Effect of bimetallic combinations of Ni, Cr, and Pb on growth, uptake of nitrate and ammonia, 14CO2 fixation, and nitrogenase activity of Nostoc muscorum

The toxicity of Cr, Ni, and Pb individually as well as in combination (Cr + Ni, Cr + Pb, Ni + Pb) on growth, final yield, uptake of NO3- and NH4+, photosynthesis, and nitrogenase activity of Nostoc muscorum has been studied. All test metals, when used individually, depicted toxicity against the above-mentioned processes of test cyanobacterium. However, their interactive effect was found to be mostly of antagonistic type. Antagonism was more prevalent in those combinations containing nickel. In an attempt to measure the toxic potential of Cr, Ni, and Pb with respect to time, photosynthetic carbon fixation was found to be the more sensitive parameter followed by N2ase, NH4+ uptake, and NO3- uptake. Contrary to this, NH4+ uptake followed by carbon fixation, nitrate uptake, and N2ase appeared to be a preferred hierarchy sequence for measuring the toxicity of Cr, Ni, and Pb individually without any consideration of time. The toxicity potentials of metal combinations are generally less significant than those of individual metals. N2ase seems to be a more critical process followed by NH4+ uptake, photosynthesis, and NO3- uptake in measuring metal combination effects with respect to time. However, if time is not given any consideration, NH4+ uptake remains the most sensitive process for measuring metal combination effects. These results suggest that exposure time and treatment types are very important factors in evaluation of heavy metal toxicity.