Lead Toxicity and Phosphate Deficiency in Chlamydomonas

The addition of lead salts to phosphate-containing Chlamydomonas reinhardtii media caused precipitation of Pb(3)(PO(4))(2), effectively removing phosphate from solution. The effect of Pb(2+) on growth of Chlamydomonas in liquid cultures depended strictly on the ratio of the equivalents of Pb(2+) to phosphate present. When the amount of Pb(2+) approached equivalency with phosphate, cell growth was initially slow as cells adhered to the surface of the precipitated Pb(3)(PO(4))(2). Later, cells grew at a normal rate, spread throughout the solution, and reached the same densities obtained in the absence of Pb(2+). Cells did not survive when the amount of Pb(2+) in the culture exceeded the equivalents of phosphate.Elemental analysis showed that in the presence of equivalent Pb(2+) and phosphate, considerable Pb(2+) remained in solution. The concentration of dissolved Pb(2+) did not vary significantly when the amount of Pb(2+) added to the culture was increased slightly, from an amount which permitted growth to an amount which completely prevented growth. The concentration of phosphate was decreased to an undetectable level when the amount of Pb(2+) approached equivalency with phosphate.In the presence of the chelating agent nitrilotriacetic acid, higher concentrations of Pb(2+) remained in phosphate-containing media. The chelated Pb(2+) did not retard the growth of Chlamydomonas.It appears that Pb(2+) is not toxic to Chlamydomonas, but kills cells by depriving them of phosphate.