Manganese uptake and Mn(II) oxidation by the alga Scenedesmus subspicatus

To examine the relationships among manganese concentration in the culture m edium, algae growth, manganese uptake and Mn(II) oxidation on the algal sur face, we exposed the unicellular alga Scenedesmus subspicatus to a broad ra nge of free Mn2+ ion concentrations. Extra- and intracellular manganese con centrations were distinguished by reducing the Mn oxides with ascorbate. A large fraction of the Mn bound by the algae occurred as reducible Mn(III/IV ) oxides formed by Mn(II) oxidation, presumably on the algal surface, altho ugh these algal cells of small size are not likely to build up a large pH g radient at their surface. Limitation of growth occurred when [Mn2+] was low er than about 1 x 10(-9) M, which corresponded to an intracellular content of 10(-7) mol Mn (g algae dry weight)(-1). Inhibition of growth occurred at high [Mn2+], at which high concentrations of Ma (>1 x 10(-5) Mol Mn (g alg ae dry weight)(-1)) were present both intracellularly and as extracellular Mn oxides. Phytoplankton collected in various lakes contained intracellular Mn and extracellular Mn oxides in similar concentrations as the Scenedesmu s cultures.