Like other plants, algae are recipients of, and respondents to, heavy metals (1-3). It has been suggested that the inhibition of photosynthesis and N2fixation by heavy metals is caused by depletion of ATP and reluctant pools by the heavy metal (4). Such cations also inhibit the cyanobacterial uptake of N03 -, NH4 +, and P043 (5-7). The rare occurrence of heavy metals as single species in any environment prompted us to investigate the interactions of Hg2 + with Nit ~, Cue +, Cd2 +, and CH3Hg+ in

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... bimetallic combinations. The present work describes the effect of Hg2 +, Cue +, and various bimetallic combinations on glutamine synthetase (transferase) and nitrogenase activity in the diazotrophic cyanobacterium Nostoc calcicola Breb. Nostoc calcicola, a local isolate was maintained in ALLEN and ARNON's combined nitrogen-free growth medium (8) at 24 + 1 °C at 250 ft c (fluorescent illumination) with an l86 6 hr light dark cycle. Protein was estimated by the method of LOwRY' et al. (9) modified by HERBERT et al. (10) using lysozyme as the standard. Chlorophyll a was measured in 80°c acetone at 665 nm in a Spectronic-20 colorimeter (Bausch and Lomb, USA). Calculations made using the specific absorption coefficient 82.04 (11). Glutamine synthetase assay was followed according to SHAPIRO and STADTMAN (12) modified by STACEY et al. (13) using a 1.0-ml algal suspension. Logphase cells of N. calcicola were inoculated into fresh growth medium (approximately 500 pg of protein/ml) containing either Hg2 + (2.5-50 pM) or Cue + (50-500 pM). For interaction experiments, equimolar (10 pM) concentrations of either CH3Hg+, Cd2 +, Cue +, or Nit + were simultaneously added to the growth medium in terms of independent bimetallic combinations with Hg2 +