Strain-Specific Impact of Titanium Dioxide Nanoparticles on Fremyella Diplosiphon Physiological and Metabolic Responses

Titanium dioxide nanoparticles (n-TiO₂) have emerged as potent modulators of photosynthetic activity in cyanobacteria; however, their strain-specific physiological effects in Fremyella diplosiphon, a model cyanobacterium, remain unexplored. In this study, we investigated the impact of n-TiO₂ on growth, pigment autofluorescence, photosynthetic capacity, reactive oxygen species (ROS) generation and ATP synthase activity in F. diplosiphon strains B481-SD (overexpressed with the sterol desaturase gene) and B481-WT (wild type). Growth as a measure of optical density was maximal in B481-SD at 2.0 mg/L (0.67 ± 0.01) and in B481-WT at both 2.0 (0.55 ± 0.01) and 16 mg/L (0.52 ± 0.01) n-TiO₂ on day 12. Pigment accumulation over 15 days revealed enhanced phycocyanin (1300 ± 2) and chlorophyll a (890 ± 5) levels in 2.0 mg/L n-TiO₂-treated B481-SD while no significant changes were observed in B481-WT. Photosynthetic efficiency (Fv/Fm) of B481-SD treated with 2.0 mg/L n-TiO₂ was significantly higher on days 6, 9, and 12. ROS quantification using the 2',7'dichlorodihydrofluorescein diacetate assay revealed significantly higher levels in B481-WT at 2.0 mg/L (260 ± 5), whereas B481-SD exhibited lower ROS levels at 2.0 (210 ± 2) and 16 mg/L (220 ± 4) n-TiO₂ on day 15. Additionally, immunodetection analysis of ATP synthase revealed significantly enhanced expression in F. diplosiphon B481-SD treated with 0.5, 2.0, and 128 mg/L n-TiO₂ compared to the untreated control. Visualization of cell-n-TiO₂ interactions using field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy revealed a strong absorption for titanium, with an atomic percentage of 0.32%. These findings demonstrate strain-specific responses of F. diplosiphon to n-TiO₂, paving the way for scale-up cultivation to enhance cyanobacteria-derived bioproducts.