The nitrogen cycle plays a major role in aquatic nitrogen transformations, including in the terrestrial subsurface. However, the variety of transformations remains understudied. To determine how nitrogen cycling microorganisms respond to different aquifer chemistries, we sampled groundwater with varying nutrient and oxygen contents. Genes and transcripts involved in major nitrogen-cycling pathways were quantified from 55 and 26 sites, respectively, and metagenomes and metatranscriptomes were analyzed from a subset of oxic and dysoxic sites (0.3-1.1 mg/L bulk dissolved oxygen). Nitrogen-cycling mechanisms (e.g. ammonia oxidation, denitrification, dissimilatory nitrate reduction to ammonium) were prevalent and highly redundant, regardless of site-specific physicochemistry or nitrate availability, and present in 40% of reconstructed genomes, suggesting that nitrogen cycling is a core function of aquifer communities. Transcriptional activity for nitrification, denitrification, nitrite-dependent anaerobic methane oxidation and anaerobic ammonia oxidation (anammox) occurred simultaneously in oxic and dysoxic groundwater, indicating the availability of oxic-anoxic interfaces. Concurrent activity by these microorganisms indicates potential synergisms through metabolite exchange across these interfaces (e.g. nitrite and oxygen). Fragmented denitrification pathway encoding and transcription was widespread among groundwater bacteria, although a considerable proportion of associated transcriptional activity was driven by complete denitrifiers, especially under dysoxic conditions. Despite large differences in transcription, the capacity for the final steps of denitrification was largely invariant to aquifer conditions, and most genes and transcripts encoding N₂O reductases were the atypical Sec-dependant type, suggesting energy-efficiency prioritization. Results provide insights into the capacity for cooperative relationships in groundwater communities, and the richness and complexity of metabolic mechanisms leading to the loss of fixed nitrogen.

氮循环在水生氮转化中起着重要作用，包括在陆地地下。然而，转换的多样性仍未得到充分研究。为了确定氮循环微生物如何对不同的含水层化学反应，我们对具有不同养分和氧气含量的地下水进行了采样。分别从 55 和 26 个位点对参与主要氮循环途径的基因和转录本进行了量化，并从一部分含氧和缺氧位点（0.3-1.1 mg/L 大量溶解氧）分析了宏基因组和宏转录组。氮循环机制（例如氨氧化、反硝化、异化硝酸盐还原为铵）普遍且高度冗余，无论位点特异性物理化学或硝酸盐可用性如何，并且存在于 40% 的重建基因组中，表明氮循环是含水层群落的核心功能。硝化、反硝化、亚硝酸盐依赖性厌氧甲烷氧化和厌氧氨氧化（厌氧氨氧化）的转录活性同时发生在好氧和缺氧地下水中，表明存在好氧-缺氧界面。这些微生物的同时活动表明通过这些界面（例如亚硝酸盐和氧气）的代谢物交换具有潜在的协同作用。片段化的反硝化途径编码和转录在地下水细菌中广泛存在，尽管相当一部分相关的转录活动是由完全反硝化菌驱动的，尤其是在缺氧条件下。尽管在转录上存在很大差异，₂ O 还原酶是非典型的 Sec 依赖型，表明能效优先。结果提供了对地下水群落合作关系能力的见解，以及导致固定氮损失的代谢机制的丰富性和复杂性。