As packed in intact fecal pellets of the surface-feeding zooplankton (Angel,1984) or in amorphous aggregates (marine snow), probably because of their seeding strategy at depleted nutrient levels in the surface layer (Smetacek, 1985), diatoms could sink to the deep ocean with a sinking rate of 100 m d 1 or more (Alldredge and Gotschalk, 1989). In such cases, the silicic frustule of diatoms played an important role as ballast for downward export of particle organic carbon (POC). A relatively low ratio of POC export to primary production was found in most oceans, except where or when large phytoplankton, particularly diatoms, dominated the plankton community structure, highlighting the importance of diatoms in the marine biological pump (Ducklow et al., 2001).