Ai-Jun Miao

Zinc oxide–engineered nanoparticles: Dissolution and toxicity to marine phytoplankton

It is now widely recognized that dissolution plays an important role in metallic nanoparticle toxicity, but to what extent remains unclear. In the present study, it was found that ZnO-engineered nanoparticle (ZnO-EN) toxicity to the marine diatom Thalassiosira pseudonana could be solely explained by zinc ion (Zn(2+) ) release. This is based on comparable inhibitive effects from ZnO-EN addition media, with or without the ultrafiltration through a 3-kD membrane, and from the media in which only Zn(2+) was added. Considering the importance of dissolution in ZnO-EN toxicity, Zn(2+) release kinetics was systematically examined under different conditions for the first time. It was found to be mainly influenced by pH as well as the specific surface area of the nanoparticles. In contrast, natural organic compounds either enhance or reduce Zn(2+) release, depending on their chemical composition and concentration. Compared with deionized water, ZnO-EN dissolution rates were accelerated in seawater, whereas ZnO-EN concentration itself only had a very small effect on Zn(2+) release. Therefore, dissolution as affected by several physicochemical factors should not be neglected in the effects, behavior, and fate of ENs in the environment.

Intracellular Uptake: A Possible Mechanism for Silver Engineered Nanoparticle Toxicity to a Freshwater Alga Ochromonas danica

The behavior and toxicity of silver engineered nanoparticles (Ag-ENs) to the mixotrophic freshwater alga Ochromonas danica were examined in the present study to determine whether any other mechanisms are involved in their algal toxicity besides Ag+ liberation outside the cells. Despite their good dispersability, the Ag-ENs were found to continuously aggregate and dissolve rapidly. When the initial nanoparticle concentration was lower than 10 µM, the total dissolved Ag+ concentration ([Ag+]T) in the suspending media reached its maximum after 1 d and then decreased suggesting that Ag+ release might be limited by the nanoparticle surface area under these conditions. Furthermore, Ag-EN dissolution extent remarkably increased in the presence of glutathione. In the Ag-EN toxicity experiment, glutathione was also used to eliminate the indirect effects of Ag+ that was released. However, remarkable toxicity was still observed although the free Ag+ concentration in the media was orders of magnitude lower than the non-observed effect concentration of Ag+ itself. Such inhibitive effects were mitigated when more glutathione was added, but could never be completely eliminated. Most importantly, we demonstrate, for the first time, that Ag-ENs can be taken in and accumulated inside the algal cells, where they exerted their toxic effects. Therefore, nanoparticle internalization may be an alternative pathway through which algal growth can be influenced.

Toxicity and bioaccumulation kinetics of arsenate in two freshwater green algae under different phosphate regimes

In the present study, the toxicity and bioaccumulation kinetics of arsenate in two green algae Chlamydomonas reinhardtii and Scenedesmus obliquus under phosphate-enriched (+P) and limited (-P) conditions were investigated. P-limitation was found to aggravate arsenate toxicity and S. obliquus was more tolerant than C. reinhardtii. Such phosphate-condition-dependent or algal-species-specific toxicity difference was narrowed when the relative inhibition of cell growth was plotted against intracellular arsenate content instead of its extracellular concentration. The discrepance was further reduced when the intracellular ratio of arsenic to phosphorus was applied. It suggests that both arsenate bioaccumulation and intracellular phosphorus played an important role in arsenate toxicity. On the other hand, arsenate uptake was induced by P-limitation and its variation with ambient arsenate concentration could be well fitted to the Michaelis-Menten model. Arsenate transporters of S. obliquus were found to have a higher affinity but lower capacity than those of C. reinhardtii, which explains its better regulation of arsenate accumulation than the latter species in the toxicity experiment. Further, arsenate depuration was facilitated and more was transformed to arsenite in C. reinhardtii or under -P condition. Intracellular proportion of arsenite was also increased after the algae were transferred from the long-term uptake media to a relatively clean solution in the efflux experiment. Both phenomena imply that algae especially the sensitive species could make physiological adjustments to alleviate the adverse effects of arsenate. Overall, our findings will facilitate the application of algae in arsenate remediation.

Cd2+ Toxicity to a Green Alga Chlamydomonas reinhardtii as Influenced by Its Adsorption on TiO2 Engineered Nanoparticles

In the present study, Cd2+ adsorption on polyacrylate-coated TiO2 engineered nanoparticles (TiO2-ENs) and its effect on the bioavailability as well as toxicity of Cd2+ to a green alga Chlamydomonas reinhardtii were investigated. TiO2-ENs could be well dispersed in the experimental medium and their pHpzc is approximately 2. There was a quick adsorption of Cd2+ on TiO2-ENs and a steady state was reached within 30 min. A pseudo-first order kinetics was found for the time-related changes in the amount of Cd2+ complexed with TiO2-ENs. At equilibrium, Cd2+ adsorption followed the Langmuir isotherm with the maximum binding capacity 31.9, 177.1, and 242.2 mg/g when the TiO2-EN concentration was 1, 10, and 100 mg/l, respectively. On the other hand, Cd2+ toxicity was alleviated in the presence of TiO2-ENs. Algal growth was less suppressed in treatments with comparable total Cd2+ concentration but more TiO2-ENs. However, such toxicity difference disappeared and all the data points could be fitted to a single Logistic dose-response curve when cell growth inhibition was plotted against the free Cd2+ concentration. No detectable amount of TiO2-ENs was found to be associated with the algal cells. Therefore, TiO2-ENs could reduce the free Cd2+ concentration in the toxicity media, which further lowered its bioavailability and toxicity to C. reinhardtii.

Effects of Engineered Nanoparticles on the Assembly of Exopolymeric Substances from Phytoplankton

The unique properties of engineered nanoparticles (ENs) that make their industrial applications so attractive simultaneously raise questions regarding their environmental safety. ENs exhibit behaviors different from bulk materials with identical chemical compositions. Though the nanotoxicity of ENs has been studied intensively, their unintended environmental impacts remain largely unknown. Herein we report experimental results of EN interactions with exopolymeric substances (EPS) from three marine phytoplankton species: Amphora sp., Ankistrodesmus angustus and Phaeodactylum tricornutum. EPS are polysaccharide-rich anionic colloid polymers released by various microorganisms that can assemble into microgels, possibly by means of hydrophobic and ionic mechanisms. Polystyrene nanoparticles (23 nm) were used in our study as model ENs. The effects of ENs on EPS assembly were monitored with dynamic laser scattering (DLS). We found that ENs can induce significant acceleration in Amphora sp. EPS assembly; after 72 hours EN-EPS aggregation reached equilibrium, forming microscopic gels of ∼4–6 µm in size. In contrast, ENs only cause moderate assembly kinetic acceleration for A. angustus and P. tricornutum EPS samples. Our results indicate that the effects of ENs on EPS assembly kinetics mainly depend on the hydrophobic interactions of ENs with EPS polymers. The cycling mechanism of EPS is complex. Nonetheless, the change of EPS assembly kinetics induced by ENs can be considered as one potential disturbance to the marine carbon cycle.

Application potential of a newly isolated indigenous aerobic denitrifier for nitrate and ammonium removal of eutrophic lake water.

The aim of this work was to evaluate the utilization potential of a newly isolated indigenous aerobic denitrifier, Pseudomonas stutzeri strain T1, for nitrogen removal from the eutrophic Lake Taihu in China. The strain was capable of conducting heterotrophic nitrification-aerobic denitrification and had both excellent nitrate and ammonium removal without nitrite build-up. The characteristics of P. stutzeri strain T1 were studied under different cultural conditions. Furthermore, under the optimized cultivation conditions, strain T1 was added into the water samples from Lake Taihu, the ammonium and nitrate removal rates of the strain reached to 60% and 75%, respectively. Via adding this strain, the water qualities of the sample ameliorated from Grade V to Grade II. Thus, the strain T1 should be an useful biological tool to remediate eutrophic lakes and do not meet acclimation problems.

Bioaccumulation of CdTe Quantum Dots in a Freshwater Alga Ochromonas danica: A Kinetics Study

The bioaccumulation kinetics of thioglycolic acid stabilized CdTe quantum dots (TGA-CdTe-QDs) in a freshwater alga Ochromonas danica was comprehensively investigated. Their photoluminescence (PL) was determined by flow cytometry. Its cellular intensity increased hyperbolically with exposure time suggesting real internalization of TGA-CdTe-QDs. This hypothesis was evidenced by the nanoparticle uptake experiment with heat-killed or cold-treated cells and by their localization in the vacuoles. TGA-CdTe-QD accumulation could further be well simulated by a biokinetic model used previously for conventional pollutants. Moreover, macropinocytosis was the main route for their internalization. As limited by their diffusion from the bulk medium to the cell surface, TGA-CdTe-QD uptake rate increased proportionally with their ambient concentration. Quick elimination in the PL of cellular TGA-CdTe-QDs was also observed. Such diminishment resulted mainly from their surface modification by vacuolar biomolecules, considering that these nanoparticles remained mostly undissolved and their expulsion out of the cells was slow. Despite the significant uptake of TGA-CdTe-QDs, they had no direct acute effects on O. danica. Overall, the above research shed new light on nanoparticle bioaccumulation study and would further improve our understanding about their environmental behavior, effects and fate.

