Physiological Regulation of Coccolith Polysaccharide Production by Phosphate Availability in the Coccolithophorid Emiliania huxleyi

摘要

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 (P_i) 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 P_i-deficient conditions but strongly suppressed under P_i-sufficient conditions. When cells were transferred from P_i-sufficient to P_i-deficient conditions the production of neutral polysaccharides (NP)—storage glucans—ceased rapidly after a temporary increase in the presence of P_i, and CP production started to increase after P_i was almost depleted. Under P_i-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 P_i depletion. [14C]CP production was maintained at almost constant, high levels under P_i-deficient conditions but gradually decreased under P_i-sufficient conditions in conjunction with cell growth. In contrast, [14C]NP production increased about 3-fold under P_i-sufficient conditions for 3 d. The present study indicates that E. huxleyi switches the direction of carbon flow toward CP and NP production under P_i-deficient and P_i-sufficient conditions, respectively.