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

The free-ion model of trace metal interactions with aquatic microorgan isms states that the biological response to a metal is proportional to the activity of the free-ion {M(z+)} in solution. The applicability o f 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 aci d (SFA), was tested in defined media (pH 5) designed to limit Al inter actions to algae and SFA. Toxicity was not proportional to the activit y of Al3+, apparent failing of the free-ion model. fulvic acid adsorbe d to cell surfaces (17 mg m(-2), pH 5) and increased membrane permeabi lity (as measured with [C-14]sorbitol) whereas Al decreased membrane p ermeability. in addition, SFA may act as a source of phosphorus to P-d eficient 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 a lgae.