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.
