Amino acid carbon isotopic fractionation patterns in oceanic dissolved organic matter: an unaltered photoautotrophic source for dissolved organic nitrogen in the ocean?

The transfer of dissolved organic carbon (DOC) and nitrogen (DON) out of the surface ocean where it is produced to storage in the oceanʹs interior creates one of the largest reservoirs of reduced carbon and organic nitrogen on earth. In nutrient-depleted surface waters of the oligotrophic ocean, dissolved nitrogenous material is of key importance as a source of fixed nitrogen for heterotrophic organisms. Recent work has increasingly indicated that, contrary to previous ideas, recalcitrant chemical structure is not the central factor underlying the preservation of DOC and DON, leaving the major preservation mechanisms largely unknown. We employ here a stable isotopic approach to examine the metabolic source and transformation signatures imprinted in carbon isotopic fractionation patterns of amino acids, which are the major components of both particulate and dissolved organic nitrogen that can be identified at the molecular level. Compound-specific isotopic signatures from central Pacific particulate and dissolved organic matter indicate a profound difference in processing histories between these two material pools. Sinking particles show a clear imprint of heterotrophic resynthesis and alteration, while the much larger and older dissolved pool retains an unaltered signature of photoautotrophic synthesis, even in samples from the abyssal ocean. In addition, δ13C signatures of enantiomers of alanine (D vs. L) in dissolved materials are indistinguishable. This isotopic data, in light of previously observed abundant D-amino acids in oceanic DOM, suggests that autotrophic prokaryotes may be a main source for dissolved nitrogenous material preserved over long time scales in the sea. Taken together, our results suggest that dissolved organic nitrogen preservation is not predominantly linked to heterotrophic reworking and resynthesis, but instead there exists a non-discriminating and rapid shunt, effectively removing recently formed autotrophic biomolecules from further recycling.