Toward a Biotic Ligand Model for Freshwater Green Algae:  Surface-Bound and Internal Copper Are Better Predictors of Toxicity than Free Cu²⁺-Ion Activity When pH Is Varied

Abstract

The freshwater green microalgae Chlorella sp. and Pseudokirchneriella subcapitata (P. subcapitata) were chronically (48 and 72 h, respectively) exposed to copper at various pH levels, i.e., pH 6-7.5 and pH 5.9-8.5, respectively. Concentrations resulting in 50% inhibition of exponential growth rate (EC50) were determined as dissolved Cu, estimated chemical activity of the free Cu2+ ion (as pCu = - log{Cu2+ activity as molarity}), and as external (surface-bound) Cu and internal Cu in the algal cells. With increasing pH, EC50dissolved decreased from 30 to 1.1 microg of Cu L(-1) for Chlorella sp. and from 46 to 18 microg of Cu L(-1) for P. subcapitata. The pH effect on copper toxicity was even more obvious when expressed as Cu2+ activity. The EC50pCu increased on average 1.4 pCu unit per pH unit for Chlorella sp. and 1.1 pCu unit per pH unit for P. subcapitata, thus indicating a marked increase of Cu2+ toxicity at higher pH (more than 1 order of magnitude per pH unit). In contrast, it was found that EC50 values expressed as surface bound or external copper (EC50external) and as internal copper (EC50internal) did not vary substantially when pH was increased. External Cu was operationally defined as the Cu fraction removable from the algal cell by short-term contact with ethylenediaminetetraacetic acid; internal copper was defined as the nonremovable fraction. For Chlorella sp. the EC50external varied between 5 and 10 fg of Cu/ cell (factor of 2 difference) and the EC50internal between 25 and 40 fg of Cu/cell (factor of 1.6 difference). For P. subcapitata the EC50external varied between 10 and 28 fg of Cu/cell (factor of 2.8 difference) and the EC50internal between 42 and 71 fg of Cu/cell (factor of 1.7 difference). Because the observed variation in EC50external and EC50internal is much less than the variation in EC50Cu2+, it is concluded that both external and internal copper are better predictors of copper toxicity than Cu2+ when pH is varied. From the perspective of toxicity modeling, this observation is the first step toward considering the use of the cell surface as the algal biotic ligand for Cu in a similar way as fish gills fulfill this role in the biotic ligand model for predicting metal toxicity to fish species.

Comparison of heavy metal accumulation by a bloom-forming cyanobacterium, Microcystis aeruginosa

“…Metal toxicity to the cells was time-dependent at higher metal concentrations, which might be attributed to the enhanced intracellular metal concentrations. Previous studies demonstrated that the intracellular metal concentration can be used to predict the metal toxicity to the algal cells [23].…”

Cadmium and Chromium Toxicity to Pseudokirchneriella subcapitata and Microcystis aeruginosa

“…Metals may associate with some ecosystem components resulting in metal complexes with inorganic or organic materials. Considering the amplitude of possibilities for the metal association in the environment, metals bioavailability may vary with environmental conditions and organisms (Lombardi et al 2002;De Schamphelaere et al 2005;Töpperwien et al 2007). Metals such as cadmium and chromium are often present in industrial wastewaters.…”

Copper and Cadmium Toxicity to Marine Phytoplankton, <i>Chaetoceros gracilis</i> and <i>Isochrysis</i> sp.

“…Binding of Cu to the biotic ligand, as yet unknown in algae but assumed to be on the plasma membrane [32]. Growth inhibition in microalgae has also been related to intracellular Cu concentration [7,33].…”