NanoTiO2 materials mitigate mercury uptake and effects on green alga Chlamydomonas reinhardtii in mixture exposure

Surface displayed MerR increases mercury accumulation by green microalga Chlamydomonas reinhardtii

Mercury is a highly toxic trace metal that can accumulate in aquatic ecosystems and when resent at high concentrations can pose risks to both aquatic life and humans consuming contaminated fish. This research explores the use of the metalloregulatory protein MerR, known for its high affinity and selectivity toward mercury, in a novel application. Through a cell surface engineering approach, MerR was displayed on cells of green alga Chlamydomonas reinhardtii. A hydroxyproline-rich GP1 protein was used as an anchor to construct the engineered strains GP1-MerR that expresses the fluorescent protein mVenus. The surface engineered GP1-MerR strain led up to five folds higher Hg2+ accumulation compared to the WT strain at concentration range from 10-9 to 10-7 M Hg2+. The binding of Hg2+ via MerR was specific and did not get significantly affected by major freshwater water quality variables such as Ca2+ and dissolved organic matter. The presence of other trace metals (Zn2+, Cu2+, Ni2+, Pb2+, Cd2+) in a same concentration range even resulted in 30-40 % increase in the accumulated Hg. Further, the engineered cells also demonstrated the ability to accumulate Hg2+ from the water extracts of the Hg-contaminated sediment samples. These results demonstrate a novel approach utilizing the cell surface display system in C. reinhardtii for its potential application in bioremediation.

Ionic and nanoparticulate silver alleviate the toxicity of inorganic mercury in marine microalga Chaetoceros muelleri

Toxicological effects of silver nanoparticles (SNPs) in different organisms have been studied; however, interactions of SNPs with other environmental pollutants such as mercury are poorly understood. Herein, bioassay tests were performed according to ΟECD 201 guideline to assess the toxic effects induced by mercury ions (mercury chloride, MCl) on the marine microalga Chaetoceros muelleri in the presence of SNPs or silver ions (silver nitrate, SN). Acute toxicity tests displayed that the presence of SNPs or SN (0.01 mg L-1) significantly reduced the toxicity of MCl (0.001, 0.01, 0.1, 1, 10, and 100 mg L-1) and increased the IC50 of MCl from 0.072 ± 0.014 to 0.381 ± 0.029 and 0.676 ± 0.034 mg L-1, respectively. In the presence of SN or SNPs, the mercury-reducing effect on algal population growth significantly decreased. Considering the increase of IC50, the mercury toxicity decreased approximately 5.44 and 9.66 times in the presence of SNPs or SN, respectively. The chlorophyll a and c contents decreased at all exposures; however, the decrease by MCl-SNPs and MCl-SN was significantly less than MCl except at 1 mg L-1. The lowering effect of MCl-SN on chlorophyll contents was less than MCl and MCl-SNPs. MCl exposure induced significant raises in total protein content (TPC) at concentrations < 0.01mg  L-1, with a maximum of ~ 70.83% attained at 100 mg L-1. The effects of MCl-SNPs and MCl-SN on TPC were significantly less than MCl. Total lipid content (TLC) at all MCl concentrations was higher than the control, while at coexposure to MCl-SN, TLC did not change until 0.01 mg L-1 compared with the control. The effects of MCl-SN and MCL-SNPs on TPC and TLC were in line with toxicity results, and were significantly less than those of MCl individually, confirming their antagonistic effects on MCl. The morphological changes of algal cells and mercury content of the cell wall at MCl-SN and MCl-SNPs were mitigated compared with MCl exposure. These findings highlight the mitigatory impacts of silver species on mercury toxicity, emphasizing the need for better realizing the mixture toxicity effects of pollutants in the water ecosystem.

Exploring the subcellular distribution of mercury in green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana: A comparative study

Mercury (Hg) is a priority pollutant of global concern because of its toxicity, its ability to bioaccumulate throughout the food web and reach significant concentrations in top predators. Phytoplankton bioconcentrate large amounts of Hg and play a key role in the entry of Hg into the aquatic food web. However, the subcellular distribution of Hg in freshwater phytoplankton, known to affect it toxicity and trophic transfer is understudied. The present study aimed at investigating the accumulation of inorganic Hg (iHg) and its subcellular distribution in freshwater phytoplankton species. To this end green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana were exposed to 10 and 100 nM of iHg for 2 h. The concentrations of Hg in the adsorbed, intracellular and subcellular (granules, debris, organelles, heat-stable peptides (HSP) and heat-denaturable proteins (HDP)) fractions were determined. The results showed that C. meneghiniana accumulated more Hg compared to C. reinhardtii at both iHg exposure concentrations (10 nM: 4.41 ± 0.74 vs. 1.10 ± 0.25 amol cell-1; 100 nM: 79.35 ± 10.78 vs. 38.31 ± 4.15 amol cell-1). The evaluation of the subcellular distribution of Hg, revealed that the majority of Hg was concentrated in the organelles fraction (59.7 % and 74.6 %) in the green algae. In the diatom, Hg was mainly found in the organelles (40.9 % and 33.3%) and in the HSP fractions (26.8 % and 40.1 %). The proportion of Hg in HDP fraction decreased in favor of the organelles fraction in C. reinhardtii when the exposure concentration increased, whereas the proportions in the debris and organelles fractions decreased in favor of HSP fraction in C. meneghiniana. This study provides pioneering information on the subcellular distribution of Hg within in freshwater phytoplankton, a knowledge that is essential to understand the toxicity and trophic transfer of Hg in contaminated aquatic environment.