Nitrogen removal by denitrification during cyanobacterial bloom in Lake Taihu

To understand the effect of cyanobacterial blooms on nitrogen transformations in eutrophic Lake Taihu, the variation in nitrate concentration during a bloom was observed in a field simulation experiment. This result showed that the cyanobacterial bloom might cause nitrate depletion in an aquatic ecosystem. To further investigate this field result, nitrate transformations after the addition of cyanobacteria collected from Lake Taihu were traced in laboratory microcosms with the 15N isotope addition method. About 81.2% and 98.4% of nitrate was lost when 2 × 109 and 4 × 109 cells L−1 of cyanobacteria were added, respectively. The nitrate concentration decrease followed first-order kinetics with the rate constant 0.23 and 1.41 d−1 in the two treatments, respectively. Conventional denitrification was found to play a major role in the nitrate removal process while other pathways (e.g., dissimilatory nitrate reduction to ammonium [DNRA] and assimilation of nitrate into microbial biomass) were negligible. It was likely that the cyanobacterial respiration as well as their decomposition resulted in anoxic conditions and that cyanobacteria also served as a carbon source for denitrification. Based on the above results, it was estimated that 109 cells of cyanobacteria were enough for denitrifiers to remove 0.53 mmol nitrate in the eutrophic lake ecosystem. Therefore, cyanobacterial blooms have the potential for nitrogen removal by denitrification and this process could cause nitrogen limitation of primary production in summer in Lake Taihu. This interactive relationship of nitrogen and cyanobacteria potentially constitutes a negative feedback for mitigating cyanobacterial blooms in this eutrophic lake environment.

Seasonal variation of phytoplankton nutrient limitation in Lake Taihu, China: A monthly study from Year 2011 to 2012

Lake Taihu is the third largest freshwater lake in China with severe eutrophication issues. However, it remains ambiguous how its phytoplankton growth is limited by various nutrients in different seasons. A series of bottle-enrichment assays in Meiliang Bay was thus performed once a month from July, 2011 to June, 2012 in the present study. The initial chlorophyll a concentration and phytoplankton cell density ranged from 4.70 to 34.6 μg/l and from 1.25×10(6) to 6.72×10(8) cells/l with three peaks in July, November, and March. Although Cyanophyta was dominant (30.9-99.2 percent) in most cases, other phyla like Chlorophyta, Bacillariophyta, and Cryptophyta could account for as much as 69.1 percent of total phytoplankton in cold seasons. The microcystin-LR content in the particulate phase followed a similar seasonal pattern as Cyanophyta. It further went up exponentially with the proportion of cyanobacteria in phytoplankton suggesting more toxigenic species and (or) upregulated microcystin synthesis when the contribution of Cyanophyta was enhanced. On the other hand, the dissolved concentrations of various nitrogen and phosphorus species reached their maxima in late spring and autumn, respectively. According to its growth response to nutrient addition, phytoplankton in Meiliang Bay was restricted by nitrogen in August, October, and November. No nutrient limitation occurred in July, September, and April, whereas phosphorus deficiency prevailed in the other months. Overall, nutrient limitation in Lake Taihu and possibly other aquatic ecosystems worldwide may be more dynamic than what we thought before, which should be considered to eliminate eutrophication.

Cd2+ toxicity as affected by bare TiO2 nanoparticles and their bulk counterpart

Toxicity of engineered nanoparticles has received extensive attention in recent years. However, nanoparticles always co-exist with other pollutants in natural environment. Whether there are any interactions between these classical pollutants and nanoparticles; and how these interactions may influence the environmental behavior, effects and fate of each other remain largely unclear. For this purpose, effects of bare titanium dioxide engineered nanoparticles (TiO(2)-NP) and their bulk counterpart (TiO(2)-BC) on Cd(2+) bioavailability and toxicity to the green alga Chlamydomonas reinhardtii were examined in the present study. We first investigated the kinetics and equilibrium isotherm of Cd(2+) adsorption on both particles in the algal culture medium. Pseudo-first-order adsorption kinetics was observed with equilibrium rate constant ranging from 0.19 to 0.33min(-1). Increase in Cd(2+) adsorption with its ambient concentration at equilibrium followed a single Langmuir isotherm for different concentrations of TiO(2). Furthermore, surface-area-based Cd(2+) adsorption by TiO(2)-BC was higher than that by TiO(2)-NP in most Cd(2+) concentration treatments suggesting that particle size was not the only cause for different adsorption. Both forms of TiO(2) could alleviate Cd(2+) inhibitive effects on C. reinhardtii. However, Cd(2+) toxicity and its bioaccumulation were comparable as long as its free ion concentration in ambient toxicity media was similar regardless the particle size and concentration of TiO(2). There was no TiO(2) inside the algal cells either. Therefore, it was Cd(2+) adsorption by TiO(2) which decreased its ambient free ion concentration and further its intracellular accumulation as well as toxicity.