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 cyclin g in the oceans still remain elemental questions in contemporary marine sci ences. The Arctic Ocean provides a model system for studying the fate of te rrigenous compounds in the ocean. We chemically characterised and traced th e discharge of dissolved organic nitrogen (DON) and its particulate counter part (PON) from the Russian rivers into the central Arctic Ocean, We focuse d on the D- and L-enantiomers of amino acids, the principal organic nitroge n compounds of living biomass. Total dissolved and particulate hydrolysable amino acids (TDAA, PAA) exhibited highest concentrations in the rivers (TD AA: 3.2 muM; PAA: 5.0 muM on average), contributing similar to 40% to DON a nd similar to 60% to PON. In the Arctic Ocean, TDAA and PAA decreased to co ncentrations of <1 muM, accounting only for similar to 10% of DON but simil ar 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, gl utamic acid, and serine. Threonine was also abundant in the rivers, and leu cine in deep seawater. Microbial-derived D-enantiomers of aspartic acid, gl utamic acid, serine, and alanine were found in significant amounts in all r iver and seawater samples, both dissolved and suspended. In riverine TDAA D -aspartic acid was most abundant (21% of total aspartic acid); in deep seaw ater 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 proport ions Of D-enantiomers than TDAA (generally < 10% of the respective amino ac id). This first direct and complete quantification Of D-amino acids dissolved an d suspended in seawater provides molecular evidence for microbial contribut ion to marine organic nitrogen. Particulate D-amino acids, present even off shore in the euphotic zone, indicated microbial biomass and fast turnover o f 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 comp ounds of terrestrial (soil) and marine origin. They behaved biogeochemicall y stable in the brackish mixing zone of the estuaries and in the Arctic Oce an over years to decades. The high amino acid content and the low D-enantio mer proportion of soil-derived DON indicate that terrestrial diagenesis is much more efficient than marine diagenesis in protecting amino acids from b acterial degradation. The huge amounts of dissolved organic nitrogen transp orted by Siberian rivers into the Arctic Ocean therefore do not substantial ly support the productivity of the Arctic Ocean. Copyright (C) 2001 Elsevie r Science Ltd.