Availability of different phosphorus forms in agricultural soil to Microcystis aeruginosa.

We investigated the availability of different forms of particulate soil phosphorus (P) to Microcystis aeruginosa by sequential extraction and bioassay. We cultured M. aeruginosa in media containing, as the sole source of P, soils sequentially extracted with 1 M NH4Cl, 0.11 M bicarbonate dithionite, 1 M NaOH, and 0.5 M HCl. Analyses of chlorophyll-a, particulate organic carbon, and particulate organic nitrogen showed that M. aeruginosa could utilize some of the P remaining in the soil after each extraction. Alkaline phosphatase (AP) assays of sequentially extracted soils showed distinct patterns that depended on the type of co-cultured soil. A direct relationship between cellular P concentrations and the level of alkaline phosphatase activity was observed in only some media, an indication that not all forms of P were equally suitable substrates for AP hydrolysis. These results imply that cyanobacterial-available P included not only HCl-extractable P, which is assumed to consist of carbonate or apatite bound-P and organic P, but also refractory P, which has been considered to be unavailable to algae. Both HCl-extracted P and refractory P enhance the production of chlorophyll a, but did not lead to the storage of P by M. aeruginosa.