Nitrogen fixation in the Arctic Ocean

北冰洋的固氮

• 批准号: NE/T001240/1
• 负责人: Claire Mahaffey
• 金额: $ 127.73万
• 依托单位: University of Liverpool
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FT001240%2F1
• 关键词: Nitrogen; fixation; Arctic; Ocean

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.

生物固氮是指将氮气（N2）转化为固定氮（如硝酸盐）。氮固定作用是全球海洋生物地球化学循环的重要组成部分。它提供了氮的主要来源，以平衡通过反硝化和厌氧反应造成的氮损失，从而控制海洋中固定氮的大小，对碳和其他营养物质的循环和储存产生影响。直到最近，人们还认为N2固定仅限于低纬度亚热带环流的温暖地表水（20 ~ 30dC）。几十年来，这种看法一直是低纬度海洋固氮研究的焦点，在低纬度海洋，铁和磷等其他营养物质限制了固氮物的活性。然而，有新的证据表明，固氮生物存在并活跃在北冰洋寒冷的表层水域中。在北冰洋西部和中部发现了固氮基因，观测到的氮固氮率与温水固氮率相当。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、光线、营养物质和铁是如何控制北冰洋固氮生物的分布和活动的。我们迫切需要更好地了解北冰洋的固氮作用，原因有三。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%。随着北冰洋变暖并无冰，预计初级产量将增加。为了支持初级生产的增加，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重要来源，并为支持未来初级生产的增长提供必要的固定氮。其次，关于北冰洋氮收支状况的争论正在进行。虽然进入和离开北冰洋的硝酸盐数量是相等的，但在北极大陆架的沉积物中有一个很大的硝酸盐汇。这意味着，要么北冰洋的硝酸盐存在大量短缺，要么目前存在一种未知的硝酸盐来源。固氮可能是平衡北冰洋氮收支所需的缺失来源。然而，观测的缺乏意味着对泛北极固氮的估计是初步的。最后，代表海洋养分循环和海洋生态系统的全球数值模式目前忽略了北冰洋的固氮作用。这意味着，这些模型预测北冰洋如何应对升温和生产力提高的能力是不准确的，因为它们没有考虑到额外的营养来源，也没有考虑到固氮对碳、磷和铁等营养物质的影响。总的来说，我们缺乏对北冰洋固氮的认识，这意味着我们不知道这个过程对全球固氮或氮收支有多重要。N-ARC提议对北冰洋东部的固氮进行首次全面研究。我们选择这个地区有两个原因；(a) N-ARC团队的新数据检测到该地区负责固氮的基因，为固氮发生在北冰洋东部提供了新的证据；(b)温度和营养物质有很强的梯度，使我们能够探索海洋条件如何控制北冰洋的固氮。