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

海洋的黄昏区（中上层，100-1000米）位于阳光照射的海洋表面之下，光线太少，无法进行光合作用，但在漆黑的深海之上，大型动物再也看不到它们的猎物。在所有从地表下沉的有机物中，有90%在中远洋被降解，只有一小部分逸入深海（约1000米）。因此，中上层代表了一个重要的屏障：大多数落入其中的物质被再矿化阻止了进一步下沉——再矿化是分解有机物质并将二氧化碳和无机营养物质释放到溶液中的降解过程。最终，物理混合或海洋环流将营养物质送回海面，为浮游植物的生长提供燃料。因此，中上层的再矿化对控制海洋生物泵至关重要，并可能影响海洋吸收大气中二氧化碳的能力。氮通常是全球海洋生物生产的限制性养分，因此它的再矿化将是生物泵效率的关键。然而，n -再矿化机制的特征很差，并且在中层中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：将有机氮降解为铵（NH4+），随后是硝化作用：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（进行硝化的微生物）可以直接利用有机氮，从而在N循环中提供了一条可能的捷径。由于不同生物对碳的摄食方式（固定或产生二氧化碳）不同，这些生物功能群的相对丰度和活性对二氧化碳平衡有不同的直接影响，潜在捷径的存在可能导致向自养（固定二氧化碳）的转变。该项目旨在利用最先进的地球化学速率测量和分子生物学分析相结合，准确地确定黄昏海洋中氮的再矿化方式和再矿化量。特别是，我们将确定上述捷径是否存在于有机氮(N)再矿化到硝酸盐（NO3-）中，并定量评估其相对于常规氨化-硝化途径对海洋N循环的潜在意义。总之，这些计划中的分析将提供有史以来在海洋中直接测量n -再矿化通量的最完整的数据集。将进行最先进的15n稳定同位素标记实验，以测量同步n转换速率，以便更准确地评估上层海洋n收支。我们将通过追踪15N （N的重稳定同位素，在自然界中很少见，与常见的14N相反）从各种改性的有机基质同时进入不同的N库来做到这一点，以确定有机N是直接进入硝化还是通过氨化。与此同时，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转化至关重要的生物标记酶的表达来确定，作为各自过程的可量化活性指标。计划沿着大西洋经向样带（AMT）从北（英国）到南大西洋（福克兰群岛/智利）进行采样，以检查不同营养体制下的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化。这样的时空覆盖和互补的跨学科数据集将产生对海洋中上层n再矿化的极具代表性的描述，并对海洋n循环中过渡带的意义进行迄今为止最全面的评估。

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.