PHOSPHORUS CYCLING AND ALGAL-BACTERIAL COMPETITION IN SANDSFJORD, WESTERN NORWAY

Microbial phosphorus-cycling, with particular emphasis on algal/bacterial competition, was studied in depth profiles through the halocline separating a brackish top layer, rich in nitrate and poor in phosphate, from the underlying coastal water, poor in both nitrate and phosphate, in the Sandsfjord area, western Norway. At 2 stations along the axis of natural freshwater outflow, physiological P-deficiency of algae and bacteria in the brackish layer was inferred from rapid luxury consumption of added PO43- by organisms in size fractions > 1 mum and 1-0.2 mum, respectively. In a branch of the fjord without natural freshwater outlets, luxury consumption in the brackish layer was less, and without a clear difference between the 2 water layers. High luxury consumption coincided with short (< 30 min) turnover times for PO43- and strong bacterial dominance of (PO43-)-P-32-uptake, suggesting bacterial superiority as competitors during P-limiting conditions. Estimation of P-specific maximum uptake rate and affinity for PO43--uptake from isotope dilution experiments indicated bacterial superiority at low and algal superiority at high concentrations. Although most of the P-32 hydrolyzed from added (gamma-ATP)-P-32 was initially liberated as free (PO4)-P-32 partitioning of incorporated P-32 between size fractions > 1 mum and 1-0.2 mum was found to depend on whether label was added as (gamma-AVP)-P-32 or as (PO43-)-P-32, indicating that coupling of uptake to hydrolysis by cell-bound enzymes could modify the outcome of algal-bacterial phosphorus competition. Disappearance rate of P-32 from the 1-0.2 mum size fraction following initial labeling and a subsequent cold chase with PO43- was used to estimate the flow-rate of phosphorus through the microbial food web. Combined with measured kinetic constants for PO43- uptake, alkaline phosphatases and 5'nucleotidases, a coherent flow-scheme could only be obtained assuming very low (< 1 nmol l-1) concentrations of PO43- and nucleotides. Chemically measured concentrations of dissolved organic phosphorus (DOP) more than 2 orders of magnitude above the estimated nucleotide level and with an estimated turnover time of ca 500 h, are consistent with the view that this large P-reservoir consists mainly of slowly hydrolyzable polymers.