Physiological regulation of coccolith polysaccharide production by phosphate availability in the coccolithophorid emiliania huxleyi

Abstract

Coccoliths of the coccolithophorid Emiliania huxleyi are calcified biomineral scales composed of calcium carbonate and coccolith polysaccharide (CPs). Coccolith production is regulated by inorganic phosphate (Pi) availability, but no information currently exists on how this process occurs. In this study CP was experimentally characterized by HPLC analysis as an acid polysaccharide of mannose, galacturonic acid, xylose and rhamnose. Both calcification (estimated from 45Ca uptake) and CP production (estimated from uronic acid quantification) were stimulated under Pi-deficient conditions but strongly suppressed under Pi-sufficient conditions. When cells were transferred from Pi-sufficient to Pi-deficient conditions the production of neutral polysaccharides (NP)storage glucansceased rapidly after a temporary increase in the presence of Pi, and CP production started to increase after Pi was almost depleted. Under Pi-sufficient conditions NP production increased, concomitant with stimulation of cell growth. Calcification increased gradually, but photosynthetic 14CO2 fixation was reduced by almost 40 for 5 d of culture during Pi depletion. [14C]CP production was maintained at almost constant, high levels under Pi-deficient conditions but gradually decreased under Pi-sufficient conditions in conjunction with cell growth. In contrast, [14C]NP production increased about 3-fold under Pi-sufficient conditions for 3 d. The present study indicates that E. huxleyi switches the direction of carbon flow toward CP and NP production under Pi-deficient and Pi-sufficient conditions, respectively.