Effects of a cationic PAMAM dendrimer on photosynthesis and ROS production of Chlamydomonas reinhardtii.

Poly(amidoamine) (PAMAM) dendrimers hold great promises for biomedicine. This study sought to examine the toxicity of generation 4 (G4) cationic PAMAM dendrimer to the green microalga, Chlamydomonas reinhardtii, using physiological and molecular biomarkers. Results revealed that the G4 dendrimer at 15 and 25 nM stimulated the photosynthetic process and the production of reactive oxygen species (ROS) in algae. However, the over-production of ROS did not induce the expression of antioxidant enzyme genes, catalase and glutathione peroxidase. In addition, genes encoding light-harvesting proteins (lhca and lhcb), a ferredoxin (fdx) and an oxygen-evolving enhancer protein (psb) involved in photosynthesis were repressed after treatment. Nevertheless, the expression of the lhcbm9 gene, encoding a major light harvesting polypeptide, was increased. These results suggest that the strong modulation of photosynthesis induced by the dendrimer could lead to elevated ROS levels in microalgae.

Comparative toxicity of a brominated flame retardant (tetrabromobisphenol A) on microalgae with single and multi-species bioassays.

The potential threat of emerging chemicals to the aquatic flora is a major issue. The purpose of the study was to develop a multispecies microalgae test in order to determine the impact of species interactions on the cytoxicity of an emergent toxic contaminant: the tetrabromobisphenol A (TBBPA). Single and multi-species tests were thus performed to study the effects of this flame retardant on two microalgae (Pseudokirchneriella subcapitata and Nitzschia palea) commonly observed in freshwater. A synthetic medium was designed to allow the growth of both species. The algae were exposed to 1.8, 4.8, 9.2, 12.9 and 16.5 µM of TBBPA for 72 h. After staining with fluorescein diacetate (FDA), viable cells of each alga species were analyzed by flow cytometry based on chlorophyll autofluorescence and intracellular esterase activity. Density and abundance of viable cells were assessed to follow the population growth and the cell viability. In TBBPA treated samples, the growth of the two microalgae was significantly inhibited at the three highest concentrations (9.2, 12.9 and 16.5 µM) in the two tests. At the end of the experiment (t=72 h), the cell viability was also significantly smaller at these concentrations. The decreases of growth rate and viable cell abundance in TBBPA treated populations of N. palea were significantly higher in multi-species test in comparison with the single-species test. No significant differences were noticed between the two tests for P. subcapitata populations exposed to TBBPA.

Monitoring of a flame retardant (tetrabromobisphenol A) toxicity on different microalgae assessed by flow cytometry.

Microalgae are key organisms in aquatic ecosystems. Emergent pollutants like the tetrabromobisphenol A (TBBPA) are potential threat for these primary producers at the base of the trophic chain. The effects of this flame retardant on three microalgae (Pseudokirchneriella subcapitata, Nitzschia palea and Chlamydomonas reinhardtii) commonly observed in freshwater ecosystems were studied using a flow cytometer. Each species was exposed to 1.8, 4.8, 9.2, 12.9 and 16.5 µmol L⁻¹ of TBBPA for 72 h. After staining with fluorescein diacetate (FDA), viable cells were discriminated in flow cytogram according to the chlorophyll autofluorescence and the intracellular enzyme activity (esterase) to assess the sensitivity of microalgae to the TBBPA with multi-parametric analysis. For P. subcapitata and N. palea, growth inhibitions of viable cells were lower when the viability was assessed with chlorophyll autofluorescence in comparison with esterase activity. These results are related to the appearance of cells presenting optimal chlorophyll fluorescence without intracellular esterase activity after exposure to TBBPA. Abundance increase of these cells was higher in N. palea than in P. subcapitata. No similar trends were observed in C. reinhardtii populations due to the very high mortality of this microalgal species exposed to TBBPA.

Toxicity of PAMAM dendrimers to Chlamydomonas reinhardtii.

