Sensitivity of Natural Bacterial Communities to Additions of Copper and to Cupric Ion Activity: A Bioassy of Copper Complexation in Seawater

Abstract

Cupric ion bioassays with natural bacterial communities were conducted in seawater samples at five stations in the Gulf of Mexico, ranging from low-productivity oceanic to high-productivity coastal. In these bioassays, we measured the incorporation of ³H-labeled amino acids by marine microbial communities following serial additions of ionic copper (CuSO₄) and cupric ion buffers (combinations of CuSO₄ and nitrilotriacetic acid, a synthetic chelator). The bioassays were designed to yield relations between (1) amino acid incorporation and total copper concentration, (2) incorporation and cupric ion activity, and (3) cupric ion activity and total copper concentration.

Microbial uptake of ³H-labeled amino acids in low-productivity seawater was reduced by 2 nmol 1⁻¹ additions of CuSO₄, an addition at or below ambient copper concentrations. Added copper was appreciably less toxic in high-productivity coastal seawater samples. Differences in the effect of added copper among samples could be attributed both to differences in the relationships between inhibition of labeled amino acid incorporation and cupric ion activity and to differences in relationships between cupric ion activity and total copper concentration. The bioassays indicated higher complexation of added copper in high-productivity coastal samples. Complexation data was modeled using chemical equilibrium theory to estimate conditional stability constants and total concentrations of strong copper complexing ligands and to estimate copper speciation at ambient copper concentrations. Conditional stability constants ranged from 10⁹ to ≥ 10¹¹ and ligandconcentrations ranged from 5 to 150 nmol 1⁻¹. Cupric ion activity at ambient levels of copper are estimated to be in the range ≤ 10⁻¹² to 10⁻¹¹ mol 1⁻¹.