| Abstrakt: |
Remineralization of labile organic nitrogen is a key part of nitrogen cycling in seawater, yet factors controlling this process remain poorly understood. The degradation and fate of amino acids in coastal waters were investigated with incubation experiments using 15N‐labeled alanine (Ala). The cycling of Ala, including the release of NH4+ through remineralization, the subsequent removal of NH4+ via assimilation and nitrification, and the formation of refractory dissolved organic nitrogen (DON), was more rapid in subsurface (mid‐depth 8 m and bottom 17 m) than in surface coastal waters (2 m) of the northern Gulf of Mexico in summer. This pattern is attributed to the different levels of soluble reactive phosphorus (SRP) observed at different depths of the water column. The addition of orthophosphate to the surface water with low SRP (<0.1 μmol L−1) significantly accelerated the remineralization of amino acid, nitrification, and the formation of refractory DON. The rapid remineralization of amino acid and further nitrification in SRP‐replete waters are linked to the rapid growth of heterotrophic bacteria and ammonia‐oxidizing archaea, respectively. Taken together, these results suggest that subsurface coastal waters enriched with SRP are pre‐primed for rapid organic N metabolisms, providing mechanistic insights into nitrogen cycling in marine waters. Plain Language Summary: In this study, we investigated the effect of phosphorus in the degradation of labile organic nitrogen in Gulf of Mexico waters. By using nitrogen stable isotope (15N) labeled amino acid, we were able to trace the fate of amino acid nitrogen at different depths of the water column. We found that subsurface water with a phosphorus concentration higher than 0.5 μmol L−1 had a more rapid decomposition rate of amino acid, with faster transformation of amino acid nitrogen to nitrate, and increased formation of relatively refractory dissolved organic nitrogen. This stimulating effect of phosphorus in subsurface water is achieved through interactions between phosphorus and the microbial community. Specifically, high concentrations of phosphorus (>0.5 μmol L−1) promote the development of ammonia‐oxidizing archaea, and therefore accelerate the transformation to nitrate. In addition, high phosphorus concentrations also enhance the growth of heterotrophic bacteria, which can convert labile amino acid to refractory bacteria‐derived organic nitrogen. Taken together, these results suggest that subsurface coastal waters, due to the high phosphorus level, are pre‐primed for rapid organic nitrogen metabolisms. Our study provides mechanistic insights into nitrogen cycling in surface and subsurface marine waters. Key Points: A high P level (≥0.5 μmol L−1) stimulates the degradation of labile organic nitrogenThe accelerated nitrification rates under high P level are attributed to the development of ammonia‐oxidizing archaeaThe formation of refractory dissolved organic nitrogen is related to fast‐growing heterotrophic bacteria [ABSTRACT FROM AUTHOR] |
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