Photo-oxidation of proteins facilitates the preservation of high molecular weight dissolved organic nitrogen in the ocean

High molecular weight (HMW) dissolved organic nitrogen (DON), especially proteinaceous materials, can persist in the deep ocean and sediment. How these materials, despite the parent source being considered labile, are transformed to be resistant to biodegradation is not yet fully understood. By conducting photo-oxidation experiments on model proteins (fetuin, cytochrome c, bovine serum albumin, and lectin) in artificial seawater, our results revealed that photo-oxidation can induce cross-linking of proteins, resulting in a change in the refractory nature of proteins and aggregation. The formation of higher molecular weight products compared to the native proteins, with simultaneously increased carbonyl content, was observed by gel electrophoresis (SDS-PAGE). The model proteins ultimately became more resistant to proteolysis. The addition of reactive oxygen species (i.e. H2O2 and center dot OH) accelerated the observed transformations. This study demonstrates that photo-oxidation can transform labile proteinaceous materials into refractory matter, providing a novel mechanism for the preservation of HMW DON in the ocean.