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 photosynthetic biomass of the oceans. Because of their small size, individual species 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 types in samples from the North Atlantic (Gulf of Maine and adjacent slope) and the subtropical North Pacific (Station ALOHA and Kaneohe Bay). Emiliania huxleyi was the most frequently recognized cell type at all sample locations throughout the euphotic zone, varying from <1 to 7% of the total eukaryotic algae. Counts include both lith-bearing and naked forms, so are the first recorded total counts for E.huxleyi. Pycnococcus 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 means to quantify this cell type. Pelagococcus subviridis was observed throughout the Gulf of Maine and at Station ALOHA, but was rarer (generally much less than 1% of total counts), and it was absent or below the limit of detection at Kaneohe Bay. Thalassiosira oceanica was also rare in the Gulf of Maine, where it occurred mainly in the mixed layer. The chlorophyte B6125, a subtropical isolate, was more abundant in Kaneohe Bay than in the Gulf of Maine. In all, only a small proportion of the total eukaryotic algae (of which >60% were cells <3 mum diameter) could be accounted for by immunofluorescently labeled cells, which suggests the presence of numerous other species and a diverse assemblage. Moreover, the presence of cell types such as E.huxleyi in a variety of geographic regions demonstrates the cosmopolitan nature of these ultraplankton species.