• Bicarbonate acquisition mechanisms and the kinetics of dissolved inorganic carbon (DIC) use in photosynthesis and calcification were investigated in Emiliania huxleyi. • Photosynthesis was measured using O2 evolution and 14 C incorporation and calcification was measured with 14 C. Noncalcifying (coccolith-free) cells were produced from calcifying (coccolith-bearing) cells of the same strain of E. huxleyi, so that photosynthesis could be monitored independently from calcification. • Neither photosynthesis nor calcification was saturated at the ambient DIC concentration of seawater. In coccolith-bearing cells, both processes showed biphasic kinetics with DIC concentration, with a hiatus located at 1 mM. The same biphasic pattern and similar rates of photosynthesis were found in the coccolith-free cells. Inhibitor experiments showed that E. huxleyi acquires bicarbonate mainly by an anion exchange protein, but external carbonic anhydrase can be activated at low concentrations of DIC. • We conclude that the biphasic kinetics of photosynthesis and calcification are caused by the presence of two bicarbonate acquisition mechanisms and also, since calcification does not enhance photosynthesis in this coccolithophore, we question the current view that the two processes are tightly coupled.
Keywords: Emiliania huxleyi; anion exchange protein; calcification; carbonic anhydrase; dissolved inorganic carbon; kinetics; photosynthesis.