# Metabolic biochemical models of N2 fixation for sulfide oxidizers, methanogens, and methanotrophs

## TL;DR

This paper models how different microbes fix nitrogen in various environments, showing how they use electrons for growth and nitrogen fixation.

## Contribution

The study introduces nine new biochemical models for chemoautotrophic N2 fixers, including sulfide oxidizers, methanogens, and methanotrophs.

## Key findings

- Aerobic organisms allocate more electrons to N2 fixation and growth compared to methanogens and sulfate users.
- The models predict whole-cell stoichiometries and electron usage for nine N2 fixers.
- The study provides insights into the environmental distribution of N2 fixers based on metabolic pathways.

## Abstract

Dinitrogen (N2) fixation provides bioavailable nitrogen to the biosphere. However, in some habitats (e.g., sediments), the metabolic pathways of organisms carrying out N2 fixation are unclear. We present metabolic models representing various chemotrophic N2 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 N2 fixers. Our results demonstrate that all modeled organisms fix sufficient N2 for growth. Aerobic organisms allocate more electrons to N2 fixation and growth, yielding more biomass and fixing more N2, while methanogens using acetate and organisms using sulfate allocate fewer electrons. This work can be applied to investigate the depth distribution of N2 fixers based on nutrient availability, complementing field measurements of biogeochemical processes and microbial communities.

N2 fixation is an important process in the global N cycle. Researchers have developed models for heterotrophic and photoautotrophic N2 fixers, but there is a lack of modeling studies on chemoautotrophic N2 fixers. Here, we built nine biochemical models for different chemoautotrophic N2 fixers by combining different types of half-chemical reactions. We include three sulfide oxidizers using different electron acceptors (O2, NO3−, and Fe3+), contributing to the sulfur, nitrogen, and iron cycles in the sediment. We have two methanogens using different substrates (H2 and acetate) and four methanotrophs using different electron acceptors (O2, NO3−, Fe3+, and SO42−). By modeling these methane producers and users in the sediment and their N2-fixing metabolic pathways, our work can provide insight for future carbon cycle studies. This study outlines various metabolic pathways that can facilitate N2 fixation, with implications for where in the environment they might occur.