Visualization and Quantification of the Impact of Humic Acid on Zinc Accumulation in Aquatic Plants Using a Low-Molecular-Weight Fluorescent Probe

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.