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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Abstract

The utilization of inorganic carbon and role of the coccoliths were investigated in intact crib 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 O2 at various concentrations of externally added NaHCO3 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 infact cells and protoplasts. The CO2-utilization reaction was, therefore, considered to involve two processes with different values of Km and Vmax. From the kinetic analysis, Km and Vmax [μmoles O2 (ml PVC)-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 incorporatioa 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 Ca14CO3 started to decrease. The decrease was accompanied by an increase ia 14C in the products of photosynthesis, suggesting thai 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.

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Sekino, K., Kobayashi, H., & Shiraiwa, Y. (1996). Role of coccoliths in the utilization of inorganic carbon by a marine unicellular coccolithophorid, Emiliania huxleyi: A survey using intact cells and protoplasts. Plant and Cell Physiology, 37(2), 123–127. https://doi.org/10.1093/oxfordjournals.pcp.a028921

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