Ecological role of nanozooplankton in the heterotrophic cycle in the Ocean

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Role of nanozooplankton was investigated in relation to the heterotrophic cycle of organic matter in the Ocean. Nanozooplankters have been hard to find by light microscopy because of its small size and colourlessness. Electron microscopy has been a useful method for taxonomy of these organisms, but is time consuming and is not acculate in quantitative estimation. In the present project, a fluorescent dye staining epifluorescence microscopy of phyto-, zoo-, bactrioplankton was developed. this method was easy and rapid for sample preparation on board and was proved most effective for both taxonomic and quantitative observations of these organisms.In the Antarctic Ocean, the abundance of the nanozooplankton was varied from 0% to 25% (7% on average) by cell volume in total plankton, including phytoplankton. Organic analyses and electron microscopic examination on fecal pellets produced by the Antarctic krill, Euphausia superba, the key species of the Antarctic ecosystems, showed that the krill fed on choanoflagellates, the abundant member of the nanozooplankton in the Antarctic Ocean. Thus, a new pathway of organic materials in the Antarctic ecosystem is proposed; a new food chain including nonliving particulate and dissolved organic materials and bacteria-choanoflagellates-krill-vertebrates, as well as traditional inorganic nutrients-diatoms-krill-vertibrates food chain.Nanozooplankton, especially choanoflagellates and amoebae, were also common and abundant in the northern North pacific and the Bering Sea.It was concluded in the present project that these nanozooplankters play an important role as an intermdeiate link which couples nonliving-bacteria level with net-zooplankton-vertibrate level in the marine food chain.