Photo-oxidation facilitating the preservation of dissolved organic nitrogen in the ocean

The high molecular weight dissolved organic nitrogen (DON) can persist into the deep ocean, and most of the deep-sea DON and sedimentary nitrogen are preserved as proteinaceous materials, despite the parent source of native proteins being considered labile. How the proteinaceous materials become more refractory during the transformation processes, and how the DON is largely preserved are not fully understood. Based on our results, we propose that photo-oxidation processes can induce cross-linking and change the refractory nature of proteinaceous materials, resulting in the preservation of DON. We conducted photo-oxidation experiments on several protein model compounds in artificial seawater to study the mechanisms leading to refractory DON. After irradiation, the byproducts were assessed for molecular weight using SDS-PAGE, and the refractory nature of proteinaceous materials was assessed by measuring their proteolysis rates with proteases. The SDS-PAGE results showed the formation of higher molecular weight compounds (>100kDa) through chemical cross-linking during irradiation. In addition, with increasing extent of oxidative modification, some proteins became more labile to proteolysis but ultimately became more refractory, and the refractory nature was closely correlated to extent of cross-linking. Our results are a first step to explain for the transformation and preservation of proteinaceous materials in the ocean.