Production of dissolved organic carbon enriched in deoxy sugars representing an additional sink for biological C drawdown in the Amazon River plume

Abstract

AbstractIn North Atlantic waters impacted by discharges from the Amazon and Orinoco Rivers, where planktonic diatom‐diazotroph associations (DDA) were active, we observed that an average (± standard deviation) of 61 ± 12% of the biological drawdown of dissolved inorganic carbon (DIC) was partitioned into the accumulating total organic carbon pool, representing a flux of up to 9 ± 4 Tg C yr−1. This drawdown corresponded with chemical alteration of ultrafiltered dissolved organic matter (UDOM), including increases in stable C isotopic composition (δ13C) and C:N. The dissolved carbohydrate component of UDOM also increased with biological DIC drawdown and diatom‐associated diazotroph (i.e., Richelia) abundance. New carbohydrates could be distinguished by distinctively high relative abundances of deoxy sugars (up to 55% of monosaccharides), which may promote aggregate formation and enhance vertical carbon export. The identified production of non‐Redfieldian, C‐enriched UDOM thus suggests a mechanism to explain enhanced C sequestration associated with DDA N2 fixation, which may be widespread in mesohaline environments.