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10 pages, 3 figures, 4 tables.-- This paper is EHUX contribution Nº 11., The patterns of carbon incorporation into proteins, polysaccharides, lipids and low molecular weight metabolites and the resulting cellular biochemical composition were examined within cultured and natural populations of the coccolithophore Emiliania huxleyi. The flows of carbon incorporated through photosynthesis were primarily directed towards the synthesis of lipids (40 to 60%) mainly neutral lipids, whereas relative carbon incorporation into proteins was low (ca 20%), regardless of irradiance levels or growth stage. Actively dividing E. huxleyi cells showed higher rates of carbon incorporation into protein during darkness than during the previous light period whereas under energy limited growth conditions proteins produced during the light period were catabolized in darkness. The observed 14C labelling patterns were consistent with both the measured biochemical composition of E. huxleyi cells growing in cultures under the same conditions and with that of coccolithophoredominated natural phytoplankton assemblages. The relative decrease in cellular density caused by a lipid-rich composition could be significant for the buoyancy of cells and, consequently, for the dynamics of blooming events. Furthermore, the high-lipid, low-protein metabolism characteristic of this species implies an increase of carbon uptake relative to nitrogen that would, to some extent, counteract the enhancing effect of calcification on the partial pressure of CO2 typical of blooms of E. huxleyi., This work was partly funded by the European Commission under contract EHUX MAS-CT92-0038. W.M.B. was supported by grants from NASA (BAGW-2426), ONR (N00014-91-J-1048) and NSF (OCE-9022227). The work of E.F was funded by a postdoctoral fellowship from the Spanish Ministry of Science and Education and by a grant from ONR (N00014-91-J-1048).