Carbohydrates in phytoplankton and freshly produced dissolved organic matter

Four taxonomically-diverse, phytoplankton cultures (Phaeocystis sp., Emiliania huxleyi, Synechococcus bacillaris, Skeletonema costatum) were grown in batch culture for 14 days, and the particulate and high-molecular-weight dissolved components of the cultures were harvested by tangential-flow ultrafiltration for bulk and molecular-level carbohydrate analyses. Bulk carbohydrates and neutral aldoses accounted for an average of 37% and 20%, respectively, of the particulate organic carbon (>0.1 μm) in the cultures. Glucose was the dominant aldose in phytoplankton cellular material. Ultrafiltered dissolved organic matter (UDOM; >1000 Da) from the cultures was rich in carbohydrates relative to cellular material. Bulk carbohydrates and neutral aldoses accounted for an average of 66% and 35%, respectively, of UDOM in the cultures. The average C/N value (21.6) for UDOM was much higher than the value (8.8) for cellular material, reflecting the carbohydrate-rich nature of UDOM. Freshly produced UDOM was characterized by similar contributions of several aldoses, including galactose, glucose, mannose, fucose, xylose, and arabinose. The aldose signatures of phytoplankton UDOM were distinct from the signatures for cellular material and were indicative of heteropolysaccharides. The UDOM produced in phytoplankton cultures was similar in nature to the UDOM isolated from various locations in the surface ocean. Surface ocean UDOM is rich in carbohydrates and is relatively depleted in nitrogen (C/N 17), and the aldose signature of marine UDOM is similar to that of phytoplankton UDOM. These observations indicate that phytoplankton extracellular releases could be a major source of the dissolved heteropolysaccharides observed throughout the surface ocean.