ALUMINUM BIOAVAILABILITY TO THE GREEN-ALGA CHLORELLA-PYRENOIDOSA IN ACIDIFIED SYNTHETIC SOFT-WATER

A unicellular green alga, Chlorella pyrenoidosa, was exposed to inorga nic Al under controlled experimental conditions to determine whether t he biological response elicited by the dissolved metal could be predic ted from the free-metal ion concentration, [Al3+]. The experimental ap proach involved concurrent measurement of both Al speciation and Al bi oavailability (bioaccumulation/growth inhibition) in buffered syntheti c solutions of defined composition. The bioassay exposure media, conta ining 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 hig hly pH dependent; [Al3+] needed to inhibit growth by 30% (EC30) increa ses markedly from 3 mug L-1 (0.1 muM) to 50 mug L-1 (1.8 muM) as the p H 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 A l3+ for binding sites at the algal surface; the effects of acidificati on at the biological surface are much more important than are its effe cts on Al speciation in solution. For systems containing both monomeri c and polynuclear Al (the AlO4Al12(OH)247+ polycation), polymeric alum inum also contributes to aluminum toxicity.