Enzymatic diversity of biological nitrogen fixation in a changing world

北方森林是世界上最大的未被破坏的陆地生态系统之一，支持着加拿大的基本经济部门(即林业、旅游业)。因此，了解北方生态系统的功能具有重要的生态和经济意义。在北方森林中，氮是最常被报道为限制初级生产力的养分。虽然北方森林土壤通常富含全氮，但这些氮以顽强的形式结合在一起，植物不容易获得。因此，森林生长在很大程度上依赖于快速循环的氮素和新的氮素投入。后者来自微生物，特别是蓝藻的生物固氮，参与与苔藓和地衣的共生和互惠关系，或在土壤中自由生活。在接下来的几十年里，由于全球气候变化，北方森林将经历前所未有的扰动(如温度、湿度增加)。最近的研究报告称，植物的氮素有效性很可能在北方森林对全球气候变化的反应中发挥关键作用。 因此，了解控制北方森林生物固氮的过程对于以可持续的方式管理北方自然资源至关重要。生物固氮依赖于固氮酶的活性，该酶能将大气中的氮素还原为生物可利用的铵。虽然固氮酶的三种亚型已被鉴定，但生物固氮主要依赖于钼亚型。我的实验室和同事最近进行的研究强调，使用钒和铁的另外两种异构体在生物固氮方面发挥着比以前接受的更重要的作用。这些发现极大地改变了传统的观点，即钼是维持生物固氮的主要金属，并邀请我们将固氮酶的多样性整合到我们的概念模型中，将金属动力学和生物固氮联系起来。 这项研究计划将解决有关北方生态系统中生物固氮的关键问题。它将极大地改进我们将微量营养素动态和生物固氮联系在一起的概念模型，通过提供关于替代固氮酶对北方森林生物固氮的贡献以及它们在北方森林对全球气候变化的反应中的潜在作用的新见解。确定固氮酶多样性在环境中的真正重要性具有潜在的变革性，因为它深刻地影响了我们理解和估计田间生物固氮的方式。这项研究最终将帮助北方森林终端用户(例如林业)和决策者改进加拿大的自然资源管理政策和做法。

The boreal forest is one of the largest untouched terrestrial ecosystem in the world and supports essential economic sectors (i.e forestry, tourism) in Canada. Understanding boreal ecosystems function is thus of prime ecological and economical importance. In boreal forests, nitrogen is the nutrient that is most often reported as limiting primary productivity. While boreal forest soils are often rich in total nitrogen, this nitrogen is bound in recalcitrant forms that are not readily available to plants. Therefore, forest growth strongly relies on rapidly cycling nitrogen and new nitrogen inputs. The last arises from biological nitrogen fixation by microorganisms, especially cyanobacteria, involved in symbiotic and mutualistic associations with mosses and lichens or living freely in soil. In the next decades, the boreal forest will undergo unprecedented perturbations (e.g. increased temperature, humidity) due to global climate change. Recent studies reported that nitrogen availability to plants will likely play a critical role in the response of the boreal forest to global climate change. Understanding processes controlling biological nitrogen fixation in boreal forest is thus essential to the management of boreal natural resources in a sustainable manner. Biological nitrogen fixation relies on the activity of the enzyme nitrogenase which reduces atmospheric dinitrogen into bioavailable ammonium. While three isoforms of the enzyme nitrogenase have been identified, biological nitrogen fixation is assumed to depend primarily on the molybdenum isoform. Recent research, conducted by my laboratory and colleagues, have highlighted that the two other isoforms, using vanadium and iron, play a more significant role on biological nitrogen fixation than previously accepted. These findings significantly alter the traditional views that molybdenum is the predominant metal sustaining biological nitrogen fixation and invite us to integrate nitrogenase diversity in our conceptual models linking metal dynamics and biological nitrogen fixation. This research program will address critical questions regarding biological nitrogen fixation in boreal ecosystems. It will significantly improve our conceptual models linking micronutrient dynamics and biological nitrogen fixation by providing new insights into the contribution of alternative nitrogenases to biological nitrogen fixation in boreal forests as well as their potential role in the response of boreal forests to global climate change. The determination of the real importance of nitrogenase diversity in the environment is potentially transformative as it deeply affects the way we comprehend and estimate biological nitrogen fixation in the field. This research will ultimately help boreal forest end-users (e.g. forestry) and decision makers to improve Canadian natural resources management policies and practices.