Uncovering novel functions of the enigmatic, abundant and active Anaerolineae in a salt marsh ecosystem

Anaerolineae, particularly uncultured representatives, are one of the most abundant microbial groups in coastal salt marshes, dominating the belowground rhizosphere, where over half of plant biomass production occurs. However, this class generally remains poorly understood, particularly in a salt marsh context. Here, novel Anaerolineae metagenome assembled genomes (MAGs) were generated from the salt marsh rhizosphere representing Anaerolineales, Promineifilales, JAAYZQ01, B4-G1, JAFGEY01, UCB3, and Caldilineales orders. Metagenome and metatranscriptome reads were mapped to annotated MAGs, revealing nearly all Anaerolineae encoded and transcribed genes required for oxidation of simple sugars to complex carbon compounds, fermentation, and carbon fixation. Further, the majority of Anaerolineae expressed genes involved in anaerobic and aerobic respiration and secondary metabolite production. The data revealed that the belowground salt marsh Anaerolineae in the rhizosphere are important players in carbon cycling, including degradation of simple carbon compounds to more recalcitrant plant material, such as cellulose, using a diversity of electron acceptors and represent an unexplored reservoir of novel secondary metabolites.