Bioremediation of chromium by Oscillatoria pseudogeminata: growth dynamics, metal uptake, and physiological responses

Hexavalent chromium, Cr(VI), is a highly toxic heavy metal that disrupts metabolic processes and threatens aquatic ecosystems. Cyanoremediation using metabolically active cyanobacteria for heavy-metal sequestration and detoxification is a sustainable, eco-friendly remediation approach. The present study evaluated the Cr(VI) removal efficiency and physiological responses of an indigenously isolated freshwater cyanobacterium, Oscillatoria pseudogeminata, under different Cr(VI) concentrations (2, 5, 7, and 10 ppm). Growth performance, photosynthetic pigments, oxidative stress markers, antioxidant responses, and chromium uptake were assessed to determine cellular responses to metal stress. Increasing Cr(VI) concentrations caused a concentration-dependent decline in growth, biomass accumulation, and pigment content, indicating impairment of photosynthetic and metabolic processes. In contrast, oxidative stress indicators and antioxidant defense responses, including malondialdehyde accumulation, superoxide dismutase activity, and proline content, were significantly elevated at lower Cr(VI) levels (2 and 5 ppm), reflecting activation of cellular defense mechanisms. These responses declined at higher concentrations, suggesting enzyme inhibition and severe metal-induced oxidative damage. Scanning electron microscopy revealed morphological alterations in treated filaments, while atomic absorption spectroscopy confirmed significant Cr(VI) absorption by the cyanobacteria, with removal efficiencies ranging from 48 to 60%. Overall, Oscillatoria pseudogeminata demonstrates notable chromium tolerance and biosorption capacity, highlighting its remediation potential for treating chromium-contaminated water.