Microbial dark carbon fixation fueled by nitrate enrichment

Anthropogenic nitrate amendment to coastal marine sediments can increase rates of heterotrophic mineralization and autotrophic dark carbon fixation (DCF). DCF may be favored in sediments where organic matter is biologically unavailable, leading to a microbial community supported by chemoautotrophy. Niche partitioning among DCF communities and adaptations for nitrate metabolism in coastal marine sediments remain poorly characterized, especially within salt marshes. We used genome-resolved metagenomics, phylogenetics, and comparative genomics to characterize the potential niche space, phylogenetic relationships, and adaptations important to microbial communities within nitrate enriched sediment. We found that nitrate enrichment of sediment from discrete depths between 0-25 cm supported both heterotrophs and chemoautotrophs that use sulfur oxidizing denitrification to drive the Calvin-Benson-Bassham (CBB) or reductive TCA (rTCA) DCF pathways. Phylogenetic reconstruction indicated that the nitrate enriched community represented a small fraction of the phylogenetic diversity contained in coastal marine environmental genomes, while pangenomics revealed close evolutionary and functional relationships with DCF microbes in other oligotrophic environments. These results indicate that DCF can support coastal marine microbial communities and should be carefully considered when estimating the impact of nitrate on carbon cycling in these critical habitats. Importance Salt marshes store carbon at one of the fastest rates of any blue carbon system and buffer coastal marine waters from eutrophication. Dark carbon fixation (DCF) conducted by microbes within the sediment can influence the carbon storage capacity, but little is known about the ecology or genomic potential of these organisms. Our study identifies a potential niche space for several functionally distinct groups of chemoautotrophs which primarily use sulfur oxidizing denitrification to fuel DCF under high nitrate concentrations. These findings fill an important gap in our understanding of microbial contributions to carbon storage within salt marsh sediments and how this critical blue carbon system responds to anthropogenic nitrate enrichment.