The main aims of this study were to investigate the impact of humic acid (HA) on zinc (Zn) accumulation in aquatic plants and to study a low-molecular-weight Zn²⁺-selective fluorescent probe to visualize and quantify the tissue-level Zn concentrations. Ceratophyllum demersum and Aldrovanda vesiculosa were exposed to solutions containing Zn (1 and 3 mg/L), HA (0.5 mg/L), and Zn with HA for nine days. The Zn accumulation (mg/g) in the plants was measured by ICP-OES and we applied a Zn²⁺-selective fluorescent probe with a low molecular weight to the analysis of Zn in C. demersum plant cells using fluorescence microscopy and ImageJ software. The application of HA reduced the Zn accumulation significantly (p < 0.05) and increased the chlorophyll concentration slightly less significantly (p > 0.05) in both plants. Results obtained from ImageJ revealed a strong positive correlation between fluorescence intensity and the Zn accumulation in C. demersum (r = 0.988). We showed that the application of HA reduced the Zn accumulation in both plants, and successfully visualized and quantified that a Zn²⁺-selective fluorescent probe with a low molecular weight can be applied to the diagnosis of Zn osmosis into a cell or tissue on the basis of fluorescence intensity.

Hydroponics, a soilless cultivation system, has received attention because of its application in industry and being thought as the option to support the increased world population by increasing crop yields with less water. A highly concentrated nutrient solution is used in the system; this solution is discharged after use because the composition becomes imbalanced, and hazardous substances and pathogens may accumulate. Water quality surveys were conducted to identify the characteristics of the hydroponic effluent, which has not been previously documented. Twenty-one effluents were collected from 16 facilities in Kochi, Japan. The concentrations of nutrients varied significantly: i.e. 9.8–526 mg N/L of nitrate, and 3.0–131 mg P/L of phosphorus. Statistical analysis revealed five properties that explained the variation in the effluents. Mass balances of nutrients were evaluated in a facility raising eggplant. The efficiencies of the nutrient investment in hydroponics seemed higher than those in soil cultivation systems: 34–41 moles (478–572 mg N) of nitrogen and 1.1–5.9 moles (35–184 mg P) of phosphorus were discharged during the cultivation of 1 tonne of eggplant.

In this study, we conducted long-term laboratory and field feeding experiments to investigate the ability of a freshwater bivalve, Nodularia douglasiae (recently renamed to N. nipponensis) to utilize cyanobacterial cells as food. In the laboratory experiment, N. douglasiae which was fed with ¹⁵N-labeled Microcystis aeruginosa cells over 80 days showed an assimilation efficiency of 47%. The fatty acid compositions in bivalves reflected a slight but apparent increase in the relative ratio of two cyanobacterial fatty acids, linoleic acid (LA) and α-linolenic acid (ALA), to eicosapentaenoic acid (EPA). In the field-feeding experiment, N. douglasiae was fed natural seston in the littoral area of the eutrophic Lake Hachiro (Akita, Japan) during the summers of 2017 and 2018 when cyanobacterial blooms occurred. The ratios of LA and ALA to EPA in bivalves increased from 0.45 to 0.70 and from 0.40 to 0.77 during the summers; the final values were consistent with those obtained in the laboratory feeding experiment. The results indicated that N. douglasiae can utilize cyanobacterial cells as food and maintain assimilation activity at least over several months, and the species may serve as a useful biomanipulation tool for controlling cyanobacterial blooms in freshwater lakes.

Di-through octachlorinated naphthalenes (DiCNs, TrCNs, TeCNs, PeCNs, HxCNs, HpCNs and OcCN) in landfill leachates can be analyzed with the same cleanup procedure as dioxins. In this study, a modified method was developed by confirming the elution order of each isomer of polychlorinated naphthalenes (PCNs) in a chromatogram of a gas chromatograph/high resolution mass spectrometer (GC/HRMS) using an Rh-12 ms capillary column, which is mainly employed for the analysis of dioxins. Using this method, PCNs in landfill leachate samples and treated leachate samples from each corresponding wastewater treatment process were quantified. PCN concentrations in landfill leachates ranged from 8.0–12,000 pg/L, and DiCNs-PeCNs were detected in almost all samples. Removal of PCNs by flocculation and active carbon treatment was greater than that achieved by activated sludge treatment in wastewater treatment plants. Furthermore, by changing the temperature of the H₂SO₄ coated silica gel column in the modified method, it became possible to measure monochlorinated naphthalenes (MoCNs), which could not be measured previously. MoCN concentrations detected were much higher compared to concentrations of DiCNs-OcCN in most of landfill leachates. Unlike PCNs, MoCNs are not POPs, but they are nevertheless important for understanding the properties of landfill leachates.

Recently, the symbiosis between microalgae and bacteria for wastewater treatment system receives more attention as microalgae and bacteria coexist symbiotically under suitable environmental condition. Moreover, the microalgae and bacteria interaction in granular form had been considered as an environmental friendly alternatives due to the synergistic cooperation between microalgae and bacteria in treating wastewater. This study aims to develop microalgae-bacteria aerobic granular sludge using low-strength domestic wastewater. A mixture of Scenedesmus obliquus and activated sludge at ratio of 17% microalgae to 83% activated sludge (v/v) was used as the seed sludge to develop microalgae-bacteria aerobic granular sludge. Upon 30 days of experimental period, granular sludge was successfully developed with largest granular diameter of 6 mm. The developed granules exhibited excellent settling properties with 62.8 m/h settling velocity. Better granular settleability indicated by low sludge volume index (SVI₃₀) was detected at 8 mL/g. Observation using field-emission scanning electron microscope (FESEM) showed the attachment of microalgae cells on the outer layer of granular sludge. Moreover, microalgae-bacteria aerobic granular sludge demonstrated good COD and ammoniacal nitrogen removal at 72% removal efficiency.