Silicon, aluminium and the biological availability of phosphorus in algae

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Abstract
Silicon is an essential element for growth in diatoms. It is, for example, used to build the silicieous frustule surrounding the diatom cell wall. Silicon starvation studies have also inferred that silicon is essential in the vital metabolic processes of DNA and chlorophyll synthesis. The mechanism of essentiality is, however, uncertain. In laboratory cultures at circumneutral pH and using a non-growth-limiting, but environmentally realistic, concentration of the essential nutrient phosphorus, we have demonstrated aluminium toxicity in the freshwater diatom Navicula pelliculosa and the amelioration of this toxicity with silicon, present in aqueous solution as silicic acid. The mechanism of aluminium toxicity was an aluminium-induced reduction in the biologically available phosphorus fraction and silicic acid protected against this effect by preferentially binding aluminium in competition with phosphorus. Silicon-stimulated growth in the presence of aluminium was also demonstrated in the non-silicon-dependent green alga, Chlorella vulgaris, in which, once again, the increase in growth rate upon the addition of silicon could be directly correlated with an increase in the biologically available phosphorus fraction. We contend that a possible hitherto unrecognized mechanism of silicon essentiality in biology is to increase the biological availability of phosphorus in the presence of aluminium.