Heavy Metal Tolerance Profiling and Differential Carotenoid Modulation in Cyanobacterial Taxa Under Lead and Cadmium Stress

Cyanobacteria are ubiquitous photosynthetic microorganisms that play essential roles in ecosystem functioning. However, activities like heavy metal pollution alter their structure and physiological processes, thereby affecting ecosystem health. Carotenoids are potentially helpful markers of physiological reactions to metal stress because of their significant roles in photoprotection and cellular defense against oxidative stress. This study investigated the effects of lead and cadmium on the carotenoid content of three cyanobacterial species: Oscillatoria subbrevis , Lyngbya martensiana , and Phormidium autumnale . The organisms were exposed to 2 and 5 ppm of Pb and Cd and carotenoid content was recorded at 0, 3, and 6 days of exposure. The results demonstrated species-specific responses to heavy metal stress. Cadmium caused a greater reduction in carotenoid content than lead in all three species, indicating its higher toxicity. O. subbrevis exhibited the greatest sensitivity, with a pronounced decline in carotenoid content under heavy metal treatments. L. martensiana showed an initial increase at the lower metal concentration, followed by a decline at the higher concentration, suggesting an intermediate tolerance to heavy metal stress. In contrast, P. autumnale was the most tolerant species, maintaining comparatively higher carotenoid levels under metal exposure, which may be associated with a more efficient antioxidant defense system and greater heavy metal biosorption capacity. Two-way ANOVA revealed significant (p < 0.05) effects of treatment, sampling day, and their interaction on carotenoid content in O. subbrevis . In L. martensiana and P. autumnale , treatment and sampling day had highly significant effects (p < 0.005), whereas the interaction between these factors was not significant. These findings demonstrate differential tolerance of cyanobacterial taxa to heavy metal stress and highlight the significance of species-specific physiological response in understanding its adaptation under stress and its potential for application in heavy metal bioremediation.