Orthophosphate Uptake Rate Constants are Mediated by the 103–104Molecular Weight Fraction in Shield Lake Waters

Abstract

Although midsummer chlorophyll concentrations can be predicted from total phosphorus, attempts to identify all bioavailable phosphorus compounds has proven difficult. Inconsistencies in determining colloidal phosphorus utilization and the discovery that orthophosphate concentration and flux are poor predictors of lake trophic status has led us to postulate that other forms of phosphorus compounds, which may not be necessarily labeled over the radioisotopic uptake assay (32P) period, are assimilated by natural phytoplankton assemblages. Using ultrafiltration methodology we test the hypothesis that the orthophosphate uptake rate constant can be physiologically varied upon changing the external concentration of selected molecular weight fractions in Shield lake waters. We determined that the fraction between molecular weight 103and 104significantly increased the orthophosphate uptake rate constant when exposed to the natural seston. Our discovery suggests that the orthophosphate uptake rate constant is subject to fluctuations depending on the availability of low molecular weight phosphorus metabolites in the external solution. The physiological basis of our observations may be a metabolic coupling between the activity of plasmalemma-bound phosphatases and the transport of PO43−across the cell membrane.