How microbial communities shape peatland carbon dynamics: New insights and implications

Peatlands are considered the most efficient ecosystem for long-term storage of atmospheric carbon (C). However, reasons for variations in C accumulation within peatlands remain largely unexplained. Using a comprehensive multi -level approach combining soil -atmosphere C exchanges, microbial extracellular enzyme activities, and genome-resolved cellular and viral metagenomics, we endeavored to decipher the microbial determinants and their role in C dynamics in the bog and fen of a European peatland. Overall, the bog exhibited a higher C content and dissolved organic carbon concentration. Despite contrasting geochemical conditions, these differences were not explained by environmental parameters nor the vegetation. Metagenomic analyses revealed varying microbial community composition, the bog being less diverse and dominated by Acidobacteriota and the fen comprising five predominant phyla (Crenarcheota, Chloroflexota, Proteobacteria, Desulfobacterota and Acidobacteriota). Both bog and fen microbial communities were stable between spring and summer. Yet, similar CO2 emissions were recorded in both bog and fen, along with similar organic matter (OM) decomposition microbial activities and potential. Ultimately, the bog harbored significantly more viruses than the fen. Most intriguingly, these viruses were predicted to target Acidobacteriota, the phyla displaying the highest OM -degrading capacity in the bog. By impairing the activity of the dominant players in OM degradation, viruses might have a significant role in C dynamics in the bog over time. In addition, we propose that low microbial diversity limited crossfeeding opportunities in the bog, further limiting C degradation. Taken together, this study deciphers the role of microbial communities driving C accumulation in peatlands and, consequently, peatland ecosystem functioning.