ESTIMATING ORGANIC-P UTILIZATION BY FRESH-WATER PLANKTON USING (P-32)ATP

We examined the use of [P-32]ATP for the determination of dissolved organic phosphorus (DOP) utilization by freshwater plankton. Uptake of P-32 from ATP was rapid; plankton exhibited a higher affinity for the P-32 from ATP than for the previously used DOP analogue, methylfluorescein phosphate (MFP). The results suggested that phosphomonoesterase (PMEase) activity measured with [P-32]ATP was greater than alkaline phosphatase (AP) activity determined with MFP. Therefore, the PMEase-enzymes responsible for DOP hydrolysis in the P-limited system we examined may include both the frequently measured AP as well the recently described 5'-nucleotidase. Uptake of tracer amounts of P-32 from ATP into different size classes of plankton demonstrated that the bacterial size fraction (0.2-1-mu-m) dominates uptake, but this domination diminishes when DOP concentrations are elevated. [P-32]ATP is an effective and convenient substrate for tracing DOP dynamics in a manner comparable to using (PO43-)-P-32 to study PO43-dynamics.