Arctic marine photoautotrophic picoplankton

Abundance and activity of picoplankton (here defined as cells passing a 1 pm Nucleporem screen) were studied in northern Foxe Basin, eastern Canadian Arctic. Substantial proportions (10 to 70 %) of the chlorophyll a content, ribulose-1,5-bisphosphate carboxylase activity (RuBPC, E.C. 4.1.1.39) and autofluorescent bodies present in whole seawater samples passed a 1 p screen in intact, photoautotrophic particles. A smaller fraction (10 to 25 %) of the light-dependent 14C fixation was found in this picoplankton fraction, the difference possibly explained by a selective effect of screening on photosynthetic activity rather than by heterotrophic uptake of algal exudates. About 10 % of the whole sample RuBPC was found to pass a 0.2 pm diameter screen, indicating the presence of autotrophy in marine ultramicrobacteria (ferntoplankton). A potential for growth in the ferntoplankton fraction was also indicated by substantial fixation of tritiated nucleic acid precursors into macromolecules.