The northern slope water of the South China Sea (SCS) is oligotrophic. The regenerated nitrogen from dissolved organic nitrogen (DON) has been suggested to play a pivotal role in sustaining primary productivity; however, DON cycling has not been investigated there. Dissolved free amino acids (DFAAs) and polyamines (DFPAs) are two major groups of labile DON that are used rapidly by heterotrophic bacteria, and important in nitrogen cycling. Here, we measured turnover rates and concentrations of representative DFAAs (arginine and glutamic acid) and DFPAs (putrescine) in waters of the northern slope of the SCS, to estimate uptake rates and evaluate their potentials to fulfill carbon and nitrogen demands of bacterial production. Furthermore, homologs of genes encoding transporter systems for arginine (aotJ) and putrescine (potD/potF) were quantified in metagenomes from four depths at two sampling stations. The taxa encoding these genes were identified to gain insight into the composition of microbial communities potentially utilizing DFAAs and DFPAs. We found that uptake rates of two DFAAs and putrescine were more rapid than measured previously in seawater with similar environmental characteristics. The uptake of arginine and glutamic acids was estimated to account for 2.4-51% (19 ± 14%) and 2.7-59% (24 ± 20%) of bacterial carbon and nitrogen demand, respectively, and putrescine uptake contributed 4.4-100% (27 ± 33%) and 13-293% (80 ± 96%), respectively. Spatial variation in uptake rates indicated that biogeochemical cycling of DFAAs was distinct from that of DFPAs. Redundancy analysis and metagenomes demonstrated that environmental variables and distinction in bacterial assembleges using DFAAs and DFPAs could both affect their dynamics in the northern slope water of the SCS.

南海北坡水域（SCS）是贫营养的。来自溶解有机氮 (DON) 的再生氮已被建议在维持初级生产力方面发挥关键作用；然而，DON 循环尚未在那里进行研究。溶解游离氨基酸 (DFAA) 和多胺 (DFPA) 是不稳定 DON 的两大类，它们被异养细菌迅速利用，并且在氮循环中很重要。在这里，我们测量了 SCS 北坡水域中代表性 DFAA（精氨酸和谷氨酸）和 DFPA（腐胺）的周转率和浓度，以估计吸收率并评估它们满足细菌生产碳和氮需求的潜力。此外，编码精氨酸转运系统的基因的同源物（aotJ) 和腐胺 ( potD / potF)) 在两个采样站的四个深度的宏基因组中被量化。鉴定了编码这些基因的分类群，以深入了解可能利用 DFAA 和 DFPA 的微生物群落的组成。我们发现两种 DFAA 和腐胺的吸收率比之前在具有相似环境特征的海水中测量的要快。精氨酸和谷氨酸的吸收估计分别占细菌碳和氮需求的 2.4-51% (19 ± 14%) 和 2.7-59% (24 ± 20%)，腐胺吸收贡献了 4.4-100% (27 ± 33%) 和 13-293% (80 ± 96%)，分别。吸收率的空间变化表明 DFAAs 的生物地球化学循环与 DFPAs 不同。