K. Sekino et Y. Shiraiwa, ACCUMULATION AND UTILIZATION OF DISSOLVED INORGANIC CARBON BY A MARINE UNICELLULAR COCCOLITHOPHORID, EMILIANIA-HUXLEYI, Plant and Cell Physiology, 35(3), 1994, pp. 353-361
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.