The Effect of pH on the Uptake and Toxicity of Copper and Zinc in a Tropical Freshwater Alga (Chlorella sp.)

Abstract

Copper and zinc toxicity to the freshwater alga Chlorella sp. was determined at a range of pH values (5.5-8.0) in a synthetic softwater (hardness 40-48 mg CaCO(3)/L). The effects of the metals on algal growth (cell division) rate were determined after 48-h exposure at pH 5.5, 6.0, 6.5, 7.0, 7.5, and 8.0. The toxicity of both metals was pH dependent. As pH decreased from 8.0 to 5.5, the copper concentration required to inhibit the algal growth rate by 50% (IC50) increased from 1.0 to 19 microg/L. For zinc, the IC50 increased from 52 to 2,700 microg/L over the same pH range. Changes in solution speciation alone did not explain the increased toxicity observed as the pH increased. Modelled Cu(2+) and Zn(2+) concentrations decreased with increasing pH, whereas toxicity was observed to increase. Measurements of extracellular (cell-bound) metal concentrations support the biotic ligand model (BLM) theory of competition between protons (H(+)) and metals for binding sites at the algal cell surface. Higher extracellular metal concentrations were observed at high pH, indicating reduced competition. Independent of pH, both extracellular and intracellular copper were directly related to growth inhibition in Chlorella sp., whereas zinc toxicity was related to cell-bound zinc only. These findings suggest that the algal cell surface may be considered as the biotic ligand in further development of a chronic BLM with microalgae. Conditional binding constants (log K) were determined experimentally (using measured intracellular metal concentrations) and theoretically (using concentration-response curves) for copper and zinc for Chlorella sp. at selected pH values. Excellent agreement was found indicating the possibility of using concentration-response data to estimate conditional metal-cell binding constants.

Effect of Copper on the Toxicity and Genotoxicity of Cadmium in Duckweed (Lemna Minor L.)

“…The biological effects of individual metals are more or less known, but even though combinations of heavy metals are common in nature their combined effects are still to be thoroughly investigated (1,2). Metals in mixtures may act independently or interact to produce additive, synergistic, or antagonistic effects, which depends on their concentrations and factors such as temperature, pH, and light (2). In general, these interaction effects do not fi t the toxicological profi les of individual metals making a combination.…”

“…Cadmium (Cd) is one of the most toxic metals, and is also genotoxic for most plant and animal organisms (1)(2)(3)(4). It is released into the environment through industrial wastewater and other waste.…”

Comparison of Cd(II), Cu(II), and Pb(II) Biouptake by Green Algae in the Presence of Humic Acid

“…The obtained results contrasted to a single uptake system with KS,Cu ) 10 6.9 M -1 obtained from Cu toxicity studies with the tropical freshwater alga Chlorella sp. (35).…”

Toxicological Responses ofChlorella vulgaristo Dichloromethane and Dichloroethane

“…Dichloromethane and dichloroethane not only disrupt the growth of bacteria and plants, but also show broad toxicity in mammals. Traditionally, the acute toxic effects of organic solvent pollutants on freshwater algae are assessed using single test compounds (Wilde et al, 2006). However, natural pools of water are usually polluted by a mixture of substances that could act synergistically (Shuhaimi-Othman and Pascoe, 2007), and this led us to test the toxic interaction of dichloromethane and dichloroethane.…”

“…Traditionally, most of our knowledge regarding the toxicity of organic solvent pollutants to freshwater algae has been based on the effects of a single tested compound on such biochemical parameters as median effective concentration (EC 50 ), chlorophyll levels, biovolume, cell count, and average cell size (Wilde et al, 2006). However, in sites polluted with solvents, two or more organic solvents are often found together in the environment.…”