'&3!JQrs -Amino m$ a ~h upfake by a natural population of Osij&toria rubescens in relation to uptake by bacterioplankton

Size fractionation and electron microscopy autoradiography were used to de- termine if, in the eutrophic Lake Nantua, the dominant species Oscillutoriu rubescens D.C. can.compete successfully with bacteria for the free amino acids. Samples were col- lected at the depth of maximal Oscillutoriu population which was lying at 10 meters just below.the thermocline and were incubated with ''Camino acid mixture at in situ tem- përature in the dark. The size fractionation process have permit to obtain larger frac- tions that were mainly algae in which O. rubescens accounted for 97% of the total algal biovolume, therefore the radioactivity incorporated in these fractions was mostly at- tributable to Oscillutoriu. Electron microscopy autoradiography of ultra-thin sections of O. rube" demonstrated their intra-cellular label and confirm the ability of O. n4bes- cens to utilize amino acids at natural concentrations and to compete with bacteria for this uptake. The significance orarnino acid uptake for O. mbescens is discu4sed. '