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 th at there are regions where it is predominantly phosphorus which determines the extent of microbial growth in drinking waters. Even a very low concentr ation of phosphorus (below 1 mu g Of P liter(-1)) can promote extensive mic robial growth, We present here a new sensitive method to determine microbia lly available phosphorus concentrations in water down to 0.08 mu g of P lit er-l. The method is a bioassay in which the analysis of phosphorus in a wat er sample is based on maximum growth of Pseudomonas fluorescens P17 when th e 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 +/- 9,40 0 CFU/mu g of PO4-P, A linear relationship was found between cell growth an d phosphorus concentration between 0,05 to 10 mu g of PO4-P Liter(-1). The content of microbially available phosphorus in Finnish drinking waters vari ed from 0.1 to 10.2 pg of P liter(-1) (median, 0.60 pg of P liter(-1)).