Cell size and the blockage of electron transfer in photosynthesis: proposed endpoints for algal assays and its application to soil alga Chlorococcum infusionum

Low-frequency ultrasound assisted contact-electro-catalysis for efficient inactivation of Microcystis aeruginosa

Frequent cyanobacterial blooms pose a serious threat to the aquatic ecosystem and human health, so developing an efficient algae removal method is a long-term goal for bloom management. Current technologies for algal bloom control need urgent improvement in terms of algicide recovery, eco-friendliness and cost. Here we propose a contact-electro-catalytic method, using polytetrafluoroethylene (PTFE) film as a reusable catalyst. This contact-electro-catalytic approach involves the generation of reactive oxygen species (e.g., O2•-, HO•, 1O2 and H2O2) through water-PTFE contact electrification under the low-frequency ultrasonic waves, facilitating the inactivation of algae. The removal rate of the cyanobacterium Microcystis aeruginosa (M. aeruginosa) exposured to the water-PTFE contact-electro-catalytic system is almost five times greater than that of ultrasound alone after 5 h. A mechanistic investigation revealed that the contact-electro-catalytic system damaged the photosynthetic activity, antioxidant system and membrane integrity of the cells. Additionally, LC-MS metabolomic analysis indicated that this system caused substantial significant disruptions in the TCA cycle, amino acid metabolism, purine metabolism and phospholipid metabolism. Three-dimensional fluorescence spectroscopy suggested contact-electro-catalysis could further availably degrade the organic matter. We anticipate that this method can provide an eco-friendly, highly efficient and economic approach for effective control of harmful algal blooms.

A study on the effect of fluorescently stained micro(nano)plastics on the full life history of Skeletonema costatum

As a new type of environmental pollutant, micro(nano)plastics have become a research hotspot in recent years, and their effects on the full life history of marine microalgae have not been studied. To investigate the effects of micro(nano)plastics on the growth, photosynthesis, physiological morphology and interaction of microalgae during the full life cycle, we selected fluorescently stained polystyrene (PS) plastic microbeads as the target pollutant. By sampling and testing the growth rate, photosynthesis and physiological morphology parameters of algal species, the influence of different concentrations of PS (10, 50 and 100 mg/L) and different particle sizes (0.1, 0.5 and 1 μm) on the full life history of Skeletonema costatum (S. costatum) was investigated. The results showed that after adding PS (particle sizes of 0.5 and 1 μm), the response of S. costatum showed a dual character, while adding the same kind of microplastics (MPs) with a particle size of 0.1 μm inhibited S. costatum throughout the full life cycle. Compared with previous studies, short-term experimental data may overestimate the true ecological risks of MPs. In addition, 0.1 μm fluorescent-stained MPs obviously accumulated around the microalgae, indicating that MPs mainly adhered to the surface of algal cells and may enter the food chain by direct or indirect ways, which can cause negative effects on the aquatic ecosystem. This study supports a more accurate assessment of the true risk of MPs to marine aquatic ecosystems.

Seasonal variation in structural and functional distribution of periphyton in a macrolide antibiotics-contaminated river

Periphyton, a microbial assemblage of autotrophic and heterotrophic organisms, is vital to aquatic ecosystems. While exposure to macrolide antibiotics has been confirmed to reduce the biodiversity and damage the critical ecological functions in indoor microcosm bioassays, the distribution of periphyton along a macrolide antibiotic pollution gradient in a river has yet to be determined. Herein, we established the spatiotemporal distribution of five major macrolides, i.e., azithromycin (AZI), roxithromycin (ROX), erythromycin (ERY), clarithromycin (CLA), and anhydro erythromycin (ERY-H2O) in water and periphyton of Zao River (Xi'an, China), after which we evaluated the effects on the structures, photosynthetic activity, and carbon utilization capacity of periphyton in March, June, and September 2023. In contrast with the reference sites, the macrolides were identified in all sewage treatment plants (STPs) impacted sites with concentrations ranging from 0.05 to 2.18 μg/L in water and from not detected - 9.67 μg/g in periphyton. Regarding community structure, the occurrence of macrolides was negatively linked to FirmicutesExiguobacterium undae and Exiguobacterium sibiricum, CyanobacteriaOscillatoriales and Vischeria sp., and ChlorophytaMonostroma grevillei, Selenastrum sp. LU21 and Desmodesmus subspicatus. At the functional level, only the metabolism of phenolic acids was significantly decreased in river reach with high antibiotic levels in June, compared to the other five carbon sources that were not altered. The overall photosynthetic activity of periphytic photosystem II remained unchanged in both reference and STPs impacted groups throughout three seasons. Overall, the macrolides released from STPs were correlated with the altered periphytic structures in the river, whereas a similar trend was not detected for the community functions owing to the functional redundancy. A mesocosm experiments warrants further consideration to validate the field results.

