ACCUMULATION AND UTILIZATION OF DISSOLVED INORGANIC CARBON BY A MARINE UNICELLULAR COCCOLITHOPHORID, EMILIANIA-HUXLEYI

The mechanism for utilization of dissolved inorganic carbon (DIC) was investigated in the marine unicellular calcareous alga Emiliania huxle yi, grown with constant aeration. The apparent K-0.5 (DIC), the concen tration of DIC which attains one-half of the maximum velocity of appar ent photosynthesis, for photosynthetic evolution of O-2, measured unde r saturating light, was 5.5 mM (55 mu M for CO2) at pH 8.0 and 25 degr ees C. The value of K-0.5 was not affected by inhibitors of carbonic a nhydrase (CA), and an electrometric assay of CA showed that the enzyme was not involved in photosynthesis in this alga. The rate of photosyn thetic fixation of C-14-DIC into acid-stable products was about 20 tim es higher than that into CaCO3, irrespective of the external concentra tion of DIC. In short-term experiments, C-14-DIC was usually incorpora ted into the internal pool of DIC (IIC) to concentrations up to 13 to 16 times higher than that of the external DIC. CO2 added externally wa s utilized mainly for fixation of CO2 and accumulation of IIC. By cont rast, HCO3- was utilized mainly for production of CaCO3 and accumulati on of IIC. Incorporation of C-14 into IIC was partially suppressed by DCMU or in darkness but its transfer to CaCO3 was unaffected. These re sults suggest that accumulation of IIC in this alga, even under ordina ry circumstances, is only partially responsible for increasing the eff iciency of utilization of DIC by photosynthetic fixation but may be mo st useful for the production of CaCO3. (Hydroxyethylidene) bisphosphon ic acid, an inhibitor of the growth of CaCO3 crystals, completely supp ressed production of CaCO3. The accumulation of IIC was also partially suppressed, but photosynthetic fixation of CO2 was enhanced. In a pul se-chase experiment with C-14-DIC, C-14 incorporated into IIC and CaCO 3 in darkness was transferred to acid-stable products of photosynthesi s in the light. These results suggest that C-14-DIC in IIC and pre-for med CaCO3 may be useful sources of carbon for fixation of CO2.