Carbon, nitrogen, and carbohydrate fluxes during the production of particulate and dissolved organic matter by marine phytoplankton

Although the principal source of marine organic matter is phytoplankton, experimental data on carbon and nitrogen mass balance during their growth cycle are lacking. Phytoplankton from diverse taxonomic groups (Synechococcus bacillaris, Phaeocystis sp., Emiliania huxleyi, Skeletonema costatum) were grown in synthetic seawater media, and changes in particulate and dissolved carbon, nitrogen, and carbohydrates were followed for 14 d. There was a close molar balance between dissolved inorganic carbon (DIC) uptake and total organic carbon (TOC) production in all phytoplankton except Emiliania, which synthesizes carbonate-containing coccoliths. Rates of dissolved organic carbon (DOC) production during phytoplankton growth ranged from 5 to 13 µM DOC d−1 (0.01–0.06 pmol DOC pM cell C−1 d−1) and constituted a substantial (10–32%) fraction of TOC production. The carbohydrate content of both the particulate and dissolved pools increased over the growth cycle and constituted 18–45% and 26–80% of TOC, respectively. The dissolved carbohydrate pool was predominantly composed of polysaccharides (70–94%). Despite some species-specific variability, phytoplankton cellular (particulate) and extracellular (dissolved) organic matter C: N ratios did not deviate far from Redfield values. However, phytoplankton synthesized compositionally distinct pools of high molecular weight dissolved organic matter (>1,000 Da, average C: N ratio ∼21) and low molecular weight dissolved organic matter (<1,000 Da, average C: N ratio ∼6.0).