Influences of Natural Dissolved Organic Matter on the Interaction of Aluminum with the Microalga Chlorella: A Test of the Free-Ion Model of Trace Metal Toxicity

Abstract

The free-ion model of trace metal interactions with aquatic microorganisms states that the biological response to a metal is proportional to the activity of the free-ion {M^z+} in solution. The applicability of the free-ion model, as it applied to the toxicity of aluminum to the green alga Chlorella pyrenoidosa in the presence of a soil fulvic acid (SFA), was tested in defined media (pH 5) designed to limit Al interactions to algae and SFA. Toxicity was not proportional to the activity of Al³⁺, an apparent failing of the free-ion model. Fulvic acid adsorbed to cell surfaces (17 mg m^-2, pH 5) and increased membrane permeability (as measured with [¹⁴C]sorbitol) whereas Al decreased membrane permeability. In addition, SFA may act as a source of phosphorus to P-deficient algae. These results emphasize the importance of considering not only the metal-complexing properties of natural dissolved organic matter but also its direct metabolic and physiological influences on algae.