IMMUNOCHEMICAL CHARACTERIZATION OF EUKARYOTIC ULTRAPLANKTON FROM THE ATLANTIC AND PACIFIC OCEANS

The eukaryotic algae are an important component of the ultraplankton ( <5 mum diameter cells) and contribute substantially to the photosynthe tic biomass of the oceans. Because of their small size, individual spe cies cannot be easily distinguished by traditional or epifluorescence microscopy. To examine the composition of the eukaryotic ultraplankton assemblage, immunofluorescence probes produced to strains thought to be representative of the ultraplankton (Emiliania huxleyi clone BT-6; Pycnococcus provasolii clone OMEGA48-23; Pelagococcus subviridis clone PELA CL2; Thalassiosira oceanica clone 13-1; unidentified chlorophyte clone B6125) were used to identify and enumerate individual cell type s in samples from the North Atlantic (Gulf of Maine and adjacent slope ) and the subtropical North Pacific (Station ALOHA and Kaneohe Bay). E miliania huxleyi was the most frequently recognized cell type at all s ample locations throughout the euphotic zone, varying from <1 to 7% of the total eukaryotic algae. Counts include both lith-bearing and nake d forms, so are the first recorded total counts for E.huxleyi. Pycnoco ccus provasolii was also observed at all sampling locations, although it appeared to be more important at offshore stations than coastal or Kaneohe Bay. In surface waters, where the prasinophyte marker pigment prasinoxanthin is below the level of detection by HPLC analysis (e.g. station ALOHA), an immunofluorescence assay provides an alternative me ans to quantify this cell type. Pelagococcus subviridis was observed t hroughout the Gulf of Maine and at Station ALOHA, but was rarer (gener ally much less than 1% of total counts), and it was absent or below th e limit of detection at Kaneohe Bay. Thalassiosira oceanica was also r are in the Gulf of Maine, where it occurred mainly in the mixed layer. The chlorophyte B6125, a subtropical isolate, was more abundant in Ka neohe Bay than in the Gulf of Maine. In all, only a small proportion o f the total eukaryotic algae (of which >60% were cells <3 mum diameter ) could be accounted for by immunofluorescently labeled cells, which s uggests the presence of numerous other species and a diverse assemblag e. Moreover, the presence of cell types such as E.huxleyi in a variety of geographic regions demonstrates the cosmopolitan nature of these u ltraplankton species.