Extracellular release of primary production by Emiliana Huxleyi: insights to structure and function of dissolved combined carbohydrates

Extracellular release (ER) by phytoplankton is a major source of dissolved organic carbon (DOC) in marine ecosystems and accompanies primary production during all growth stages. Size and composition of released molecules, and the extent to which ER occurs passively, by leakage, or actively, by exudation is largely unknown. We report on ER by the widespread and bloom-forming coccolithophore Emiliania huxleyi grown in phosphorus controlled chemostats. 14C incubations were accomplished to determine primary production (PP) and the release of dissolved organic carbon (DO14C). High molecular weight dissolved combined carbohydrates (HMW-dCCHO) as major components of ER were determined using HPAEC-PAD. Fractionation of ER products was obtained for distinct size classes (<0.40 µm, <1000 kDa, <100 kDa and <10 kDa) of DO14C and HMW-dCCHO. Despite similar ER rates, composition of released carbohydrates varied largely between size fractions. We discuss pathways of algal carbohydrate release that could explain different molecular signatures, and how differences in ER composition may be related to distinct ecological and biogeochemical functions.