Aluminium effects on Scenedesmus obtusiusculus with different phosphorus status. I. Mineral uptake

Abstract

Synchronously growing cultures of the unicellular green alga Scenedesmus obtusiusculus were cultivated for 24 and 72 h in the presence or absence of phosphorus. Aluminium chloride (37, 74, 111, 148, 185, or 222 μmol) was added daily to 1 l cell suspension at the end of the cell division phase. As AlCl3 decreases the pH of the growth medium, controls were run in media with low pH in the absence of AlCl3. Samples for analysis of the internal (net uptake) and external (bound to cell surface) levels of Al, Mg, P, Ca, and Fe were taken every second hour during a 24 h period or once after 72 h.The investigation shows that the intracellular aluminium in Scenedesmus affects the nutrient status of the cells. A high intracellular level of Al is in consort with an enhancement of the intracellular fractions of Mg, P, Ca and Fe. The increase in net uptake of the minerals measured in the presence of Al is not due to an Al‐induced lowering of the pH, caused by Al. The concentrations of Al, Mg, P, Ca and Fe in the cells are generally lower during the dark period of the cell cycle, when the cells are dividing, than during the light period. A peak in mineral concentration of the cells could be monitored in the middle of the 24 h life cycle of the cells. The intracellular Al level is higher when the growth medium is low in P than in phosphorus‐rich medium, due to precipitation of aluminium‐phosphate both in medium and at cell surfaces. The extracellular Al and P fractions are thus higher in the presence than in the absence of P. The highest Al content monitored in the cells is about 100 nmol Al (106 cells)−1. A large fraction of Al initially taken up after addition to the medium is subsequently released from the cells during the 24 h cell cycle. The results are interpreted as Al effects on the plasma membrane, thus indirectly affecting various mechanisms for ion transport across the membrane. There are also indications that a surface covered with aluminium‐phosphate, formed at high P level in the medium, may prevent ion uptake.