Cu uptake in the diazotrophic cyanobacterium Nostoc calcicola Breb. was accompanied by inhibitions in the in vivo activities of photosystem (PS) II, PS I (CO2)-C-14-fixation, and decline in the ATP pool. Cyanobacterial cells, while saturated for Cu uptake within 1 h at 40-mu-M Cu, showed more than 50% inhibition of PS II and 95.4% of (CO2)-C-14 fixation compared with only 15.5% decrease in the PS I activity. The total extractable ATP content also declined by 32.3% within 1 h. In a subsequent follow-up study lasting 72 h, PS II activity and (CO2)-C-14 fixation showed complete inhibition, in contrast to 34.4% of PS I activity and 4.2% of ATP still remaining unaffected. The results have been discussed in the light of multiple effects of Cu during and subsequent to its uptake by the cyanobacterium.