Banana pseudostem-derived hydrochar mitigates aluminum oxide nanoparticle toxicity to microalgae: implications for sustainable phosphate binder application in contaminated waters

Aluminum oxide nanoparticles (Al2O3) are used as a phosphate binder for controlling harmful cyanobacterial blooms in waters; however, concerns related to their toxic effects on non-target microalgae limit their long-term field application. In the present study, banana pseudostem-derived hydrochar was combined with Al2O3 to form a composite (HCAL). The hydrochar and Al2O3 concentrations in HCAL (0.5:2) were selected based on preliminary dosage-optimization trials. Then, toxicity assessment was done by exposing pre-grown microalgae Graesiella emersonii MN877773 at three concentrations of HCAL (115, 230, 460 mg L−1) and compared with negative (Al2O3) and positive control (BG11 media). Microalgae grown in HCAL treatment showed a 29–49% higher growth rate than Al2O3 treatment. At all tested concentrations, HCAL treatment resulted in significantly higher (p ≤ 0.05; 19–64%) chlorophyll content in microalgae compared to the control. A significant upregulation of genes involved in carbon fixation and photosynthesis, such as rbcL, psaB and psbc genes, was observed at 230 mg L−1 HCAL exposure. Microalgae were found to overcome the stress via increasing lipid synthesis and a strong antioxidant response. These findings demonstrate the potential of utilizing banana pseudostem-derived hydrochar as an eco-engineering amendment to mitigate the toxicity of aluminum oxide as a phosphate-binding agent in contaminated waters.