Photosynthesis-induced phosphate precipitation in seawater: ecological implications for phytoplankton

A relationship between nutrient concentration, cell density and pH increase due to photosynthesis was established for 3 phytoplankton species, Prorocentrum minimum (Dinophyceae), Phaeodactylum tricornutum (Bacillariophyceae) and Tetraselmis sp. (Prasinophyceae), using batch cultures. Experiments were carried out to find out whether changes in pH induced by photosynthesis can cause precipitation of phosphate (PO4) in seawater. Nutrient concentrations similar to those of North Atlantic water induced phytoplankton growth, which in turn caused a pH increase of 0.75 U. At pH approximate to 9, up to 20% of the phosphate in our nutrient solutions precipitated. The precipitation was positively correlated with the carbonate concentration. At pH > 9.7, another precipitation reaction, not correlated with carbonate concentration, caused flocculation of particles and a highly efficient PO4 removal, presumably involving Mg(OH)(2), In contrast to the other 2 species that grew at pH <= 10.2, P. minimum only grew at pH <= 9.7, thus surviving high pH without itself causing catastrophic precipitation of phosphate. This could be one reason why P. minimum is able to bloom in eutrophic estuarine waters.