Alkaline phosphatase activity in two planktonic desmid species and the possible role of an extracellular envelope

Cosmarium abbreviatum var. planctonicum , a desmid from oligo‐mesotrophic lakes, had a higher maximum alkaline phosphatase activity (APA) and affinity constant under both continuous and pulsed inorganic phosphorus (P i ) limitation than Staurastrum chaetoceras , a desmid predominantly encountered in eutrophic lakes. 2. APA of both species increased when measured in starved cells subjected to pulsed P i conditions when compared to continuous P i limitation. 3. The portion of extracellular relative to cellular APA was higher in S. chaetoceras than in C. abbreviatum , indicating that S. chaetoceras secreted the enzymes more readily into its environment. 4. The difference in APA could explain the dominance of C. abbreviatum during competition between these two species under conditions of continuous organic P (P o ) limitation, but not the outcome under a pulsed P o shortage. The dominance of S. chaetoceras in the latter experiment can, however, be explained by species‐specific P i ‐uptake characteristics. 5. After a saturating pulse of P i , no increase in P i in the extracellular mucus layer of C. abbreviatum was found and it was therefore concluded that the mucilage sheath does not store P. However, the sheath could have a main function as an accumulation site of cellular APA, providing the cell with P i .