Zero-Valent Iron Nanoparticles Induce Reactive Oxygen Species in the Cyanobacterium, Fremyella diplosiphon

Nanoscale zero-valent iron nanoparticles (nZVIs) are known to boost biomass production and lipid yield in Fremyella diplosiphon, a model biodiesel-producing cyanobacterium. However, the impact of nZVI-induced reactive oxygen species (ROS) in F. diplosiphon has not been evaluated. In the present study, ROS in F. diplosiphon strains (B481-WT and B481-SD) generated in response to nZVI-induced oxidative stress were quantified and the enzymatic response determined. Lipid peroxidation as a measure of oxidative stress revealed significantly higher malondialdehyde content (p < 0.01) in both strains treated with 3.2, 12.8, and 51.2 mg L^-1 nZVIs compared to untreated control. In addition, ROS in all nZVI-treated cultures treated with 1.6-25.6 mg L^-1 nZVIs was significantly higher than the untreated control as determined by the 2',7'-dichlorodihydrofluorescein diacetate fluorometric probe. Immunodetection using densitometric analysis of iron superoxide dismutase (SOD) revealed significantly higher SOD levels in both strains treated with nZVIs at 51.2 mg L^-1. In addition, we observed significantly higher (p < 0.001) SOD levels in the B481-SD strain treated with 6.4 mg L^-1 nZVIs compared to 3.2 mg L^-1 nZVIs. Validation using transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS) revealed adsorption of nZVIs with a strong iron peak in both B481-WT and B481-SD strains. While the EDS spectra showed strong signals for iron at 4 and 12 days after treatment, a significant decrease in peak intensity was observed at 20 days. Future efforts will be aimed at studying transduction mechanisms that cause metabolic and epigenetic alterations in response to nZVIs in F. diplosiphon.