High Light Intensity Augments Mercury Toxicity in Cyanobacterium Nostoc muscorum

The present study is aimed at investigating the role of growth irradiance in determining the extent of mercury (Hg) toxicity on various physiological parameters viz. growth, pigment contents, photosynthesis, respiration, ¹⁴CO₂ fixation, photosynthetic electron transport, photorespiration and enzyme activity of cyanobacterium Nostoc muscorum. A general decline was observed in all these parameters with increasing concentration of Hg except for carotenoids content and respiratory activity which exhibited significant enhancement. This effect was more pronounced in high light (130???mol photon m^?2?s^?1) exposed cells as compared to normal (70???mol photon m^?2?s^?1) and low (10???mol photon m^?2?s^?1) light exposed cells. Among the photosynthetic electron transport activities, whole chain was found to be more sensitive than photosystem II (PSII) and photosystem I (PSI). ¹⁴CO₂ fixation was more affected as compared to O₂ evolution when exposed to Hg and different light intensities. Photorespiratory activity, which is an index of protecting organisms from light-induced damage, also showed a similar declining trend. Enzyme assay revealed that among the carboxylating enzymes, activity of RUBISCO was more severely inhibited than PEPCase. Thus, these results suggest that Hg itself was toxic at all tested concentrations and high light intensity augmented its toxicity in N. muscorum inhibiting the growth, pigment contents and photosynthetic activity of the organism.