caa a22 4500 465810888 CHVBK 20180323112116.0 cr unu---uuuuu 170327e19901201xx s 000 0 eng 10.1007/BF02123506 doi (NATIONALLICENCE)springer-10.1007/BF02123506 Kanda Jota Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, 164, Tokyo, Japan aut Studies of nitrogen assimilation by marine phytoplankton and its implication in the nitrogen cycling in the sea [Elektronische Daten] Lecture by the member awarded the Okada Prize of the Oceanographical Society of Japan for 1990 [Jota Kanda] I present here a review of my work concerning nitrogen assimilation by marine phytoplankton. This opportunity was provided to me as the recipient of the Okada Prize for 1990 from the Oceanographical Society of Japan. Assimilation of nitrogenous nutrients by phytoplankton has received considerable research effort since it is an essential process in organic matter production in the sea surface. The use of15N technique is necessary for tracing nitrogen assimilation by natural marine phytoplankton, but nitrogen metabolism of heterogenous natural populations significantly complicates flow of isotope. Dilution of15N isotope by heterotrophic regeneration of ammonium causes underestimates of uptake rates. I made an evaluation of isotope dilution effects in available data sets of15N-ammonium uptake experiments in literature. Incorporated15N in particulates might revert back to dissolved organic or inorganic nitrogen. I conducted pulse-chase experiments which can quantify such loss of tracer. From these studies, a short term experiment with sufficient amount of tracer enrichment is found to overwhelm these problems. In such an experiment, however, the elevation of nutrient concentration by tracer addition may likely perturb the uptake process. An initial rapid uptake is expected if the population is nitrogen deficient, but I found that this phenomenon is not common to surface oligotrophic open oceans. Uptake rate from such an experiment, or capacity of nitrogen uptake, was obtained using surface waters from an extended area in the North Pacific, and its regional variability was discussed. In addition to overall15N uptake, time series analysis of intracellular15N partitioning between hot ethanol soluble and insoluble fractions was found to be useful. When15N-ammonium is added to nitrogen deficient cells of phytoplankton,15N is accumulated in the ethanol soluble fraction. Using cultured strains of marine phytoplankton, this accumulation was proved to be caused by the difference of rates of nitrogen uptake and nitrogenous macromolecule synthesis. Uptake rate per cell is relatively constant irrespective of nutritional status, but macromolecule synthesis decreases with nitrogen deficiency. This accumulation of15N in the ethanol soluble fraction was used as an index of nutritional status with respect to nitrogen of the natural populations of phytoplankton from the western North Pacific. The uptake capacity of nitrate was observed to be higher than that of ammonium in the regional upwelling around Izu Islands and during the spring bloom in Alaskan coastal water. The15N partitioning technique revealed that nitrate taken up was rapidly incorporated in the macromolecule fraction. This suggests that ammonium uptake is suppressed to be smaller than intracellular nitrogen assimilation, rather than that nitrate is taken up in excess and accumulates within the cell. Regulation of nitrate uptake by light intensity was also discussed in detail for the Alaskan data. Several other studies currently conducted are also mentioned. Oceanographical Society of Japan, 1989 Journal of the Oceanographical Society of Japan Kluwer Academic Publishers 46/6(1990-12-01), 314-327 0029-8131 46:6<314 1990 46 10872 https://doi.org/10.1007/BF02123506 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF02123506 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- Kanda Jota Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, 164, Tokyo, Japan aut NATIONALLICENCE 773 0- Journal of the Oceanographical Society of Japan Kluwer Academic Publishers 46/6(1990-12-01), 314-327 0029-8131 46:6<314 1990 46 10872 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer