GROWTH KINETIC-PARAMETERS OF 2 PLANKTONIC DESMID SPECIES UNDER FLUCTUATING PHOSPHORUS CONDITIONS IN CONTINUOUS-FLOW CULTURE

Two planktonic desmid taxa, Staurastrum chaetoceras and Cosmarium abbr eviatum var. planctonicum, were examined under pulsed phosphorus (P)-l imited conditions in continuous culture. Two pulse regimes were applie d, i.e. 2.5 mu M P twice a week and 10 mu M P once every 2 weeks. Unde r both pulse regimes, S.chaetoceras showed a higher maximum P uptake r ate (V-max) and affinity for P uptake (V-max/affinity constant K-m) th an C.abbreviatum. Affinity for P uptake in S.chaetoceras just before p ulsing was higher than shortly after pulsing, whereas C.abbreviatum di d not show any significant difference. Cell densities and cellular P q uota in S.chaetoceras and C.abbreviatum fluctuated in a comparable way , but fluctuations in C.abbreviatum were less pronounced than in S.cha etoceras. The mean cell volume of both species was greater under the b imonthly than under the biweekly pulse regime, and the lowest under co ntinuous P-limited conditions. Competition between S.chaetoceras and C .abbreviatum under the bimonthly pulse regime was won by S.chaetoceras , but displacement was less fast than under more frequent pulsing (equ alling the same total P supply). The above-mentioned results are discu ssed in relation to possible ecological strategies.