Role of coccoliths in the utilization of inorganic carbon by a marine unicellular coccolithophorid, Emiliania huxleyi: A survey using intact cells and protoplasts

The utilization of inorganic carbon and role of the coccoliths were investigated in intact cells and protoplasts of a marine unicellular calcareous alga, Emiliania huxleyl. 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 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 K-m and V-max. From the kinetic analyses, K-m and V-max [mu moles O-2 (ml PCV)(-1) h(-1)] were deduced to be 92 mu M 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 C-14-DIC and the incorporation of C-14 into acid-stable products of photosynthesis and the internal pool of DIC, determined as acid-labile compounds, under CO2-limiting conditions (80 mu M) were very similar for intact cells and protoplasts. However, incorporation of C-14 into CaCO3 apparently occurred more slowly in protoplasts than in intact cells, (3) In long term (24-h) experiments, patterns of incorporation of C-14 were almost same for intact cells and protoplasts, with the exception that the amount of C-14 incorporated into CaCO3 was much smaller in the former than the latter, The production of (CaCO3)-C-14 increased during the course of 10 h after a 4-h lag, However, after 10 h the level of (CaCO3)-C-14 started to decrease, The decrease was accompanied by an increase in C-14 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.