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Abstract; Tiny but diverse microbes are earth engines that drive global biogeochemical cycles, which shape our environment and impact all life. One such cycle is the nitrogen cycle. Nitrogen is an essential nutrient for all organisms. Most surface ocean is nitrogen limited. Thus, the amount of bioavailable nitrogen sets the limits for the ocean’s productivity and its capacity to fix CO2. Nitrite oxidation catalyzed by nitrite-oxidizing bacteria (NOB) retains nitrogen in the ocean by oxidizing one form of bioavailable nitrogen to another. This process was thought to be restricted to oxic waters. Surprisingly, I recovered genomes of novel NOB from anoxic seawaters via metagenomics. Due to experimental constraints, whether these novel NOB can perform nitrite oxidation without oxygen has been a long debate. I obtained the first experimental evidence of anaerobic nitrite oxidation and demonstrated that implementing this new microbial process decreased the estimate of marine nitrogen loss by up to 62%. This is one of my research programs highlighting the importance of unexpected microbial diversity in characterizing the current earth and predicting its future. However, due to the lack of quantitative frameworks and theories, microbes are rarely incorporated into predictive models. To address this challenge, I explored general rules that can quantitatively predict microbial diversity. I discovered that individual cell-level strategies and trade-offs can predict community-level responses to environmental changes. Guided by these rules and my previous findings, I established novel theoretical frameworks that directly incorporate microbes into models. These frameworks successfully explained observed microbial diversity, gene abundance, and biogeochemical dynamics, setting the stage for considering microbes in global models. These studies also generate hypotheses for my future experiments and theoretical analyses. These findings motivate me to make the unexpected microbial diversity and their biogeochemistry in a changing environment expected and predictable by pioneering novel frameworks using experiments, models, and ecological theories.

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