Nitrogen fixation in the Arctic Ocean

生物固氮是将氮气(N_2)转化为固定氮(如硝酸盐)。氮气固定是全球海洋生物地球化学循环的重要组成部分。它提供了通过反硝化和Anamox平衡氮损失所需的主要氮源，从而控制了海洋中固定氮的数量，从而导致碳和其他营养物质的循环和储存。直到最近，氮气的固定还被认为仅限于低纬副热带环流的温暖表层水域(20至30dC)。这种看法使几十年来关于固氮的研究集中在低纬海洋，那里的其他营养物质，如铁和磷，限制了固氮菌的活性。然而，有新的证据表明，北冰洋冰冷的表层水域中存在并活跃着固氮菌。在北冰洋西部和中部发现了固氮基因，观测到的固氮速度与温水固氮相当。这一意想不到的结果意味着，我们目前对全球固氮的理解是不完整的，因为它忽略了冷水中的固氮。此外，我们不知道温度、光照、营养物质和铁是如何控制北冰洋固氮菌的分布和活动的。出于三个原因，迫切需要更好地了解北冰洋的固氮情况。首先，北冰洋的初级生产力已经受到硝酸盐供应的限制。自20世纪70年代以来，北冰洋变暖导致海冰每十年减少9%，导致初级生产力增加25%。随着北冰洋变暖并变得无冰，初级生产量预计将增加。为了支持初级产量的增加，必须有一个额外的硝酸盐来源，但这个来源仍然难以捉摸。来自河流、大气和溶解有机氮的硝酸盐输入不能满足与初级生产增加相关的硝酸盐需求。因此，固氮可能是当代和未来北冰洋氮的一个重要来源，并提供支持未来初级生产增加所需的固定氮。其次，关于北冰洋氮预算状况的辩论仍在继续。尽管进出北冰洋的硝酸盐数量相等，但北冰洋大陆架上的沉积物中有一个巨大的硝酸盐下沉。这意味着，要么北冰洋的硝酸盐存在巨大的短缺，要么目前有一种来源的硝酸盐下落不明。固氮可能是平衡北冰洋氮收支所需的缺失来源。然而，缺乏观测意味着泛北极地区对固氮的估计是粗略的。最后，代表海洋营养循环和海洋生态系统的全球数值模型目前忽视了北冰洋的固氮。这意味着，这些模型预测北冰洋将如何应对气候变暖和生产力增加的能力是不准确的，因为它们没有考虑额外的营养源或固氮对碳、磷和铁等营养物质的影响。总体而言，我们缺乏关于北冰洋固氮的知识，这意味着我们不知道这一过程对全球固氮或氮收支有多重要。N-ARC提议在北冰洋东部进行第一个关于固氮的整体研究。我们选择这个地区有两个原因：(A)来自N-ARC团队的新数据检测到了负责该地区固氮的基因，为北冰洋东部正在进行固氮提供了新的证据；(B)温度和营养物质存在强烈的梯度，使我们能够探索海洋条件如何控制北冰洋的固氮。

Biological nitrogen fixation is the conversion of nitrogen gas (N2) to fixed nitrogen (e.g. nitrate). N2 fixation is a crucial component of global ocean biogeochemical cycles. It provides the major source of nitrogen necessary to balance nitrogen loss via denitrification and annamox and thus controls the magnitude of fixed nitrogen in the ocean, with consequences for the cycling and storage of carbon and other nutrients. Until recently, N2 fixation was thought to be restricted to the warm surface waters (20 to 30dC) of the low latitude subtropical gyres. This perception has focused decades of research on nitrogen fixation to the low latitude ocean, where other nutrients such as iron and phosphorus limit the activity of nitrogen fixers. However, there is new evidence that nitrogen fixing organisms are present and active in the cold surface waters of the Arctic Ocean. The nitrogen fixing gene has been detected in the western and central Arctic Ocean and observed rates of nitrogen are comparable to warm water nitrogen fixation. This unexpected result means that our current understanding of global nitrogen fixation is incomplete as it neglects to account for nitrogen fixation in cold waters. In addition, we do not know how temperature, light, nutrients and iron control the distribution and activity of nitrogen fixing organisms in the Arctic Ocean. There is an urgent need to better understand nitrogen fixation in the Arctic Ocean for three reasons. Firstly, the primary productivity of the Arctic Ocean is already limited by the availability of nitrate. Warming of the Arctic Ocean has caused sea ice to decline by 9% per decade since the 1970s, causing primary productivity to increase by 25%. As the Arctic Ocean warms and becomes ice free, primary production is predicted to increase. To support increased primary production, there must be an additional source of nitrate, but this source remains elusive. Inputs of nitrate from rivers, the atmosphere and dissolved organic nitrogen do not meet the nitrate demand associated with increased primary production. Nitrogen fixation may therefore be a crucial source of nitrogen in the contemporary and future Arctic Ocean and provide the fixed nitrogen necessary to support the future increase in primary production. Secondly, there is ongoing debate on the status of the nitrogen budget in the Arctic Ocean. Although the amount of nitrate entering and leaving the Arctic Ocean is equal, there is a large nitrate sink in the sediments on the Arctic shelves. This implies that either there is a large deficit in nitrate in the Arctic Ocean, or there is currently a source of nitrate that is unaccounted for. Nitrogen fixation may be the missing source required to balance the Arctic Ocean nitrogen budget. However, the lack of observations means that pan-Arctic estimates of nitrogen fixation are rudimentary. Finally, global numerical models that represent the oceanic nutrient cycling and the marine ecosystem currently ignore nitrogen fixation in the Arctic Ocean. This means that the ability of these models to predict how the Arctic ocean will respond to increased warming and increased productivity is inaccurate because they don't account for the additional nutrient source or the effect nitrogen fixation has on nutrients such as carbon, phosphorus and iron. Overall, our lack of knowledge on nitrogen fixation in the Arctic Ocean means we do not know how important this process is to global nitrogen fixation or nitrogen budget. N-ARC proposes to conduct the first holistic study of nitrogen fixation in the eastern Arctic Ocean. We chose this region for two reasons; (a) new data from N-ARC team has detected the gene responsible for nitrogen fixation in this region, providing new evidence that nitrogen fixation is occurring in the eastern Arctic Ocean and (b) there are strong gradients in temperature and nutrients, allowing us to explore how the ocean conditions control nitrogen fixation in the Arctic Ocean.