The effects of extracellular algicidal compounds of Bacillus sp. B1 on Heterosigma akashiwo: a metabolomics approach

Heterosigma akashiwo (H. akashiwo), a harmful algal species, has been a global environmental problem. Extracellular algicidal compounds (EACs) extracted from Bacillus sp. B1 exhibited algicidal effects against H. akashiwo. However, little is known about the algicidal mechanism and metabolic process. In this study, metabolomics and physiological analyses were combined to investigate the cellular responses of H. akashiwo when treated with EACs. The results indicated that EACs at 10% (v_EACs/v_sample) showed more than 90% inhibition of H. akashiwo. EAC treatment resulted in excessive reactive oxygen species (ROS) production in algal cells, causing stress responses such as inhibition of photosynthetic pigment synthesis, reduction of sugar synthesis, imbalance of osmotic pressure in the cell membrane, disruption of cell size and morphology, and eventual cell death. The results reveal the underlying mechanism of the algicidal process and provide new insights into algae-bacteria interactions and the application of metabolomics to algal research.

Applicability Evaluation of Soil Algae Pipe Assay in Silver Nanoparticle-Contaminated Soils

Due to pervasive and resilient soil contaminants, heterogeneously contaminated soil poses unpredictable potential threats to ecosystems. In this study, the extension of a previously developed soil algae pipe assay for evaluating heterogeneously contaminated soil under an open system is described. The assay can be used in soil that is heterogeneously contaminated with silver nanoparticles in combination with the examination of morphological changes (e.g., in vivo chlorophyll a, cell granularity, cell size, and mucilaginous sheath) and lipid contents. In addition, we attempted to extend the exposure duration under an open system. We evaluated the applicability of this soil algae pipe assay using green alga Chlamydomonas reinhardtii exposed to heterogeneous and homogeneous polyvinylpyrrolidone capping silver nanoparticles in contaminated soils. The results demonstrated that this method is an applicable bioassay that can be employed to better evaluate soil algal toxicity under an open system, with significant changes in the measured endpoints. The developed assay showed decent predictivity, which can be a useful tool when evaluating heterogeneous soil algae contamination.

Estimation of hazardous concentration of toluene in the terrestrial ecosystem through the species sensitivity distribution approach

Toluene is a highly flammable and commonly used industrial chemical with severe health consequences on humans upon exposure and ingestion. In this study, multispecies bioassays were conducted using a species sensitivity distribution approach to determine acute and chronic hazardous concentrations of toluene in soil. Acute and chronic toluene toxicity tests were conducted with seven soil species from four taxonomic groups. The results from the toxicity tests were used to estimate the acute and chronic HC₅ (hazardous concentration for 5 % of species) of toluene in the terrestrial environment at 58.9 (5.4-639.6) mg kg^-1 and 2.2 (0.2-19.8) mg kg^-1, respectively. To the best of our knowledge, this is the first study to estimate the hazardous concentration of toluene in soil by conducting a battery of bioassays. These values can be used as references for the environmental risk assessment of chemical accidents involving toluene and estimating its impact on soil to protect the terrestrial environment.

Azithromycin induces dual effects on microalgae: Roles of photosynthetic damage and oxidative stress

Antibiotics are frequently detected in aquatic ecosystems, posing a potential threat to the freshwater environment. However, the response mechanism of freshwater microalgae to antibiotics remains inadequately understood. Here, the impacts of azithromycin (a broadly used antibiotic) on microalgae Chlorella pyrenoidosa were systematically studied. The results revealed that high concentrations (5-100 μg/L) of azithromycin inhibited algal growth, with a 96-h half maximal effective concentration of 41.6 μg/L. Azithromycin could weaken the photosynthetic activities of algae by promoting heat dissipation, inhibiting the absorption and trapping of light energy, impairing the reaction centre, and blocking electron transfer beyond Q_A. The blockage of the electron transport chain in the photosynthetic process further induced the generation of reactive oxygen species (ROS). The increases in the activities of superoxide dismutase, peroxidase and glutathione played important roles in antioxidant systems but were still not enough to scavenge the excessive ROS, thus resulting in the oxidative damage indicated by the elevated malondialdehyde level. Furthermore, azithromycin reduced the energy reserves (protein, carbohydrate and lipid) and impaired the cellular structure. In contrast, a hormesis effect on algal growth was found when exposed to low concentrations (0.5 and 1 μg/L) of azithromycin. Low concentrations of azithromycin could induce the activities of the PSII reaction centre by upregulating the mRNA expression of psbA. Additionally, increased chlorophyll b and carotenoids could improve the absorption of light energy and decrease oxidative damage, which further contributed to the increase in energy reserves (protein, carbohydrate and lipid). The risk quotients of azithromycin calculated in this study were higher than 1, suggesting that azithromycin could pose considerable ecological risks in real environments. The present work confirmed that azithromycin induced dual effects on microalgae, which provided new insight for understanding the ecological risk of antibiotics.

Exploring the biophysicochemical alteration of green alga Asterococcus superbus interactively affected by nanoparticles, triclosan and illumination

Toxic effects on Asterococcus superbus were studied based on different combinations of P25-TiO₂, nano-ZnO and triclosan under multiple illumination conditions. A full factorial design (2 × 2×2 × 3) was implemented to explore interactive effects, and to identify significant factors. The results showed illumination is the most important factor with significance and becomes one of the main reasons to affect chlorophyll pigments, photosynthesis activity, unsaturated fatty acids, mitochondria function, and cause oxidative stress. Triclosan considerably affects cell viability, photosynthesis activity, lipid peroxidation and protein structure, for which triclosan is more significant than nano-ZnO. P25 is significant for oxidative stress, antioxidant enzyme, and lipid peroxidation. P25 * nano-ZnO is the only significant interaction of pollutants, affecting macromolecules, lipid peroxidation, and photosynthesis activity. High-order interactions play significant roles in affecting multiple molecular components. Two groups of endpoints are best to reflect alga responses to interactively effects from P25, nano-ZnO, and triclosan. One is ROS, chlorophyll pigments, TBARS, area, MTT, and MMP, and the other one is chlorophyll pigments, ROS, TBARS, CAT, MTT and SOD. Our findings can be instructive for a comprehensive comparison among interactions of multiple pollutants and environmental factors in natural waters, such that more robust environmental toxicity analyses can be performed.

Combined effects of 17β-estradiol and copper on growth, biochemical characteristics and pollutant removals of freshwater microalgae Scenedesmus dimorphus

Contamination by estrogens and heavy metals can cause great environment concern and necessitate efficient approaches for their removals. In this study, the combined effects of 17β-estradiol (E2) and Cu(II) on microalgae growth and biochemical characteristics were investigated. Results showed that 1 mg/L Cu(II) promoted the growth of Scenedesmus dimorphus, while 2 mg/L Cu(II) exhibited growth inhibition, compared with the same concentration of E2. Biochemical characteristics including enzyme activities as well as the contents of chlorophyll, protein and carbohydrate were significantly affected by the coexistence of E2 and Cu(II) after 12 d of cultivation. S. dimorphus exhibited high E2 and Cu(II) removal efficiencies (89.9% of E2 and 76.6% Cu(II) under the coexistence of 0.5 mg/L E2 and 1 mg/L Cu(II), respectively). Lower concentration of Cu(II) might serve as a bridge during E2 removal by S. dimorphus while competitive adsorption of Cu(II) and E2 occurred under the condition of excessive Cu(II). Results could confirm that S. dimorphus was a potential bioresource for the effective removal of E2 and Cu(II).

Influence of polystyrene microplastics on the growth, photosynthetic efficiency and aggregation of freshwater microalgae Chlamydomonas reinhardtii

Microplastics are ubiquitous in aquatic ecosystems worldwide, but knowledge on their impacts on phytoplankton, especially freshwater microalgae, is still limited. To investigate this issue, microalgae Chlamydomonas reinhardtii was exposed to polystyrene (PS) microplastics with 4 concentration gradients (5, 25, 50 and 100 mg/L), and the growth, chlorophyll a fluorescence, photosynthetic activities (Fv/Fm), the contents of malondialdehydes (MDA), soluble proteins, extracellular polymeric substances (EPS) and settlement rate were accordingly measured. Results showed that the density of microalgae decreased as the increase of PS microplastics concentrations, and the highest inhibitory rate (IR) was 45.8% on the 7th day under the concentration of 100 mg/L. The high concentration (100 mg/L) of microplastics evidently inhibited the content of EPS released by microalgae into the solution. PS under all dosages tested could reduce both the chlorophyll a fluorescence yields and photosynthetic activities. The scanning electron microscope (SEM) images demonstrated that microplastic beads were wrapped on the surface of microalgae and damaged their membranes, which could suggest the reduction of photosynthetic activities and the increase of soluble proteins and MDA content. The results also showed that PS microplastics could inhibit the settlement of microalgae at the later stage, which also indicated the recovery of microalgae from the toxic environment. Our findings will contribute to understanding the effects of microplastics on freshwater microalgae, as well as evaluating the possible influences of microplastics on aquatic ecosystems.

Sustainable Iron Recovery and Biodiesel Yield by Acid-Adapted Microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, Grown in Synthetic Acid Mine Drainage

Sustainable resource recovery is the key to manage the overburden of various waste entities of mining practices. The present study demonstrates for the first time a novel approach for iron recovery and biodiesel yield from two acid-adapted microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, grown in synthetic acid mine drainage (SAMD). Virtually, there was no difference in the growth of the strain MAS3 both in Bold's basal medium (control) and SAMD. Using the IC50 level (200 mg L-1) and a lower concentration (50 mg L-1) of iron in SAMD, the cell granularity, exopolysaccharide (EPS) secretion, iron recovery, and biodiesel were assessed in both the strains. Both cell granularity and accumulation of EPS were significantly altered under metal stress in SAMD, resulting in an increase in total accumulation of iron. Growth of the microalgal strains in SAMD yielded 12-20% biodiesel, with no traces of heavy metals, from the biomass. The entire amount of iron, accumulated intracellularly, was recovered in the residual biomass. Our results on the ability of the acid-adapted microalgal strains in iron recovery and yield of biodiesel when grown in SAMD indicate that they could be the potential candidates for use in bioremediation of extreme habitats like AMD.

Effects on photosynthesis and polyphenolic compounds in crop plant mung bean (Vigna radiata) following simulated accidental exposure to hydrogen peroxide

Hydrogen peroxide (H₂O₂) is a strong oxidizer and bleaching agent included in the list of substances requiring accident preparedness by the National Chemical Information System, Korea. Although chemical accidents related to H₂O₂ frequently occur globally, few studies have evaluated its toxicity and risk to soil ecosystems. Herein, accidental exposure to H₂O₂ was simulated in a microcosm including crop plant mung bean (Vigna radiata), and its long-term effects on photosynthetic activities and polyphenolic compounds were measured. Plants were evaluated based on the concentration and amount of H₂O₂ exposure, distance from H₂O₂ source, and duration post exposure. Plants exposed to high concentrations and large amounts of H₂O₂ at a close distance were most damaged; their photosynthetic activities and polyphenolic compound levels significantly decreased compared to the controls. H₂O₂ consistently damaged plants and affected their activities, but plants with minor damage recovered their photosynthetic activities and polyphenolic compound levels. Additionally, moderate oxidative stress from H₂O₂ exposure induced the synthesis of polyphenolic antioxidants including flavonol and anthocyanin. Thus, we suggest that flavonol and anthocyanin levels are the most sensitive indicators of adverse effects of H₂O₂ exposure in V. radiata. Our results highlight the risk of H₂O₂ and serve as a reference for chemical accidents.

Quantitative assessment of photosynthetic activity of Chlorella (Class Trebouxiophyceae) adsorbed onto soil by using fluorescence imaging

In the present study, we evaluate our previously developed non-destructive soil algal toxicity method using species from a different class of algae; Class Trebouxiophyceae (Chlorella vulgaris and Chlorella sorokiniana), and directly measure the photosynthetic activity of these species adsorbed onto the soil as a new toxicity endpoint. This study shows that non-destructive soil algal toxicity method is applicable to non-specific test species, including those of Class Trebouxiophyceae as well as Class Chlorophyceae (Chlorococcum infusionum and Chlamydomonas reinhardtii). Furthermore, by performing photosynthesis image analysis, we verify that it is possible to measure the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soils without the need to extract algal cells from the soil. We propose that the non-destructive soil algal toxicity method represents a novel technique for 1) evaluating pollutants in soil using non-specific algae and 2) conveniently and rapidly assessing the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soil as a new toxicity endpoint.

Changes in soil properties after remediation influence the performance and survival of soil algae and earthworm

Previous research on soil remediation focused on soil properties and not on its effects on soil ecosystems. The present study investigated the adverse effects of soil physicochemical changes due to remediation on the biological indicators Chlorococcum infusionum and Chlamydomonas reinhardtii (algae) and Eisenia andrei (earthworm). Soil physicochemical properties, concentrations of total, bioavailable, and water-soluble heavy metals in soil were measured before and after remediation. Changes in soil pH, electrical conductivity, total nitrogen, and total phosphorous immediately after soil remediation were the primary causes of the biomass and photosynthetic activity inhibition observed in C. infusionum and C. reinhardtii, and the survival, normality, and burrowing behavior decrease observed in E. andrei in remediated soils showing dramatic changes in those properties. These findings suggest that remediated soils need some time to recover before restoring their functions, although heavy metals are no longer contaminating the soil.

Size- and shape-dependent toxicity of silver nanomaterials in green alga Chlorococcum infusionum

Silver nanomaterials (AgNMs) of different shapes and sizes are potentially toxic to aquatic organisms. However, studies on the toxicity of AgNMs and on their shape-dependent effects on algae are scarce. The present study evaluated the effects of three AgNMs (silver nanospheres, AgNPs; silver nanowires, AgNWs; silver nanoplates, AgPLs) with different shapes coated with polyvinylpyrrolidone on the growth and photosynthetic performance of an alga, Chlorococcum infusionum. We used growth measurements and determined the photosynthetic parameters based on chlorophyll fluorescence transients in the algal cells exposed to different concentrations of the three AgNMs. The effective concentrations at 50% (EC₅₀) of AgNPs, AgNWs, and AgPLs were calculated to be 0.1, 0.045, and 0.021 mg/L, respectively. The results showed that the toxicity of AgNMs in C. infusionum was in the order, AgPLs (40 nm diameter) > AgNWs (21,000 nm length × 42 nm diameter) > AgNPs (57 nm diameter), based on the decrease in growth and three photosynthetic activities. We propose that the toxic potential of AgNMs is primarily dependent on their diameter and secondarily on their shape. Overall, this study provides, for the first time, a comparison of the growth and photosynthetic activities of C. infusionum exposed to AgNMs of three different shapes.

Effects of bisphenol A in soil on growth, photosynthesis activity, and genistein levels in crop plants (Vigna radiata)

The compound bisphenol A (BPA), an endocrine-disrupting compound that can act as an estrogen, is widely used in the industrial manufacture of plastic products and epoxy resins. Because of the widespread use of the compound and its use in soil amendments, there is concern regarding its effects on crop plants, although comparatively little information is available on the ecotoxicity and potential risk of bisphenol. Here, we investigated the toxicity of BPA on mung bean (Vigna radiata) by evaluating growth, photosynthesis parameters, and phytoestrogen changes. Adverse effects on shoot growth were observed at a dose of 750 mg BPA/kg dry soil after acute (14 days) and chronic (21 days) exposure, and inhibition of root development was confirmed at a dose of 1000 mg BPA/kg dry soil. Chlorophyll content and stomatal size decreased at doses of 250 and 500 mg BPA/kg dry soil, respectively, and leaf spots due to leaf necrosis were observed in the groups that received 250 mg BPA/kg dry soil. Photosynthetic activity appeared to decrease in the groups that received the highest exposure, although it was not statistically significant. Meanwhile, exposure to bisphenol A increased the level of the phytoestrogen genistein. We propose that changes in genistein levels due to endocrine-disrupting compounds can be considered as a specific toxicity endpoint for endocrine-disrupting chemicals; further studies should explore this effect. This study confirmed the phytotoxicity of BPA at various endpoints and the results provide a basis for future ecological risk assessment for BPA.

Assessing applicability of the paper-disc method used in combination with flow cytometry to evaluate algal toxicity

Soil algal bioassays have been limited by their inability to evaluate several toxic endpoints because it is difficult to collect pure soil algae growing on and beneath the soil surface. This study describes the extension of a previously developed paper-disc method for analyzing soil toxicity to algae. The method can be used in conjunction with flow cytometric analysis and facilitates the assessment of previously proposed toxicity endpoints, such as the growth zone, biomass, and photosynthetic activity. We assessed the applicability of this paper-disc soil method using the green algae Chlamydomonas reinhardtii and Pseudokirchneriella subcapitata exposed to nickel-contaminated soil; examined cell sizes, cell granularity, enzyme activity, and oxidative stress as new toxicity endpoints using flow cytometry; and identified morphological changes in green algae assayed. The results showed that, used in conjunction with flow cytometry, the extended paper-disc soil method is sufficiently sensitive to detect decreases in cell granularity in C. reinhardtii and esterase activity in P. subcapitata. The method also revealed decreases in growth zone, biomass, and electron transfer from the reaction center to the quinone pool. Collectively, the results of this study indicate that soil algal bioassays using nonspecific algae can be used to assess soil quality, to derive several toxicity endpoints for individual cells, and to evaluate previously established flow cytometric toxicity endpoints.

Quantification of silver nanoparticle toxicity to algae in soil via photosynthetic and flow-cytometric analyses

Soil algae, which have received attention for their use in a novel bioassay to evaluate soil toxicity, expand the range of terrestrial test species. However, there is no information regarding the toxicity of nanomaterials to soil algae. Thus, we evaluated the effects of silver nanoparticles (0-50 mg AgNPs/kg dry weight soil) on the soil alga Chlamydomonas reinhardtii after six days, and assessed changes in biomass, photosynthetic activity, cellular morphology, membrane permeability, esterase activity, and oxidative stress. The parameters measured were markedly affected by AgNP-induced stress at 50 mg AgNPs/kg dry weight soil, where soil algal biomass, three measures of photosynthetic activity (area, reaction center per absorption flux, and reaction center per trapped energy flux), and esterase activity decreased. AgNPs also induced increases in both cell size and membrane permeability at 50 mg AgNPs/kg dry weight soil. In addition to the increase in cell size observed via microscopy, a mucilaginous sheath formed as a protective barrier against AgNPs. Thus, the toxicity of AgNPs can be effectively quantified based on the physiological, biochemical, and morphological responses of soil algae, where quantifying the level of toxicity of AgNPs to soil algae could prove to be a useful method in terrestrial ecotoxicology.

