A NEW PARADIGM FOR THE MARINE DISSOLVED ORGANIC NITROGEN RESERVOIR: REFRACTORY HETEROCYCLIC NITROGEN OF BACTERIAL ORIGIN?

Marine dissolved organic nitrogen (DON) is one of the largest standing reservoirs of fixed N.  However, most ocean DON appears resistant to biological utilization, representing a central control on N biogeochemical cycling in the modern ocean. However, the molecular composition of this refractory DON (RDON), and the reasons for its long-term persistence, have never been understood. Past characterization focused almost entirely on the high molecular weight fraction, which is now recognized as younger and more labile, consistent with its  amide functionality. In contrast, the vastly larger low molecular weight (LMW), RDON pool has received relatively little attention. Here we synthesize a suite of new results focused on the quantitatively dominant, refractory LMW DON pool, pointing toward a fundamentally new view of the ocean’s dominant DON composition. We show that solid-state ¹⁵N NMR using multi-cross polarization pulse sequences yields good quantitation, even for non-protonated N functionalities.  Data acquired with these sequences indicates that essentially the entire LMW  DON pool, representing most DON in the world ocean, is composed of previously unknown N heterocyclic material.   Amino acid biomarkers from the same samples indicate bacterial origin, and suggest that most RDON may be “preformed” as refractory N compounds in the surface ocean. Finally, these data suggest that most amino acids in RDON may not be proteinaceous, as has been long assumed, but could derive from unknown bacterial natural products.  

Presentation Preference: Oral

Primary Presenter: Matthew McCarthy, UC Santa Cruz (mdmccar@ucsc.edu)

Authors:

Hope Ianiri, US Geological Survey  (hianiri@usgs.gov)