Kinetic Aspects of the Interactions between TiO2 Nanoparticles, Mercury and the Green Alga Chlamydomonas reinhardtii

Aquatic organisms are exposed to mixtures of environmental pollutants, including engineered nanoparticles; however, the interactions underlying cocktails’ effects are poorly understood, in particular, the kinetic aspects. The present study explored the time course of the interactions between nano-sized titanium dioxide (nTiO2) with different primary particle sizes, algae and inorganic mercury (Hg) over 96 h under conditions that were representative of a highly contaminated environment. The results showed that nTiO2 with smaller primary particle size and higher concentrations rapidly reduced the adsorption and internalization of mercury by green alga Chlamydomonas reinhardtii. Such a mitigating effect on mercury bioavailability could be explained by the strong adsorbing capacity of nTiO2 for Hg and the aggregation and sedimentation of nTiO2 and bound Hg. The present study highlighted the key processes determining the bioavailability of mercury to the algae in mixture exposure under conditions representative of a highly contaminated environment, such as industrial wastewater effluents.

Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure

Titanium dioxide nanoparticles (nTiO₂) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO₂ on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO₂ led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO₂ pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO₂ led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO₂ for IHg. In addition, high concentrations of nTiO₂ favored the trophic transfer of IHg/MeHg through feeding on nTiO₂ pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL^-1 nTiO₂. By contrast, an increase of the AE of MeHg through feeding on nTiO₂ and alga was found in the presence of higher concentration of 200 mgL^-1 nTiO₂. The present results will help to better understand the role of nTiO₂ on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.

Metabolic alterations in alga Chlamydomonas reinhardtii exposed to nTiO2 materials

Nano-sized titanium dioxide (nTiO₂) is one of the most commonly used materials, however the knowledge about the molecular basis for metabolic and physiological changes in phytoplankton is yet to be explored. In the present study we use a combination of targeted metabolomics, transcriptomics and physiological response studies to decipher the metabolic perturbation in green alga Chlamydomonas reinhardtii exposed for 72 h to increasing concentrations (2, 20, 100 and 200 mg L⁻¹) of nTiO₂ with primary sizes of 5, 15 and 20 nm. Results show that the exposure to all three nTiO₂ materials induced perturbation of the metabolism of amino acids, nucleotides, fatty acids, tricarboxylic acids, antioxidants but not in the photosynthesis. The alterations of the most responsive metabolites were concentration and primary size-dependent despite the significant formation of micrometer-size aggregates and their sedimentation. The metabolic perturbations corroborate the observed physiological responses and transcriptomic results and confirmed the importance of oxidative stress as a major toxicity mechanism for nTiO₂. Transcriptomics revealed also an important influence of nTiO₂ treatments on the transport, adenosine triphosphate binding cassette transporters, and metal transporters, suggesting a perturbation in a global nutrition of the microalgal cell, which was most pronounced for exposure to 5 nm nTiO₂. The present study provides for the first-time evidence for the main metabolic perturbations in green alga C. reinhardtii exposed to nTiO₂ and helps to improve biological understanding of the molecular basis of these perturbations.

Combination of transcriptomics, metabolomics and physiology studies highlighted the nanoparticle size- and concentration-dependent disturbance in algal metabolism induced by nTiO₂.

A density gradient centrifugation method for rapid separation of nanoTiO2 and TiO2 aggregates from microalgal cells in complex mixtures with mercury

In natural environment, the microorganisms are exposed to complex mixtures of contaminants, including manufactured nanoparticles and their aggregates. Evaluation of the toxicant accumulation in biota exposed to such cocktails is a challenging task because the microorganisms need to be separated from nanomaterial aggregates often of a comparable size. We propose a method for separation of TiO₂ aggregates from green microalga Chlamydomonas reinhardtii and subsequent determination of cellular Hg concentration in algae exposed to mixture of Hg with nanoTiO₂, known also to adsorb Hg. The method is based on differences in specific weight of algae and TiO₂ aggregates, using medium speed centrifugation on a step gradient of sucrose. The efficiency of the separation method was tested with nanoTiO₂ of three different primary sizes at four concentrations: 2, 20, 100 and 200 mg L⁻¹. The method gives a possibility to separate nanoTiO₂ and their aggregates from the algae with a mean recovery of 83.3% of algal cells, thus allowing a reliable determination of Hg accumulation by microalgae when co-exposed to Hg and nanoTiO₂.

• A rapid and reliable method to separate algal cells and nanoparticle aggregates of comparable size.

• A method to measure the cellular amount of Hg in green alga co-exposed to Hg and nanoTiO₂.