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

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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 (pH 5.5). (1) The kinetics of the photosynthetic evolution of O_2 at various concentrations of externally added NaHCO_3 were the same for intact cells and protoplasts, in-dicating 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 CO_2-utilization reaction was, therefore, considered to involve two processes with different values of K_m and V_<max>. From the kinetic analyses, K_m and V_<max> [μmoles O_2 (ml PCV)^<-1> h^<-1>] were deduced to be 92 μM and 76.3 for a "low-K_m" reaction and 4.1 mM and 252 for a "high-K_m" reaction, respectively. (2) In short-term (40-min) experiments, time courses of the total uptake of ^<14>C-DIC and the incorporation of ^<14>C into acid-stable products of photosynthesis and the internal pool of DIC, determined as acid-labile compounds, under CO_2-limiting conditions (80 μM) were very similar for intact cells and protoplasts. However, incorporation of ^<14>C into CaCO_3 apparently occurred more slowly in protoplasts than in intact cells. (3) In long term (24-h) experiments, patterns of incorporation of ^<14>C were almost same for intact cells and protoplasts, with the exception that the amount of ^<14>C incorporated into CaCO_3 was much smaller in the former than the latter. The production of Ca^<14>CO_3 increased during the course of 10 h after a 4-h lag. However, after 10 h the level of Ca^<14>CO_3 started to decrease. The decrease was accompanied by an in-crease in ^<14>C in the products of photosynthesis, suggesting that CaCO_3 was reutilized for the photosynthetic fixation of CO_2 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.
日本植物生理学会の論文

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

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