Project description:Anthropogenic activities such as urbanization and agriculture can potentially pose a threat to neighboring freshwaters through nitrate and phosphorous contamination, which over time may lead to lake eutrophication. In such nitrogen-polluted environments, oxygen is depleted, and plants die and decompose. This enhances denitrifying microbes that respire under hypoxic/anoxic conditions by reducing nitrate instead of molecular oxygen and using plant remnants (lignocellulose) as carbon source. Microbial lignocellulose degradation has been well-studied for both aerobic- and anaerobic conditions; however, its degradation during denitrification remains largely unknown. Here we have applied a combination of gas kinetics and meta-omics techniques to enrich and analyze microbial communities from 10 eutrophic lakes to identify a set of core microbial metagenome-assembled genomes (MAGs) present in all the eutrophic lakes. We have further investigated their strategies and enzyme profiles for degrading lignocellulose under denitrifying conditions. We identified Pseudomonadota, Bacteroidota, Verrucomicrobiota, and Actinomycetota as the most abundant phyla and they were present in enrichments from all eutrophic lakes having a key role in denitrification and fermentation. Lignocellulose degradation was, however, dominated by species outside the core microbiome, i.e., there were differing key degraders between lakes, suggesting some level of lake-specialization. Among these we observed potential respiratory DNRA pathways, and they expressed a broad range of CAZymes targeting the various lignocellulose subfractions. Interestingly, many of the detected MAGs contained NO dismutases, enzymes postulated to convert NO to molecular oxygen and dinitrogen gas.