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

Microbial phosphorus-cycling, with particular emphasis on algal/bacter ial competition, was studied in depth profiles through the halocline s eparating 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 an d bacteria in the brackish layer was inferred from rapid luxury consum ption 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 witho ut a clear difference between the 2 water layers. High luxury consumpt ion coincided with short (< 30 min) turnover times for PO43- and stron g bacterial dominance of (PO43-)-P-32-uptake, suggesting bacterial sup eriority as competitors during P-limiting conditions. Estimation of P- specific maximum uptake rate and affinity for PO43--uptake from isotop e dilution experiments indicated bacterial superiority at low and alga l superiority at high concentrations. Although most of the P-32 hydrol yzed 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 (gamm a-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-bac terial 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 thro ugh the microbial food web. Combined with measured kinetic constants f or PO43- uptake, alkaline phosphatases and 5'nucleotidases, a coherent flow-scheme could only be obtained assuming very low (< 1 nmol l-1) c oncentrations of PO43- and nucleotides. Chemically measured concentrat ions of dissolved organic phosphorus (DOP) more than 2 orders of magni tude above the estimated nucleotide level and with an estimated turnov er time of ca 500 h, are consistent with the view that this large P-re servoir consists mainly of slowly hydrolyzable polymers.