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Abstract Marine dissolved organic carbon and nitrogen (DOC and DON) are major global carbon and nutrient reservoirs, and their characterization relies on extraction methods for preconcentration and salt removal. Existing methods optimize for capturing and describing DOC. Here, we report an optimized analytical strategy to recover marine DON for subsequent molecular characterization. Retention efficiencies between 5% and 95% are reported for seven solid phase extraction (SPE) sorbents, with PPL recovering 23% of marine DON compared to 95% recovered with C₁₈. Additional comparisons of the effect of varying sample volumes and elution speed, and the resulting molecular composition of DON extracts, were investigated using C₁₈and PPL sorbents. Sample volumes > 200 mL decreased DON retention efficiency independent of SPE sorbent, and gravity elution recovered 1.7‐ to 4.2‐fold more DON compared to vacuum elution. Characterization of extracted DON by negative‐ion electrospray ionization Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR MS) highlights compositional differences between DON species recovered by each method. DON isolated with optimized methods includes low molecular weight (< 600 Da) peptide‐like compounds with low O:C ratios (0.2 to 0.5) that are not detected by other SPE sorbents (e.g., PPL). The majority of additional DON isolated with this approach was undetectable by direct infusion negative mode FT‐ICR MS analysis. 

Because nitrogen availability limits primary production over much of the global ocean, understanding the controls on the marine nitrogen inventory and supply to the surface ocean is essential for understanding biological productivity and exchange of greenhouse gases with the atmosphere. Quantifying the ocean’s inputs, outputs, and internal cycling of nitrogen requires a variety of tools and approaches, including measurements of the nitrogen isotope ratio in organic and inorganic nitrogen species. The marine nitrogen cycle, which shapes nitrogen availability and speciation in the ocean, is linked to the elemental cycles of carbon, phosphorus, and trace elements. For example, the majority of nitrogen cycle oxidation and reduction reactions are mediated by enzymes that require trace metals for catalysis. Recent observations made through global-scale programs such as GEOTRACES have greatly expanded our knowledge of the marine nitrogen cycle. Though much work remains to be done, here we outline key advances in understanding the marine nitrogen cycle that have been achieved through these analyses, such as the distributions and rates of dinitrogen fixation, terrestrial nitrogen inputs, and nitrogen loss processes. 

The cyanobacterium  Trichodesmium  plays an essential role supporting ocean productivity by relieving nitrogen limitation via dinitrogen (N 2 ) fixation. The two common Trichodesmium clades,  T. erythraeum  and  T. thiebautii , are both observed in waters along the West Florida Shelf (WFS). We hypothesized that these taxa occupy distinct realized niches, where  T. thiebautii  is the more oceanic clade. Samples for DNA and water chemistry analyses were collected on three separate WFS expeditions (2015, 2018, and 2019) spanning multiple seasons; abundances of the single copy housekeeping gene  rnpB  from both clades were enumerated via quantitative PCR. We conducted a suite of statistical analyses to assess Trichodesmium  clade abundances in the context of the physicochemical data. We observed a consistent coastal vs. open ocean separation of the two clades:  T. erythraeum  was found in shallow waters where the concentrations of dissolved iron (dFe) and the groundwater tracer Ba were significantly higher, while  T. thiebautii  abundance was positively correlated with water column depth. The Loop Current intrusion in 2015 with entrained Missisippi River water brought higher dFe and elevated abundance of both clades offshore of the 50 m isobath, suggesting that both clades are subject to Fe limitation on the outer shelf. Whereas, previous work has observed that  T. thiebautii  is more abundant than  T. erythraeum  in open ocean surface waters, this is the first study to examine  Trichodesmium  niche differentiation in a coastal environment. Understanding the environmental niches of these two key taxa bears important implications for their contributions to global nitrogen and carbon cycling and their response to global climate change.