Aluminum bioavailability to the green alga Chlorella pyrenoidosa in acidified synthetic soft water

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Abstract
A unicellular green alga, Chlorella pyrenoidosa, was exposed to inorganic Al under controlled experimental conditions to determine whether the biological response elicited by the dissolved metal could be predicted from the free‐metal ion concentration, [Al3+]. The experimental approach involved concurrent measurement of both Al speciation and Al bioavailability (bioaccumulation/growth inhibition) in buffered synthetic solutions of defined composition. The bioassay exposure media, containing neither phosphate nor trace metals, covered the pH range 4.3 to 6. For systems at a given pH, containing only inorganic monomeric Al, aluminum bioavailability varies predictably as a function of the free Al3+ concentration. However, the effect of Al3+ on algal growth is highly pH dependent; [Al3+] needed to inhibit growth by 30% (EC30) increases markedly from 3 μg L−1 (0.1 μM) to 50 μg L−1 (1.8 μM) as the pH is decreased from 6 to 5. This decreased toxicity of Al at low pH is partly explained by an effective competition between the H+ ion and Al3+ for binding sites at the algal surface; the effects of acidification at the biological surface are much more important than are its effects on Al speciation in solution. For systems containing both monomeric and polynuclear Al (the AlO4Al12(OH) polycation), polymeric aluminum also contributes to aluminum toxicity.