In recent decades, a new class of polymeric materials, PAMAM dendrimers, has attracted marked interest owing to their unique nanoscopic architecture and their hopeful perspectives in nanomedicine and therapeutics. However, the potential release of dendrimers into the aquatic environment raises the issue about their toxicity on aquatic organisms. Our investigation sought to estimate the toxicity of cationic PAMAM dendrimers on the green alga, Chlamydomonas reinhardtii. Algal cultures were exposed to different concentrations (0.3-10 mgL(-1)) of low dendrimer generations (G2, G4 and G5) for 72 h. Potential adverse effects on Chlamydomonas were assessed using esterase activity (cell viability), photosynthetic O2 evolution, pigments content and chlorophyll a fluorescence transient. According to the median inhibitory concentration (IC50) appraised from esterase activity, toxicity on cell viability decreased with dendrimer generation number (2, 3 and 5 mgL(-1) for G2, G4 and G5 dendrimers, respectively). Moreover, the three generations of dendrimers did not induce the same changes in the photosynthetic metabolism of the green alga. O2 evolution was stimulated in cultures exposed to the lowest generations tested (i.e. G2 and G4) whereas no significant effects were observed with G5. In addition, total chlorophyll content was increased after G2 treatment at 2.5 mgL(-1). Finally, G2 and G4 had positive effects on photosystem II (PSII): the amount of active PSII reaction centers, the primary charge separation and the electron transport between Q(A) and Q(B) were all increased inducing activation of the photosynthetic electron transport chain. These changes resulted in stimulation of full photosynthetic performance.

Effects of pesticides on freshwater diatoms.

The study of pesticide effects on algae, and diatoms in particular, was focused on photosynthesis and biomass growth disturbances. Few studies have been performed to investigate the effects of these toxic agents on intracellular structures of diatom cells. Nuclear alterations and cell wall abnormalities were reported for diatoms exposed to toxic compounds. Nevertheless, the cellular mechanisms implicated in the development of such alterations and abnormalities remain unclear. Sensitivity to pesticides is known to be quite different among different diatom species. Eutrophic and small species are recognized for their tolerance to pesticides exposure. More pronounced cell defenses against oxidative stress may explain this absence of sensitivity in species of smaller physical size. Notwithstanding, on the whole, explaining the rationale behind tolerance variations among species has been quite difficult, thus far. In this context, the understanding of intracellular toxicity in diatoms and the relation between these intracellular effects and the disturbance of species composition in communities represent a key target for further research. The original community species structure determines the response of a diatom community to toxic agent exposure. Diatom communities that have species capable of switching from autotrophic to heterotrophic modes, when photosynthesis is inhibited (e.g., after pesticide exposure), can continue to grow, even in the presence of high pesticide pollution. How diatoms respond to toxic stress, and the degree to which they respond, also depends on cell and community health, on ecological interactions with other organisms, and on general environmental conditions. The general structural parameters of diatom communities (biomass, global cell density) are less sensitive to pesticide effects than are the specific structural parameters of the unicellular organisms themselves (cell density by species, species composition). For benthic species, biofilm development and grazing on this matrix as a source of food for invertebrates and fishes may also modify the response of diatom communities. Environmental parameters (light exposure, nutrient concentrations, and hydraulic conditions) affect, and often interfere with, the response of diatoms to pesticides. Therefore, the complexity of aquatic ecosystems and the complexity of pesticide to easily detect the effects of such pollutants on diatoms. Clearly more research will be required to address this problem.

Comparison between the polar organic chemical integrative sampler and the solid-phase extraction for estimating herbicide time-weighted average concentrations during a microcosm experiment.

Polar organic chemical integrative samplers (POCIS) were exposed for 9 days in two different microcosms that contained river waters spiked with deethylterbuthylazine, terbuthylazine and isoproturon. The experiment was performed with natural light and strong turbulence (flow velocities of about 15-50cms(-1)) for reproducing natural conditions. The concentrations were kept relatively constant in the first microcosm (2.6-3.6microgl(-1)) and were variable in the second microcosm (peak concentrations ranged from 15 to 24microgl(-1) during the 3 day pulse phase). The time-weighted average (TWA) concentrations were determined with both POCIS and repetitive grab sampling followed by solid-phase extraction. The results showed a systematic and significant overestimation of the TWA concentrations with the POCIS most probably due to the use of sampling rates derived under low flow scenario. The results showed also that peak concentrations of pollutants are fully integrated by this passive sampler. Even if the POCIS should not provide very accurate concentration estimates without the application of adequate sampling rate values or the use of performance reference compounds, it can be a really useful tool for detecting episodic or short-term pollution events (e.g. increased herbicide concentrations during a flood), which may be missed with classical and low frequency grab sampling.