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

氮循环是地球上最复杂、扰动最严重的全球生物地球化学循环之一。自然地，微生物将气态大气中的氮“固定”为植物和随后的其他生物使用的形式。其他微生物可以通过一系列的转化，将固定的氮以气态的形式返回到大气中，完成这个循环。然而，人类正在通过肥料合成人为地固定大量的氮来丰富地球，从而导致广泛的氮污染。尽管内陆水域的氮污染在加拿大和全球范围内日益受到关注，但人们对淡水中微生物多样性和微生物氮率转化的机制控制知之甚少。湖泊内不平衡的氮转化可能对水生生态系统健康产生负面影响，因为在这个复杂的循环中，不同氮形态的可用性可能调节浮游植物群落结构，促进有害蓝藻的增殖，并产生强效的温室气体一氧化二氮。我们建议在几种不同生态系统类型的不同调查尺度上表征多种N代谢率转化以及淡水N循环中的微生物多样性。这项独特的综合研究将通过结合几种新的生理、同位素、分类学和分子技术来完成，其中许多技术尚未在淡水中以系统的方式使用。我们将沿着湖泊类型的梯度研究固氮、硝化、反硝化和藻类氮营养的时空变化，并从群落结构变化的角度和与碳等其他主要生物地球化学循环相关的角度，描述作为气候变化、点和非点源氮污染增加的函数的N处理过程。这一创新项目的发现有望改变我们目前对淡水中氮循环的范式，为帮助证实控制内陆水域氮污染的必要性提供证据，并为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.