UC Santa Cruz

Fixed nitrogen is the key limiting nutrient for primary production in much of the world ocean. While the concentration of dissolved inorganic nitrogen is below detection in open ocean subtropical gyres, micro-molar concentrations of dissolved organic nitrogen (DON) persist throughout the surface ocean. Similar to the broader dissolved organic matter (DOM) pool, the origin and molecular structure of this refractory material are largely unknown. The “microbial carbon pump” has emerged as a proposed framework for understanding microbial production of refractory DOM (R-DOM), but whether the same mechanisms also form R-DON remains unknown. Spectroscopy, radiocarbon (Δ14C), and concentration depth profiles of common amide-containing biomolecules together suggest that R-DON originates from proteinaceous material. Therefore, if an analogous microbial N pump exists, it most likely operates through microbial transformations of proteinaceous DON. Amino acids provide multiple potential tracers for this process, as the likely monomers contributing to amide R-DON material while their D-enantiomers (D-AAs) are unambiguous biomarkers for prokaryotic organisms.

Here, for the first time, we couple 14C ages, %D-AAs, and AA molar percent abundance (Mol%) data within DOM size-reactivity fractions, in order to assess the importance of bacterial sources in the formation of R-DON. We analyzed DOM isolates representing opposite ends of the size-reactivity continuum, from surface to deep ocean in the North Pacific Subtropical Gyre. Results from this first comparison of prokaryotic contributions to DON in isolated size-reactivity fractions strongly support the idea of a microbial N pump, with higher %D in R-DOM for almost all AAs. Quantification based on mass spectral data also allowed the unambiguous identification of three new D-AAs which had not previously been definitively identified in DOM, with oceanographically consistent depth profiles and relationships with Mol% and Δ14C, expanding the suite of environmentally relevant D-AAs from four to seven. Our results also suggest that several novel D-AA subgroupings could be developed as tracers: Mol% and %D trends with Δ14C for Alanine (Ala) suggest that D-Ala has independent cycling mechanisms from other D-AAs, and may be strongly linked to bacterial peptidoglycan sources and cycling, while D-Leucine and D-Valine have a particularly strong relationships with Δ14C, with a %D maximum in the twilight zone suggesting a link to sinking particles. Interestingly, traditional degradation indices based on Mol% changes appear to indicate bacterial inputs rather than age, though we show that consistent changes in Mol% are observed with age in both DOM size-reactivity fractions as well. Overall, our results are largely consistent with the dominant production of R-DON by prokaryotic organisms, and support a greater mechanistic understanding of degradation and recalcitrance through distinct relationships between %D, Mol%, and Δ14C for our expanded suite of environmentally relevant chiral AAs.