Shortcuts in the Oceanic Nitrogen Cycle: Fluxes and Microbial Pathways of Nitrogen Remineralization in the Ocean's Twilight Zone

海洋的暮光区(中层，100-1000米)位于阳光充足的表层海洋之下，光合作用的光线太少，但在漆黑的深海之上，大型动物不再能看到猎物。在所有从表层下沉的有机物中，90%在中层海洋中被降解，只有一小部分逃逸到深海(&gt；1000米)。因此，中大气层代表着一个重要的障碍：落入其中的大多数物质通过再矿化作用防止进一步下沉--分解有机物并向溶液中释放二氧化碳和无机营养物质的降解过程。最终，物理混合或海洋环流将营养物质带回表面，为浮游植物的生长提供动力。因此，中层海洋的再矿化对控制海洋生物泵至关重要，并可能影响海洋封存大气二氧化碳的能力。氮往往是全球海洋生物生产的限制性营养物质，因此，氮的再矿化作用将是生物泵效率的关键。然而，氮再矿化的机制还没有得到很好的描述，而且在中层海洋中也没有对这一过程的速率测量。海洋中氮(N)的再矿化包括氨化作用：将有机N降解为铵(NH4+)，随后的硝化作用：将NH4+氧化为亚硝酸盐，然后是硝酸盐。然而，最近的一项研究表明，一些硝化菌(进行硝化作用的微生物)可以直接利用有机N，从而在N循环中提供了一条可能的捷径。由于不同的生物对于碳的摄食方式不同(固定或产生二氧化碳)，这些生物功能群的相对丰度和活动将对二氧化碳平衡产生不同的直接影响，潜在捷径的存在可能会导致向自养(固定二氧化碳)的转变。该项目旨在结合最先进的地化速率测量和分子生物学分析，准确确定在暮光海洋中氮的再矿化方式和数量。特别是，我们将确定在有机氮(N)到硝酸盐(NO3-)的再矿化过程中是否存在上述捷径，并定量评估其相对于传统的氨化-硝化途径对海洋N循环的潜在意义。总而言之，这些计划中的分析将提供海洋中有史以来尝试过的最完整的直接测量的N再矿化通量的数据集。将进行最先进的15N稳定同位素标记实验来测量同时发生的N转化速率，以便更准确地评估上层海洋N的收支：氨化、硝化、同化(将N并入生物体中)和溶解的有机N的释放。我们将通过跟踪15N(氮的重稳定同位素，在自然界中很少见，而不是常见的14N)同时进入不同的N库来实现这一点，以确定有机氮是直接进入硝化作用还是通过氨化作用。同时，主要的再矿化途径将通过阐明在基因转录和蛋白质水平上对这些N-转化起关键作用的生物标志物酶的表达作为各自过程的可量化活动指标来确定。计划沿大西洋子午线(AMT)从北(英国)到南大西洋(福克兰/智利)进行采样，以检查不同营养状况下的N-再矿化，同时通过百慕大大西洋时间序列(BATS)站点的季节性采样来探索时间变异性。这种时空覆盖和互补性的跨学科数据集将产生对海洋中层N再矿化的高度代表性的描述，并产生迄今关于海洋N循环中黄昏带重要性的最全面评估。

The Ocean's twilight (mesopelagic, 100-1000m) zone lies beneath the sunlit surface ocean, with too little light for photosynthesis but above the pitch-black deep ocean, where large animals can no longer see their prey. Of all organic matter that sinks out from the surface, >90% is degraded in the mesopelagic with only a small fraction escaping into the deep ocean (>1000 m). The mesopelagic thus represents an important barrier: most material falling into it is prevented from sinking further by remineralisation - the degradation process that breaks down organic matter and releases CO2 and inorganic nutrients to solution. Eventually, physical mixing or ocean circulation deliver the nutrients back to the surface to fuel phytoplankton growth. Hence, remineralisation in the mesopelagic is critical to controlling the oceanic biological pump, and can affect the ocean's ability to sequester atmospheric CO2. Nitrogen is often the limiting nutrient for biological production in global oceans, its remineralisation would thus be key to biological pump efficiency. However, the mechanisms of N-remineralisation are poorly characterized, and there are no rate measurements of this process in the mesopelagic. The remineralisation of nitrogen (N) in the oceans encompasses ammonification: the degradation of organic N to ammonium (NH4+), and subsequently nitrification: the oxidation of NH4+ to nitrite and then nitrate. However, a recent study suggests that some nitrifiers (microorganisms conducting nitrification) can utilise organic N directly, thus presenting a possible shortcut in the N-cycle. Because the respective organisms have different feeding styles regarding carbon (CO2- fixing or producing), the relative abundance and activities of these functional groups of organisms will have different direct impacts on the CO2 balance, and the existence of the potential shortcut will likely cause a shift towards autotrophy (CO2 fixation).This project aims to determine exactly how and how much nitrogen is remineralised in the twilight ocean, using a combination of state-of-the-art geochemical rate measurements and molecular biological analyses. In particular, we will determine whether the above-mentioned shortcut exists in the remineralisation of organic nitrogen (N) to nitrate (NO3-), and quantitatively assess its potential significance to the oceanic N-cycle relative to the conventional ammonification-nitrification pathway. Together, these planned analyses will give the most complete dataset of directly measured N-remineralisation fluxes ever attempted in the oceans.State-of-the-art 15N-stable-isotope-labeling experiments will be conducted to measure rates of concurrent N-conversions for a more accurate assessment of upper ocean N-budget: ammonification, nitrification, assimilation (incorporation of N into biomass) and release of dissolved organic N. We will do this by tracing 15N (the heavy stable isotope of N that is rare in nature, as opposed to the common 14N) from various amended organic substrates into different N-pools at the same time, to determine whether organic-N is directly channeled to nitrification or via ammonification. In parallel, major remineralisation pathways will be identified by elucidating the expression of biomarker enzymes key to these N-conversions at both gene transcript and protein levels, as quantifiable activity indicators for the respective processes.Sampling is planned along the Atlantic Meridional Transect (AMT) from the north (UK) to south Atlantic (Falklands/Chile) to examine N-remineralisation in diverse nutrient regimes, while temporal variability is explored via seasonal sampling at the Bermuda Atlantic Time Series (BATS) site. Such spatiotemporal coverage and complementary, interdisciplinary dataset would yield a highly representative depiction of mesopelagic N-remineralisation in the oceans, and the most comprehensive assessment to date on the significance of the twilight zone in oceanic N-cycle.