Origin and biogeochemical cycling of organic nitrogen in the eastern Arctic Ocean as evident from D- and L-amino acids

The chemical structure of organic nitrogen and the mechanisms of its cycling in the oceans still remain elemental questions in contemporary marine sciences. The Arctic Ocean provides a model system for studying the fate of terrigenous compounds in the ocean. We chemically characterised and traced the discharge of dissolved organic nitrogen (DON) and its particulate counterpart (PON) from the Russian rivers into the central Arctic Ocean, We focused on the D- and L-enantiomers of amino acids, the principal organic nitrogen compounds of living biomass. Total dissolved and particulate hydrolysable amino acids (TDAA, PAA) exhibited highest concentrations in the rivers (TDAA: 3.2 muM; PAA: 5.0 muM on average), contributing similar to 40% to DON and similar to 60% to PON. In the Arctic Ocean, TDAA and PAA decreased to concentrations of <1 muM, accounting only for similar to 10% of DON but similar to 80% of PON. Dominant amino acids in TDAA were glycine and alanine (in the rivers, 35% of TDAA; in deepwater, 49%) followed by aspartic: acid, glutamic acid, and serine. Threonine was also abundant in the rivers, and leucine in deep seawater. Microbial-derived D-enantiomers of aspartic acid, glutamic acid, serine, and alanine were found in significant amounts in all river and seawater samples, both dissolved and suspended. In riverine TDAA D-aspartic acid was most abundant (21% of total aspartic acid); in deep seawater D-alanine predominated (44% of total alanine). The proportions of all D-enantiomers were significantly higher in oceanic versus riverine TDAA and increased with depth in the Arctic Ocean. PAA exhibited much lower proportions Of D-enantiomers than TDAA (generally < 10% of the respective amino acid). This first direct and complete quantification Of D-amino acids dissolved and suspended in seawater provides molecular evidence for microbial contribution to marine organic nitrogen. Particulate D-amino acids, present even offshore in the euphotic zone, indicated microbial biomass and fast turnover of decaying phytoplankton. However, recognisable microbial-derived compounds contributed only a minor fraction to marine DON. The amino acid signature of DON can be explained largely by conservative mixing of recalcitrant compounds of terrestrial (soil) and marine origin. They behaved biogeochemically stable in the brackish mixing zone of the estuaries and in the Arctic Ocean over years to decades. The high amino acid content and the low D-enantiomer proportion of soil-derived DON indicate that terrestrial diagenesis is much more efficient than marine diagenesis in protecting amino acids from bacterial degradation. The huge amounts of dissolved organic nitrogen transported by Siberian rivers into the Arctic Ocean therefore do not substantially support the productivity of the Arctic Ocean. Copyright (C) 2001 Elsevier Science Ltd.