The Formation of Refractory Dissolved Organic Nitrogen From Free Amino Acids in the Ocean

Dissolved organic nitrogen (DON) is actively involved in N cycling and transformation processes in the ocean, yet how this large N pool is formed remains elusive. Here we incubated N-15-labeled individual amino acids (Ala, Val, and Phe) in seawater and monitored their fates over 21 days. About 25%-45% of Phe-N and Val-N were transformed to "uncharacterizable DON," as compared to only about 6% for Ala-N, indicating the formation of refractory DON is related to specific amino acids. Through a stable isotope probing approach, 5 Phe-derived DON molecules from the incubations were found to be present in natural waters, and their possible structures were proposed. These results shed new lights on the formation mechanisms of refractory DON, including the roles of specific amino acids and particular chemical structures that may resist decomposition in millennial time scales. Plain Language Summary The formation of refractory dissolved organic nitrogen through microbial degradation was investigated through a series of lab incubations using amino acids as substrates. We found that a higher fraction of nitrogen (25%-45%) from phenylalanine and valine was transformed into refractory dissolved organic nitrogen, as compared to alanine (6%). Such contrasting results suggest that the formation of refractory dissolved organic nitrogen may be tightly connected with specific amino acids. We also constructed a database with possible structures of the amino acids-derived dissolved organic nitrogen molecules based on stable isotope and high-resolution mass spectrometry techniques. With the database, we further confirmed that at least five of amino acids-derived dissolved organic nitrogen molecules were present in natural waters (including the deep ocean). Overall, our study offered new insights into the formation mechanisms of refractory dissolved organic nitrogen, demonstrating the roles of specific amino acids and particular chemical structures in the long-term stability of dissolved organic nitrogen in the ocean.