Photo-Transformation of Dissolve Organic Nitrogen in Coastal Regions

Dissolved organic nitrogen (DON) plays an important role in the nitrogen biogeochemical cycle in the coastal regions as it represents the major fraction of total dissolved nitrogen (TDN). Photo-oxidation if one of the major degradation pathways of DON. Photo-ammonification via DON degradation can take place throughout the euphotic zone of the water column in coastal and marine waters and could link DON to significant NH₄⁺sources for primary production. Despite this link between DON and the production of NH₄⁺fueling primary production, the chemical degradation mechanism of DON and its organic photoproducts are often overlooked in studies of the nitrogen budget in coastal areas. In this study, we conducted laboratory photo-oxidation experiments on surface water collected from six stations along a semi-arid Bay (Baffin Bay, Texas). We measured the photoproduction of ammonium and changes in the DON chemical composition using an Ultra Performance Liquid Chromatography Orbitrap Fusion Tribrid Mass Spectrometer (UPLC-OT-FTMS) with electrospray ionization in both the positive and negative mode. We observed a continuous production of NH4⁺ concentration (up to 8.6 ppm) during the photo-oxidation experiments. Photo-production of ammonia was mainly linked to photo-hydrolysis of deamination peptides into free amino acids and small organic acids. The biogeochemical implications of this DON photo-degradation pathway will be discussed.