Microbial consortia in salt marsh sediments are sequentially buried over millennia and genomic complementarity analysis indicates an important role in complex carbon decomposition

Salt marsh sediments contain high levels of microbial diversity and functions that reflect the heterogeneity of available nutrients, dynamic hydrology, and layers of organic matter developed over millennia. However, the microbially mediated processes involved in the cycling of complex carbon are still largely undescribed. We used genome reconstruction, co-occurrence networks, and genome scale metabolic modeling to identify the functional capacity for organic matter decomposition and transformation among groups of co-occurring microbes from 5-240 cm of sediment in a Spartina patens salt marsh, representing over 2000 years of sediment accumulation. We identified four consortia with similar taxonomic and functional profiles, but distinctly different distributions within the sedimentary layers. Microbial members common to each of these consortia included novel members of Bathyarchaeia BA1, Desulfatiglandales, Chloroflexota, Caldatribacteriota (JS1), Planctomycetota, and WOR3. The collective metabolic potential of these core consortia contained the capacity to decompose complex carbon and aromatics through syntrophic interactions. The composition and functional potential of these co-occurring members are most similar to communities inhabiting sediment hundreds of meters beneath the surface of deep-sea sediments. Our results suggest that the assembly of communities within the salt marsh sediment are similarly governed by burial and nutrient limitation. The burial processes and putative syntrophy identified here provide a mechanistic understanding of how microbial life can persist under energy limited conditions and contribute to the transformation of carbon within salt marsh sediment.