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ABSTRACT Dinitrogen (N₂) fixation provides bioavailable nitrogen to the biosphere. However, in some habitats (e.g., sediments), the metabolic pathways of organisms carrying out N₂fixation are unclear. We present metabolic models representing various chemotrophic N₂fixers, 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 N₂fixers. Our results demonstrate that all modeled organisms fix sufficient N₂for growth. Aerobic organisms allocate more electrons to N₂fixation 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 N₂fixers based on nutrient availability, complementing field measurements of biogeochemical processes and microbial communities.IMPORTANCEN₂fixation is an important process in the global N cycle. Researchers have developed models for heterotrophic and photoautotrophic N₂fixers, but there is a lack of modeling studies on chemoautotrophic N₂fixers. Here, we built nine biochemical models for different chemoautotrophic N₂fixers 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 (H₂and 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 N₂fixation, with implications for where in the environment they might occur. 

ABSTRACT Microbial nitrogen fixation (diazotrophy) is a critical ecological process. We curated DiazoTIME (Diazotroph Taxonomic Identity and MEtabolism), a comprehensive database of diazotroph genomes including taxonomic annotation and metabolic prediction. DiazoTIME is unique among databases for classifying diazotrophs because it resolves both taxonomy and metabolic functionality. 

Abstract Marine Group II Euryarchaeota ( Candidatus Poseidoniales), abundant but yet-uncultivated members of marine microbial communities, are thought to be (photo)heterotrophs that metabolize dissolved organic matter (DOM), such as lipids and peptides. However, little is known about their transcriptional activity. We mapped reads from a metatranscriptomic time series collected at Sapelo Island (GA, USA) to metagenome-assembled genomes to determine the diversity of transcriptionally active Ca . Poseidoniales. Summer metatranscriptomes had the highest abundance of Ca . Poseidoniales transcripts, mostly from the O1 and O3 genera within Ca . Thalassarchaeaceae (MGIIb). In contrast, transcripts from fall and winter samples were predominantly from Ca . Poseidoniaceae (MGIIa). Genes encoding proteorhodopsin, membrane-bound pyrophosphatase, peptidase/proteases, and part of the ß-oxidation pathway were highly transcribed across abundant genera. Highly transcribed genes specific to Ca . Thalassarchaeaceae included xanthine/uracil permease and receptors for amino acid transporters. Enrichment of Ca . Thalassarchaeaceae transcript reads related to protein/peptide, nucleic acid, and amino acid transport and metabolism, as well as transcript depletion during dark incubations, provided further evidence of heterotrophic metabolism. Quantitative PCR analysis of South Atlantic Bight samples indicated consistently abundant Ca . Poseidoniales in nearshore and inshore waters. Together, our data suggest that Ca . Thalassarchaeaceae are important photoheterotrophs potentially linking DOM and nitrogen cycling in coastal waters.