Rapid in situ assessment for predicting soil quality using an algae-soaked disc seeding assay

The potential of Euglena species as a bioindicator for soil ecotoxicity assessment

Currently, there are no standard international test methods for assessing aquatic and soil toxicity, with aquatic toxicity tests based on limited Euglena species. Here, we proposed Euglena species as extended test species, especially as new soil test species for a paper-disc soil method, considering its ecologically important roles in providing highly bioavailable in-vivo nutrients to upper trophic level organisms. We conducted experiments to identify the optimal exposure duration for two Euglena species (Euglena viridis and Euglena geniculata). We demonstrated the toxic effects of nickel (model contaminant) on their photosynthetic parameters and growth in freshwater. The growth and photosynthetic activity of three Euglena species were significantly inhibited in nickel-contaminated soil during paper-disc soil tests, especially the test species adsorbed onto paper-disc soil. Euglena gracilis was more sensitive to nickel than E. viridis and E. geniculata in freshwater and soil. Thus, E. viridis and E. geniculata have potential as additional test species for improving test species diversity, while all three species have potential as new soil test species for soil toxicity assessment. Thus, results these species may be suitable for routine aquatic toxicity testing and new soil toxicity testing, addressing the current paucity of test species in freshwater and soil toxicity assessment.

Validation of the paper-disc soil method using soil alga Chlorococcum infusionum to quantitatively determine the toxicity of heavy metals

The paper-disc soil method is a soil algal bioassay used to assess soil water transfer in polluted soils. Although this ecotoxicological method was first established in 2016 to assess soil pollution, quantitative toxicity assessments of Cu- or Ni-treated soil and in situ or ex situ soils polluted with other heavy metals (HMs) have not been widely conducted. In this study, the paper-disc soil method was validated under different test conditions. The validated paper-disc soil method was used to quantitatively investigate the toxicity of four HMs (As, Cd, Cr, and Zn). Based on the results, the test species, initial inoculation concentration in the algal-dispersed solution, water content, light intensity, and exposure duration were proposed as Chlorococcum infusionum, 3 × 10⁶ cells/mL in Bold's basal medium, 80% water-holding capacity, 4000 ± 500 lx, and 6 days, respectively. The toxicity of HMs was quantified using the validated paper-disc soil method, and the half-maximal effective concentration values for biomass were calculated as 22.49 (21.26-23.78) mg Cr/kg dry weight (dw) soil, 42.72 (38.64-47.24) mg Cd/kg dw soil, 57.79 (55.46-60.21) mg As/kg dw soil, and 183.06 (175.38-191.06) mg Zn/kg dw soil. The paper-disc soil method using the soil alga C. infusionum was validated by quantitatively evaluating the test conditions and toxicity of HMs. Our results provide important quantitative toxicity data for soil algae exposed to HMs and a basis for standardizing the paper-disc soil method using soil algae.

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.

Perspectives on microalgae as model organisms toward the standardization of soil algal toxicity test methods

When considering test species for soil ecotoxicity, the development of new model organisms is often suggested to increase the reliability of ecological risk assessments. Ubiquitous soil algae could offer potential test species for assessing various soil pollution levels. Currently, there are few reviews offering comprehensive perspectives on stressors-based toxicological studies using microalgae in soil media, with the majority of scholarly attention paid to the toxicological effects of freshwater algae or marine algae in aquatic ecosystems. In this review, we focus on current toxicological studies of microalgae assessed in soil-related media and suggest considerations for using microalgae in soil toxicity tests based on 22 publications (1998-2021). In addition, we analyzed characteristics of soil algae based on criteria for selecting test species and suggest that future research should be directed toward the standardization of soil algal toxicity test methods. This review discusses a promising method using soil algae as new test species for soil toxicity assessment as cost-effective and environmentally sound soil quality bioindicators. The review also addresses the lack of understanding behind how soil algae can serve as important test species for soil ecotoxicity.

Rapid In Situ Biomonitoring of Subsoil Contamination by Applying an Algae-Soaked Disc Seeding Assay

Various pollutants are pervasive in soil environments due to human activities, thereby damaging soil ecosystems. In this study, extension of a previously developed algae-soaked disc seeding assay for periodic evaluation of subsoil contamination over time was described. The assay can be used in different contamination configurations of silver nanoparticles in combination with examination of cell morphology, esterase activity, oxidative stress, and membrane permeability. In addition, we periodically attempted to repeat the algae-soaked disc seeding assay every three weeks. We evaluated applicability of this algae-soaked disc seeding assay using alga Chlamydomonas reinhardtii exposed to heterogeneous silver nanoparticle-contaminated soils. The results demonstrated that this assay is applicable for monitoring a change of subsoil contamination by periodic evaluation over time. The developed assay was identified as a periodically rapid in situ biomonitoring technique to measure subsoil contamination over time.

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.

Soil algae pipe assay: ex situ method for the evaluation of soil quality based on soil algae and its application to the pot test

We aimed to develop an ex situ method for evaluating soil quality using soil algae. The soil algae pipe assay involves application and sampling of algae and flow cytometry. As it can be performed in a greenhouse, the assay was effective for pot tests and conducive for soil quality assessment. This novel assay will enable broader toxicity testing.