A new sensitive bioassay for determination of microbially available phosphorus in water.

The content of assimilable organic carbon has been proposed to control the growth of microbes in drinking water. However, recent results have shown that there are regions where it is predominantly phosphorus which determines the extent of microbial growth in drinking waters. Even a very low concentration of phosphorus (below 1 &mgr;g of P liter-1) can promote extensive microbial growth. We present here a new sensitive method to determine microbially available phosphorus concentrations in water down to 0.08 &mgr;g of P liter-1. The method is a bioassay in which the analysis of phosphorus in a water sample is based on maximum growth of Pseudomonas fluorescens P17 when the energy supply and inorganic nutrients, with the exception of phosphorus, do not limit bacterial growth. Maximum growth (CFU) in the water sample is related to the concentration of phosphorus with the factor 373,200 +/- 9,400 CFU/&mgr;g of PO4-P. A linear relationship was found between cell growth and phosphorus concentration between 0.05 to 10 &mgr;g of PO4-P liter-1. The content of microbially available phosphorus in Finnish drinking waters varied from 0.1 to 10.2 &mgr;g of P liter-1 (median, 0.60 &mgr;g of P liter-1).