Biological nitrogen uptake and microbial transformations in aquatic ecosystems under conditions of rapid environmental change

氮(N)循环是地球上最复杂和最受干扰的全球生物地球化学循环之一。自然而然地，微生物将大气中的气态氮“固定”成植物所使用的形式，然后是其他生物所使用的形式。其他微生物可以通过完成循环的一系列转化，将这种固定的氮以气体的形式返回到大气中。然而，人类正在通过人工合成化肥来固定大量的氮，从而使地球变得更加肥沃，导致了广泛的氮污染。尽管氮对内陆水域的污染在加拿大和全球范围内日益受到关注，但对淡水这一关键营养循环的微生物多样性和微生物氮速率转化的机制控制知之甚少。湖泊内氮转化的不平衡可能会对水生生态系统的健康产生负面影响，因为在这个复杂的循环中，不同形态的N的有效性可能会调节浮游植物群落结构，促进有害蓝藻的繁殖，并产生强有力的温室气体一氧化二氮。我们建议在几个不同生态系统类型的不同调查范围内表征淡水氮循环中的多重氮代谢率转换以及微生物多样性。这种对氮循环的独特综合观察将通过几种新的生理、同位素、分类和分子技术的组合来完成，其中许多技术尚未在淡水中系统地使用。我们将研究氮固定、硝化、反硝化和藻类氮营养在不同湖泊类型梯度上的时空变化，并将从群落结构变化的角度以及与其他主要生物地球化学循环(如碳)的关系来描述氮的处理过程，作为气候变化、点源和非点源氮污染的函数。这一创新计划的发现有望改变我们目前淡水中氮循环的范式，提供证据帮助证实控制内陆水域氮污染的必要性，并对不同过程的可变性提供重要见解，以帮助指导未来的海洋学研究。

The nitrogen (N) cycle is one of the most complex and heavily perturbed global biogeochemical cycles on earth. Naturally, microorganisms "fix" gaseous atmospheric N into a form used by plants and subsequently other organisms. Other microorganisms can return this fixed N to the atmosphere in gaseous form through a series of transformations completing the cycle. Humans however are enriching the earth by artificially fixing large amounts of N through fertilizer synthesis resulting in widespread N pollution. Although N pollution to inland waters is a growing concern in Canada and around the globe, little is known about the microbial diversity and the mechanistic controls of the microbial N rate transformations of this critical nutrient cycle in freshwaters. Unbalanced N transformations within lakes may have negative repercussions on aquatic ecosystem health as the availability of different N forms within this complex cycle may regulate phytoplankton community structure, contribute to the proliferation of nuisance cyanobacteria and produce the potent greenhouse gas nitrous oxide. We propose to characterize multiple N metabolic rate transformations as well as the microbial diversity in the freshwater N cycle at different scales of inquiry across several different ecosystem types. This unique integrative look into the N cycle will be done by using a combination of several novel physiological, isotopic, taxonomical and molecular techniques, many of which have yet to be used in a systematic fashion in freshwaters. We will examine the spatial and temporal variation of N-fixation, nitrification, denitrification and algal N nutrition along a gradient of lake types and will characterise the processing of N, as a function of climate change, increased point and non-point source N pollution, from a perspective of changes in community structure and in relation to other major biogeochemical cycles like carbon. The findings of this innovative program are expected to alter our current paradigms of N cycling in freshwaters, supply evidence to help substantiate the need to control N pollution to inland waters and provide important insights in the variability of the different processes to help direct future oceanographic studies.