ALKALINE-PHOSPHATASE ACTIVITY IN 2 PLANKTONIC DESMID SPECIES AND THE POSSIBLE ROLE OF AN EXTRACELLULAR ENVELOPE

1. Cosmarium abbreviatum var. planctonicum, a desmid from oligo-mesotr ophic lakes, had a higher maximum alkaline phosphatase activity (APA) and affinity constant under both continuous and pulsed inorganic phosp horus (Pi) limitation than Staurastrum chaetoceras, a desmid predomina ntly encountered in eutrophic lakes. 2. APA of both species increased when measured in starved cells subjected to pulsed Pi conditions when compared to continuous Pi limitation. 3. The portion of extracellular relative to cellular APA was higher in S, chaetoceras than in C, abbre viatum, indicating that S, chaetoceras secreted the enzymes more readi ly into its environment. 4. The difference in APA could explain the do minance of C. abbreviatum during competition between these two species under conditions of continuous organic P (P-0) limitation, but not th e outcome under a pulsed P, shortage. The dominance of S, chaetoceras in the latter experiment can, however, be explained by species-specifi c P-1-uptake characteristics. 5. After a saturating pulse of Pi, no in crease in Pi in the extracellular mucus layer of C, abbreviatum was fo und and it was therefore concluded that the mucilage sheath does not s tore P. However, the sheath could have a main function as an accumulat ion site of cellular APA, providing the cell with Pi.