Urea sinks are mainly associated with assimilation by phytoplankton. However, recent studies have shown that there is a process by which nitrifiers convert urea-derived nitrogen (urea-N) into nitrate. We examined these two processes in the shelf and off-shelf regions of the Arctic Ocean. Urea concentration was high near the bottom in the shelf region, while it was depleted throughout the water column in the off-shelf region. Urea-N assimilation was generally higher in the upper euphotic zone than the lower euphotic zone. By contrast, urea-N oxidation was low in the upper euphotic zone and increased with depth. These results indicate that urea sinks consist of a two-layer system. We further examined the organisms involved in urea-N oxidation and found a dominance of shallow clade ammonia-oxidizing archaea, whose abundance was low in the upper euphotic zone and increased with depth. The abundances of archaeal ureC and amoA genes of shallow clade ammonia-oxidizing archaea were well correlated (ρ = 0.96, Spearman's correlation), suggesting that most of shallow clade ammonia-oxidizing archaea could use urea as a source of ammonia oxidation. However, we found that the urea-N oxidation rate often exceeded the ammonia oxidation rate while kinetics experiments suggested that ammonia oxidizers use urea less actively than ammonia. Network analyses indicated that ammonia oxidizers were closely related to other prokaryotes with the ability to decompose urea. These results suggested that ammonia-oxidizing archaea may not necessarily use urea-N directly.

中文翻译：

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夏季北冰洋尿素氮的同化与氧化

尿素汇主要与浮游植物的同化有关。然而，最近的研究表明，有一个过程，硝化菌将尿素衍生的氮 (urea-N) 转化为硝酸盐。我们研究了北冰洋大陆架和大陆架外地区的这两个过程。货架区域底部附近的尿素浓度很高，而货架外区域的整个水体中尿素浓度都很高。尿素氮同化在上部透光区普遍高于下部透光区。相比之下，尿素-N 氧化在上部透光区较低，并且随着深度的增加而增加。这些结果表明尿素汇由两层系统组成。我们进一步检查了参与尿素氮氧化的生物，发现浅层氨氧化古细菌占主导地位，其丰度在上部透光区较低，并随深度增加。丰富的古菌浅层氨氧化古菌的ureC和amoA基因相关性很好（ρ  =0.96，Spearman相关），表明大部分浅层氨氧化古菌都可以利用尿素作为氨氧化的来源。然而，我们发现尿素-N 氧化速率通常超过氨氧化速率，而动力学实验表明氨氧化器使用尿素的活性低于氨。网络分析表明，氨氧化剂与其他具有分解尿素能力的原核生物密切相关。这些结果表明氨氧化古细菌不一定直接使用尿素氮。