Metabolic biochemical models of N ₂ fixation for sulfide oxidizers, methanogens, and methanotrophs

Dinitrogen (N 2 ) fixation provides bioavailable nitrogen to the biosphere. However, in some habitats (e.g., sediments), the metabolic pathways of organisms carrying out N 2 fixation are unclear. We present metabolic models representing various chemotrophic N 2 fixers, which simulate potential pathways of electron transport and energy flow, resulting in predictions of whole-cell stoichiometries. By balancing mass, electrons, and energy for metabolic half-reactions, we quantify the electron usage for nine N 2 fixers. Our results demonstrate that all modeled organisms fix sufficient N 2 for growth. Aerobic organisms allocate more electrons to N 2 fixation and growth, yielding more biomass and fixing more N 2 , while methanogens using acetate and organisms using sulfate allocate fewer electrons. This work can be applied to investigate the depth distribution of N 2 fixers based on nutrient availability, complementing field measurements of biogeochemical processes and microbial communities. IMPORTANCE N 2 fixation is an important process in the global N cycle. Researchers have developed models for heterotrophic and photoautotrophic N 2 fixers, but there is a lack of modeling studies on chemoautotrophic N 2 fixers. Here, we built nine biochemical models for different chemoautotrophic N 2 fixers by combining different types of half-chemical reactions. We include three sulfide oxidizers using different electron acceptors (O 2 , NO 3 − , and Fe 3+ ), contributing to the sulfur, nitrogen, and iron cycles in the sediment. We have two methanogens using different substrates (H 2 and acetate) and four methanotrophs using different electron acceptors (O 2 , NO 3 − , Fe 3+ , and SO 4 2− ). By modeling these methane producers and users in the sediment and their N 2 -fixing metabolic pathways, our work can provide insight for future carbon cycle studies. This study outlines various metabolic pathways that can facilitate N 2 fixation, with implications for where in the environment they might occur.