Size-fractionated dissolved primary production and carbohydrate composition of the coccolithophore Emiliania huxleyi

Extracellular release (ER) by phytoplankton is the major source of fresh dissolved organic carbon (DOC) in marine ecosystems and accompanies primary production during all growth phases. Little is known, so far, on size and composition of released molecules, and to which extent ER occurs passively, by leakage, or actively, by exudation. Here, we report on ER by the widespread and bloom-forming coccolithophore Emiliania huxleyi grown under steady state conditions in phosphorus controlled chemostats (N : P = 29, growth rate of μ = 0.2 d−1). 14C incubations were accomplished to determine primary production (PP), comprised by particulate (PO14C) and dissolved organic carbon (DO14C), and the concentration and composition of particulate combined carbohydrates (pCCHO), and of high molecular weight (>1 kDa, HMW) dissolved combined carbohydrates (dCCHO) as major components of ER. Information on size distribution of ER products was obtained by investigating distinct size classes ( 10 kDa was significantly different with higher Mol% of arabinose. Mol% of acidic sugars increased and Mol% glucose decreased with increasing size of HMW-dCCHO. We conclude that larger polysaccharides follow different production and release pathways than smaller molecules, potentially serving distinct ecological and biogeochemical functions.