Major biochemical composition of dissolved high molecular weight organic matter in seawater

Abstract

Abstract Ultrafiltered dissolved organic matter (UDOM) was isolated from surface, oxygen minimum, and deep waters of three ocean basins and its elemental as well as molecular-level aldose and amino acid compositions were determined. Surface concentrations account for 23–33% of total dissolved organic carbon, and are a factor of 2–3 greater than those in deeper waters. Oceanic UDOM has an extremely characteristic organic composition, clearly distinct from other marine materials such as fresh plankton, sinking particles or humic substances. Polysaccharides appear to be the major reactive component of UDOM. They have a distinctive aldose distribution rich in galactose and deoxy sugar that is almost ubiquitous regardless of depth or location, suggesting that UDOM carbohydrate is dominated by a very similar suite of polysaccharide throughout the ocean. In contrast, amino acids account for a relatively minor component 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. In O2 minimum and deep ocean 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 unreactive throughout the subsurface ocean. Taken together with large compositional differences between UDOM and sinking particles, this observation suggests that dynamic aggregation is probably not an important formation or removal process for UDOM in the deep ocean. Amino acid and especially carbohydrate concentrations are lower in deep UDOM, but the overall molecular-level compositions remain similar to those from surface waters. This molecular-level homogeneity suggests that the UDOM biopolymers reflected in amino acid and carbohydrate data persist relatively unaltered in the deep ocean.