DISCRIMINATION OF ALGAL AND BACTERIAL ALKALINE-PHOSPHATASES WITH A DIFFERENTIAL-INHIBITION TECHNIQUE

Both phytoplankton and bacterioplankton produce alkaline phosphatases, but the techniques currently available for discriminating between the two sources are poor, especially when samples are from turbid waters. A novel approach, based on the differential inhibition of alkaline ph osphatases by various physical and chemical treatments, was assessed a s a rapid and inexpensive technique for determining whether phytoplank ton or bacterioplankton were the more important producers of alkaline phosphatases in turbid rivers of south-eastern Australia. Eight phytop lankton strains and 14 bacterial strains (eight isolated from the Oven s River and six isolated as bacterial contaminants of the phytoplankto n cultures) were grown in laboratory culture. Rates of alkaline phosph atase activity in the bacterial cultures varied from <1 to 21 fmol cel l-1 day-1. Rates for phytoplankton were usually <5 mumol (mug chla)-1 day-1 but could reach 128 mumol (mug chla)-1 day-1, depending on wheth er inorganic or organic phosphorus was supplied. Differential-inhibiti on profiles were determined for the 22 isolates, using seven chemical treatments (L-cysteine, EDTA and L-levamisole, and Zn2+ and Cu2+ each at two concentrations) plus one physical treatment (thermal deactivati on). The alkaline phosphatases of the three microbial groups (i.e. Ove ns River bacteria, bacteria isolated as contaminants from algal cultur es, and phytoplankton) could be classified with a predictive accuracy of better than 90% when these data were analysed with Discriminant Fun ction Analysis. L-Cysteine, Zn2+ and Cu2+ were the best predictors of class membership; thermal deactivation and EDTA sometimes were also si gnificant. Inhibition profiles were then determined for the alkaline p hosphatases of various river-water samples. These environmental sample s usually (>70% of cases) grouped separately from those of the laborat ory cultures of bacteria and phytoplankton, perhaps because the microb es studied in laboratory culture were not representative of native ass emblages or because the culture conditions did not mimic those in natu re. Nevertheless, differential-inhibition techniques have much potenti al for determining the origin of the alkaline phosphatases found in na tural waters, with the major factor limiting their application being t he collection of valid inhibition profiles for native bacterial and al gal communities.