularity), deterioration of the photosynthetic apparatus (chlorophyll synthesis and photochemical reactions of photosynthesis), and oxidative stress (ROS production). The change in cellular viability demonstrated to be a very sensitive biomarker of NiO-NPs toxicity with EC50 of 13.7 mg L-1. Analysis by TEM and X-ray confirmed that NiO-NPs were able to cross biological membranes and to accumulate inside algal cells. Therefore, this study provides a characterization of both physicochemical and toxicological properties of NiO-NPs suspensions in tested media. The use of the freshwater strain of C. vulgaris demonstrated to be a sensitive bioindicator of NiO-NPs toxicity on the viability of green algae.

Toxic effects of nickel oxide bulk and nanoparticles on the aquatic plant Lemna gibba L.

The aquatic plant Lemna gibba L. was used to investigate and compare the toxicity induced by 30 nm nickel oxide nanoparticles (NiO-NPs) and nickel(II) oxide as bulk (NiO-Bulk). Plants were exposed during 24 h to 0-1000 mg/L of NiO-NPs or NiO-Bulk. Analysis of physicochemical characteristics of nanoparticles in solution indicated agglomerations of NiO-NPs in culture medium and a wide size distribution was observed. Both NiO-NPs and NiO-Bulk caused a strong increase in reactive oxygen species (ROS) formation, especially at high concentration (1000 mg/L). These results showed a strong evidence of a cellular oxidative stress induction caused by the exposure to NiO. Under this condition, NiO-NPs and NiO-Bulk induced a strong inhibitory effect on the PSII quantum yield, indicating an alteration of the photosynthetic electron transport performance. Under the experimental conditions used, it is clear that the observed toxicity impact was mainly due to NiO particles effect. Therefore, results of this study permitted determining the use of ROS production as an early biomarker of NiO exposure on the aquatic plant model L. gibba used in toxicity testing.

Effect of soluble copper released from copper oxide nanoparticles solubilisation on growth and photosynthetic processes of Lemna gibba L.

Copper oxide nanoparticles (CuO NPs) are used as a biocide in paints, textiles and plastics. Their application may lead to the contamination of aquatic ecosystems, where potential environmental effects remain to be determined. Toxic effects may be related to interactions of NPs with cellular systems or to particles' solubilisation releasing metal ions. In this report, we evaluated CuO NPs and soluble copper effects on photosynthesis of the aquatic macrophyte Lemna gibba L to determine the role of particle solubility in NPs toxicity. When L. gibba plants were exposed 48 h to CuO NPs or soluble copper, inhibition of photosynthetic activity was found, indicated by the inactivation of Photosystem II reaction centers, a decrease in electron transport and an increase of thermal energy dissipation. Toxicity of CuO NPs was mainly driven by copper ions released from particles. However, the bioaccumulation of CuO NPs in plant was shown, indicating the need to evaluate organisms of higher trophic level.