Protective effects of Ca2+, Mg2+, Cu2+, and Ni2+ on mercury and methylmercury toxicity to a cyanobacterium

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+ greater than Mg2+ much greater than Cu2+ greater than Ni2+.