ELECTRON-MICROSCOPY OF NANOPLANKTON FROM THE NORTH PACIFIC CENTRAL GYRE

Nanoplankton was collected from the North Pacific Central Gyre during a cruise of R/V "Melville" in August 1985. Water samples from the surface and the deep chlorophyll maximum layer were fixed with glutaraldehyde and prepared for electron microscopy observation using formvar coated grids. The grids were examined first by epifluorescence microscopy under blue light to differentiate chloroplast and non-chloroplast containing cells. The surface and deep samples were dominated strongly by members of the Prymnesiophyceae, accounting for 55% of the total number of organisms identified. Prymnesiales and Coccosphaerales were equally abundant although coccolith-bearing cells tended to dominate in the deep chlorophyll maximum layer. Heterotrophic choanoflagellates were also abundant at depth and were characterized by a high species diversity. Micromonas pusilla (Butcher) Manton & Parke was the major representative of the Prasinophyceae and was observed commonly within the deep chlorophyll maximum. Our observations reveal several new species, including autotrophic and heterotrophic specimens, and demonstrate the importance of knowing the structure of the planktonic community for ecological purposes. Indeed, the presence of a large number of heterotrophic organisms in the deep water suggests an active microbial food chain which may play an important role in regulating plankton processes in oligotrophic waters.