Effect of fluoride on the cell viability, cell organelle potential, and photosynthetic capacity of freshwater and soil algae

Although fluoride occurs naturally in the environment, excessive amounts of fluoride in freshwater and terrestrial ecosystems can be harmful. We evaluated the toxicity of fluoride compounds on the growth, viability, and photosynthetic capacity of freshwater (Chlamydomonas reinhardtii and Pseudokirchneriella subcapitata) and terrestrial (Chlorococcum infusionum) algae. To measure algal growth inhibition, a flow cytometric method was adopted (i.e., cell size, granularity, and auto-fluorescence measurements), and algal yield was calculated to assess cell viability. Rhodamine123 and fluorescein diacetate were used to evaluate mitochondrial membrane potential (MMA, ΔΨ_m) and cell permeability. Nine parameters related to the photosynthetic capacity of algae were also evaluated. The results indicated that high concentrations of fluoride compounds affected cell viability, cell organelle potential, and photosynthetic functions. The cell viability measurements of the three algal species decreased, but apoptosis was only observed in C. infusionum. The MMA (ΔΨ_m) of cells exposed to fluoride varied among species, and the cell permeability of the three species generally decreased. The decrease in the photosynthetic activity of algae may be attributable to the combination of fluoride ions (F^-) with magnesium ions (Mg²⁺) in chlorophyll. Our results therefore provide strong evidence for the potential risks of fluoride compounds to microflora and microfauna in freshwater and terrestrial ecosystems.

Paper-disc method: An efficient assay for evaluating metal toxicity to soil algae

The probabilistic ecological risk assessment using terrestrial toxicity data has been mainly based on microfauna or mesofauna. Soil algae, which are food source for microfauna and mesofauna, may be alternatively used for assessing soil toxicity. However, there are no internationally recommended guidelines for soil algal bioassays, and the collection of algae from the test soils has some limitations. In this study, we suggested the paper-disc method as an easy-to-use alternative. This method has been widely used for testing the antibacterial toxicity of various chemicals in agar media by measuring the diameter of the inhibition zone around the disc. We adapted the paper-disc method for screening the toxicity of copper (Cu) and nickel (Ni) to the soil alga Chlorococcum infusionum using various evaluation endpoints, such as growth zone, chlorophyll fluorescence, and photosynthetic activity. Chlorophyll fluorescence and photosynthetic activity decreased with the increasing concentrations of Cu(+2) or Ni(+2) contaminated soils. Algal growth zone was analyzed visually and showed similar results to those of chlorophyll fluorescence. The direct ethanol extraction method and indirect culture medium extraction method were similarly effective; however, the former was easier to perform, while the latter might facilitate the analysis of additional endpoints in future studies. Overall, the results suggested that the paper-disc method was not only a user-friendly assay for screening soil toxicity, but also effective due to its association with indirect soil quality indicators.

The effect of lead on the growth, content of primary metabolites, and antioxidant response of green alga Acutodesmus obliquus (Chlorophyceae)

Green unicellular alga Acutodesmus obliquus (Turpin) Hegewald et Hanagata (SAG strain no. 276-6) (Chlorophyceae) was used for determination of phytotoxicity of lead (Pb) at the range of concentrations 0.01-500 μM during 7 days of culture. The accumulation of Pb in algal cells was found to be increased in a concentration- and duration-dependent manner. The highest Pb uptake value was obtained in response to 500 μM Pb on the seventh day of cultivation. The decrease in the number and the size of cells and the contents of selected primary metabolites (photosynthetic pigments, monosaccharides, and proteins) in A. obliquus cells were observed under Pb stress. Heavy metal stimulated also formation of reactive oxygen species (hydrogen peroxide) and oxidative damage as evidenced by increased lipid peroxidation. On the other hand, the deleterious effects of Pb resulting from the cellular oxidative state can be alleviated by enzymatic (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) and non-enzymatic (ascorbate, glutathione) antioxidant systems. These results suggest that A. obliquus is a promising bioindicator of heavy metal toxicity in aquatic environment, and it has been identified as good scavenger of Pb from aqueous solution.