Browsing by Author "Chandra Bhushan Singh"

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Assessment of Hg2+ toxicity to a N2-fixing cyanobacterium in long- and short-term experiments
(Kluwer Academic Publishers, 1992) Chandra Bhushan Singh; S.P. Singh
Toxicological responses of the filamentous N2-fixing cyanobacterium Nostoc calcicola Bréb. towards Hg2+ were studied to enumerate the decisive lethal events. In low-dose, long-term experiments (0.05-0.25 μm Hg2+, 10 days), photoautotrophic growth was severely inhibited with concurrent loss of photosynthetic pigments (phycocyanin>chlorophyll α>carotenoids) and nucleic acids. The termination of growth after a day 4 exposure to 0.25 μm Hg2+ has been attributed to the complete inhibition of in vivo photosynthetic activity in the cyanobacterium (O2 evolution>14CO2 incorporation). The elevated Hg2+ concentrations irreversibly damaged the cell membrance as observed under light microscopy, and as indicated by the leakage of intracellular electrolytes and phycocyanin. In high-dose, short-term experiments (0.5-20.0 μm Hg2+, up to 6 h), the in vivo activities of selected enzymes (glutamine synthetase > nitrate reductase > nitrogenase) were less inhibited by Hg2+ than the uptake of nutrient ions (NH4+>NO3->PO43-). © 1992 Rapid Communications of Oxford Ltd.
Hg2+-induced leakage of electrolytes and inhibition of NO3- utilization in Nostoc calcicola - Role of interacting cations
(Springer-Verlag, 1990) Chandra Bhushan Singh; S.P. Singh
The effect of mercury (Hg2+) in the absence and presence of methylmercury (CH3Hg+), cadmium (Cd2+), copper (Cu2+), nickel (Ni2+) and calcium (Ca2+) on Nostoc calcicola Bréb. has been studied in terms of electrolyte leakage, NO3- uptake and in vivo nitrate reductase (NR) activity to discover any possible correlation among such parameters under Hg2+ stress. Leakage of electrolytes from Hg2+-treated cyanobacterial cells was directly proportional to Hg2+ concentrations and exposure time. In comparison to NO3- uptake, an about 60-fold slower rate of NR activity was observed in the untreated cultures, the former being five times more Hg2+-sensitive. A non-competitive synergistic interaction of Hg2+ with CH3Hg+ or Cd2+ and antagonistic with that of Ni2+ or Ca2+ has been observed for both the processes of NO3- utilization. The antagonistic interaction of Cu2+ with Hg2+ in terms of NO3- uptake and synergistic with respect to NR activity, has been attributed to the dual bonding preference of Cu2+ for cellular ligands. These findings suggest that (a) a statistically significant correlation exists among such parameters; (b) Hg2+ predominantly attacks the cyanobacterial cell membrane; (c) Hg2+ inhibits NO3- utilization; (d) the presence of other cations increases or decreases the inhibitory actions of Hg2+. © 1990 Springer-Verlag.
Protective effects of Ca2+, Mg2+, Cu2+, and Ni2+ on mercury and methylmercury toxicity to a cyanobacterium
(1992) Chandra Bhushan Singh; S.P. Singh
Toxicological investigations of the impact of inorganic mercury (Hg2+) and methylmercury (CH3Hg+) in terms of growth, NH4+ uptake, in vivo glutamine synthetase (transferase) activity, and regulation of toxicity by Ca2+, Mg2+, Cu2+, and Ni2+ in the diazotrophic cyanobacterium Nostoc calcicola Bréb. have been completed. Photoautotrophic growth of the cyanobacterium was extremely sensitive to both mercury compounds, CH3Hg+ being 2.5 times more toxic than Hg2+. Although NH4+ uptake was 6 times more sensitive than in vivo GS activity against the two mercurials, both processes had a greater susceptibility toward CH3Hg+. On the basis of Km and Vmax, it is suggested that both mercury species inhibit such metabolic events noncompetitively. Ca2+, Mg2+, Cu2+, and Ni2+ did not change the nature of inhibition and effectively antagonized the Hg2+ and CH3Hg+ toxicities in the sequence Ca2+ > Mg2+ ≪ Cu2+ > Ni2+. © 1992.