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Plant and Cell Physiology, 1996, Vol. 37, No. 2 123-127
© 1996

Role of Coccoliths in the Utilization of Inorganic Carbon by a Marine Unicellular Coccolithophorid, Emiliania huxleyi: a Survey Using Intact Cells and Protoplasts

Keisuke Sekino, Hiroshi Kobayashi and Yoshihiro Shiraiwa1

Department of Biology, Faculty of Science, Niigata University Ikarashi 2-8050, Niigata, 950-21 Japan

1Corresponding should be addressed to Y. Shiraiwa.

The utilization of inorganic carbon and role of the coccoliths were investigated in intact cells and protoplasts of a marine unicellular calcareous alga, Emiliania huxleyi. Protoplasts with high photosynthetic activity were obtained by artificial decalcification with 50 mM MES-NaOH (pH5.5). (1) The kinetics of the photosynthetic evolution of O2 at various concentrations of externally added NaHCO3 were the same for intact cells and protoplasts, indicating that the kinetic properties with respect to dissolved inorganic carbon (DIC) were not affected by the presence or absence of the coccoliths on the cell surface. Double-reciprocal plots and plots of the concentration of substrate divided by velocity (s/v) against the concentration of substrate (s) were biphasic in the case of both intact cells and protoplasts. The CO2-utilization reaction was, therefore, considered to involve two processes with different values of Km and Vmax. From the kinetic analyses, Km and Vmax [µmoles O2 (ml PCV)–1 h–1] were deduced to be 92 µM and 76.3 for a "low-Km" reaction and 4.1 mM and 252 for a "high-Km" reaction, respectively. (2) In short-term (40-min) experiments, time courses of the total uptake of 14C-DIC and the incorporation of 14C into acid-stable products of photosynthesis and the internal pool of DIC, determined as acid-labile compounds, under CO2-limiting conditions (80 µM) were very similar for intact cells and protoplasts. However, incorporation of 14C into CaCO3 apparently occurred more slowly in protoplasts than in intact cells. (3) In long term (24-h) experiments, patterns of incorporation of 14C were almost same for intact cells and protoplasts, with the exception that the amount of 14C incorporated into CaCO3 was much smaller in the former than the latter. The production of Ca14CO3 increased during the course of 10 h after a 4-h lag. However, after 10 h the level of Ca14CCO3 started to decrease. The decrease was accompanied by an increase in 14C in the products of photosynthesis, suggesting that CaCO3 was reutilized for the photosynthetic fixation of CO2 and, therefore, that the coccoliths function as sites of storage of DIC. However, the internal level of DIC remained at the same level even after the supply of external DIC has been almost completely depleted.

(Received July 25, 1995; Accepted December 11, 1995)
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