Integrating geochemical and microbiological information for better modeling of the N-cycle – past and present

Cycling of N occurs through a multitude of microbial reactions used by microorganisms to harness energy and generate growth. These microbial reactions are the main controls on the availability of fixed-N and can often limit primary production in marine ecosystems. The microorganisms involved in the N-cycle are diverse and the metabolic pathways are further distributed across many taxa, rendering the modeling of the N-cycle complex. Indeed, models of N-cycling fall short of making robust and explicit predictions, in part due to a lack of ecophysiological information describing the relevant processes at a molecular scale. Direct ecophysiological information is obtained from process rate measurements, yet these generally lack coupled information on microbial community composition limiting their extensibility across multiple environments. This dissertation creates a new framework for the modeling of the N-cycle by measuring the rates and pathways of N-cycling in anoxic pelagic environments. This new and quantitative knowledge is incorporated into models of N-cycling to improve reconstructions of past and future N-cycle. I describe the rates and pathways of Fe-dependent NO¯₃ reduction in a ferruginous pelagic environment, analogous to the Proterozoic oceans. I then describe the nutrients status and the implications of NO¯₃ reduction through DNRA and denitrification for biological production through a flux-balance model for ancient oceans. I also study the environmental factors that influence the partitioning of N-loss between anammox and denitrification in an anoxic fjord (Saanich Inlet). A flux-balance model was built to describe the competition between anammox and denitrification based on the rates of N₂ production as well as changes in microbial community composition and ecophysiological parameters. We show that recycling of N through DNRA, rather than N-loss, dominates annual NO¯₃ reduction in Saanich Inlet, challenging current assumptions that DNRA does not need to be considered as an important pathway of N-cycling in the ocean. Overall, the work presented here offers a new and integrated approach that combines geochemical information such as nutrient profiles and process rate measurements, microbiological information such as microbial community composition, structure and functions analysis, and applies it to quantitative models that can be used to further test hypotheses about the N-cycle.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat