Microbial consortia mediating lignocellulose turnover and denitrification in eutrophic lake sediment enrichments

ABSTRACT Lignocellulose is a major component of plant biomass and is recalcitrant, with efficient degradation typically requiring oxygen-dependent oxidative and carbohydrate-active enzymes (CAZymes). Anaerobic turnover is slower but can be supported by microbes capable of nitrate respiration, including denitrifiers and dissimilatory nitrate reduction to ammonium (DNRA) bacteria, which may use nitrate or nitric oxide as alternative oxidants. Anoxic layers beneath the oxic zones of eutrophic lake sediments, where nitrate penetrates from surface waters, provide a natural habitat for such organisms. To investigate these processes, we established nitrate-amended enrichments from organic-rich sediments of 10 eutrophic lakes and applied gas kinetics alongside metagenomics and metaproteomics to characterize the microbial communities. We identified a set of core microbial metagenome-assembled genomes (MAGs) present in all enrichments, dominated by Pseudomonadota , Bacteroidota , Verrucomicrobiota , and Actinomycetota , which played key roles in denitrification and fermentation. Lignocellulose degradation, however, was largely carried out by species outside the core microbiome—that is, different key degraders between lakes, suggesting lake-specific specialization. Among these, we observed potential respiratory DNRA pathways and a broad repertoire of CAZymes targeting various lignocellulose subfractions. Interestingly, many MAGs also encoded nitric oxide dismutases (NODs), enzymes postulated to convert NO to molecular oxygen and dinitrogen gas. Together, these findings advance our understanding of anaerobic biomass degradation and nitrogen cycling in eutrophic freshwater sediments, while highlighting the unexplored functional diversity of NOD-containing bacteria as an intriguing open question for future research. IMPORTANCE Lignocellulose, the main structural component of plant biomass, represents a vast reservoir of organic carbon in natural environments. Although lignocellulose breakdown is commonly associated with oxygen-rich conditions, it also occurs in oxygen-depleted habitats such as lake sediments, where the responsible microbes and processes are poorly understood. This study reveals how diverse microbial communities can degrade lignocellulose while respiring nitrate, linking carbon turnover to nitrogen cycling in anoxic environments. By identifying shared and lake-specific microbial strategies, as well as a widespread but poorly characterized class of enzymes associated with nitric oxide metabolism, our work advances our understanding of anaerobic biomass degradation. These insights have implications for ecosystem functioning in nutrient-rich waters and for the development of sustainable, oxygen-free biotechnological processes.