Enzymatic diversity of biological nitrogen fixation in a changing world

北方森林是世界上最大的未受破坏的陆地生态系统之一，支持着加拿大重要的经济部门（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，但这些氮以不易被植物利用的顽固形式结合在一起。因此，森林生长强烈依赖快速循环的氮和新的氮输入。最后一种来自微生物的生物固氮作用，特别是与苔藓和地衣或自由生活在土壤中的蓝藻。在未来几十年里，由于全球气候变化，北方森林将遭受前所未有的扰动（例如温度、湿度升高）。最近的研究报道，植物的氮有效性可能在北方森林对全球气候变化的响应中发挥关键作用。

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.