Calcification and utilization of inorganic carbon by the coccolithophorid <i>Emiliania huxleyi</i> Lohmann

Abstract

summary The relationship between inorganic‐carbon dependent photosynthetic oxygen evolution and calcification was investigated in high‐ and low‐calcifying strains of Emiliania huxleyi showing a ten‐fold difference in calcification rate. Unlike the low‐calcifying strain calcifying cultures showed a four‐fold increase in inorganic carbon (1 mM) dependent photosynthetic oxygen over the pH range 5‐8.3 resulting in a 20 fold difference in photosynthetic rate between the two strains at pH 8.3. Calcifying cells have a high affinity for HCO 3 − , the concentration of dissolved inorganic carbon [DIC] required for half‐maximal rate of photosynthetic O 2 evolution ( K 0.5 [DIC]) being 200 μM at pH 8‐3. In mid‐exponential phase cultures the stoichiometry between 14 CO 2 fixation and calcification was 1 : 1 so it is likely that the high photosynthetic rate at pH 8.3 is sustained by 14 CO 2 , released from H 14 CO 2 during calcification. Measurement of bicarbonate transport by the silicone‐oil‐layer centrifugal filtering technique demonstrated a rapid uptake and achievement of equilibrium (less than 3 s) between the intracellular and external inorganic carbon concentrations in low and high‐calcifying cells. Subsequent metabolism of the 14 C intracellular inorganic carbon pool did not occur in low‐calcifying cells suggesting the block in calcification occurs either in transport into or within the coccolith vesicle.