l Vol. 20: 207-220. 1985 MARINE ECOLOGY - PROGRESS SERIES 1 Published J a n u q 2 l Mar. Ecol. Prog. Ser. 1 Arctic marine photoautotrophic picoplankton John Clegg Smith, Trevor Platt, W. K. W. Li, E. P. W. Horne, W. G. Harrison, D. V. Subba Rao and B. D. Irwin Marine Ecology Laboratory. Bedford Institute of Oceanography, P. 0. Box 1006, Dartmouth Nova Scotia B2Y 4.42. Canada ABSTRACT: 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 mac- romolecules. INTRODUCTION It has become increasingly apparent that very small organisms (less than 1 pm in diameter) constitute an important fraction of the photoautotrophic biomass of the sea. Holmes and Anderson (1963) demonstrated for Puget Sound waters that about 35 % of the photoauto- trophically fixed 14C passed a 0.45 pm pore diameter Millipore filter and was retained by a 0.22 pm Milli- pore filter while Saijo (1964), also using Millipore filters, showed that up to 37 % of the 14C fixed in light bottles in the Indian Ocean was associated with parti- cles between 0.8 and 0.45 pm in diameter. The extent to which this production might be due either to the retention of cell fragments by the smaller filters or to the heterotrophic uptake of radioactive algal exudates was unclear, however. Berman (1975) reported for the Gulf of California that about 25 % of the post-fraction- ated chlorophyll and apparent light-dependent 14C fixing activity passed a 1 pm Nuclepore filter, but concluded, based on a personal communication from Azam, that this was due to chloroplast fragments incapable of photosynthesis. Similarly, Harrison et al. (1977) showed for a small flagellate dominated community in Saanich Inlet that 18 % of the chlorophyll and 25 % of the photoassimi- lated 14C passed a l pm Nuclepore filter. Subse- quently, Azam and Hodson (1977) found for samples off Scripps pier and the California Bight that from 10 to 40 % of the chlorophyll in whole seawater samples passed into l pm Nuclepore filtrates and that, on enrichment, these filtrates produced 2 to 3 pm micro- flagellates which were filterable through 1 Km Nucle- pore screens but which were not observable in glutaraldehyde-fixed 1 pm filtrates. Azam and Hodson (1977) also showed that these l pm-filterable organ- isms photoassimilated 14C02 at rates correlated with the chlorophyll content of the filtrate. Larsson and Hagstrom (1982) documented for the northern Baltic Sea that the biomass of cells < 2 Fm in diameter could account for as much as 25 % of the total phytoplankton biomass, but they attributed about half of the 14C assimilation in this fraction to heterotrophic re-fixation of algal exudates. The size fraction between 2.0 and 0.2 pm diameter has been called 'picoplankton' (Sieburth et al., 1978) and direct microscopical observations show that amongst the constituents are chroococcacean cyano- bacteria (Johnson and Sieburth, 1979; Waterbury et al., 1979) and prasinophytes (Johnson and Sieburth, 1982). These findings are supported by an analysis of pico- plankton pigments (Yentsch, 1983). We have previ- ously shown for the eastern tropical Pacific Ocean (Li et al., 1983) and sub-tropical Atlantic Ocean (Platt et al., 1983), that up to 90 % of the chlorophyll biomass, up to 80 % of the photoautotrophic inorganic carbon O Inter-Research/Printed in F. R. Germany 0171-8630/85/0020/0207/$ 02.50