MAJOR BIOCHEMICAL-COMPOSITION OF DISSOLVED HIGH-MOLECULAR-WEIGHT ORGANIC-MATTER IN SEAWATER

Ultrafiltered dissolved organic matter (UDOM) was isolated from surfac e, oxygen minimum, and deep waters of three ocean basins and its eleme ntal as well as molecular-level aldose and amino acid compositions wer e determined. Surface concentrations account for 23-33% of total disso lved organic carbon, and are a factor of 2-3 greater than those in dee per waters. Oceanic UDOM has an extremely characteristic organic compo sition, clearly distinct from other marine materials such as fresh pla nkton, sinking particles or humic substances. Polysaccharides appear t o be the major reactive component of UDOM. They have a distinctive ald ose distribution rich in galactose and deoxy sugar that is almost ubiq uitous regardless of depth or location, suggesting that UDOM carbohydr ate is dominated by a very similar suite of polysaccharide throughout the ocean. In contrast, amino acids account for a relatively minor com ponent of both total UDOM and of its organic nitrogen component. Amino acid distributions are similar to those from unfractionated seawater, and are not preferentially remineralized.Tn O-2 minimum and deep ocea n water, ultrafiltered material accounts for 18-25% of total dissolved organic carbon. Compositions are nearly invariant in these subsurface isolates, suggesting that ultrafiltered material is stable and unreac tive throughout the subsurface ocean. Taken together with large compos itional differences between UDOM and sinking particles, this observati on suggests that dynamic aggregation is probably not an important form ation or removal process for UDOM in the deep ocean. Amino acid and es pecially carbohydrate concentrations are lower in deep UDOM, but the o verall molecular-level compositions remain similar to those from surfa ce waters. This molecular-level homogeneity suggests that the UDOM bio polymers reflected in amino acid and carbohydrate data persist relativ ely unaltered in the deep ocean.