Higher Levels of Soluble Reactive Phosphorus Promote Labile Organic Nitrogen Cycling in Subsurface Coastal Waters

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 N-15-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 mu mol 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.