| 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 (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.
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