Margarita Kniaziuk

• Methane is the most abundant hydrocarbon on Earth, and plays a vital role in the global carbon cycle. In marine ecosystems, large quantities of produced methane are oxidized by methane-oxidizing microorganisms before it reaches the atmosphere. Aerobic methanotrophic bacteria, which consume methane in the upper oxic layers of marine sediments and the water column, represent the final oceanic methane filter. To date, the majority of marine aerobic methanotrophs remains uncultivated, with currently only nine formally described cultures. This hinders our understanding of their physiology that ultimately controls their activity and affects the dispersal of these methanotrophs in nature. In order to fill this gap in knowledge, the present work was focused on the isolation and characterization of marine methanotrophs from the North Sea and the Western Scheldt estuary sediments. The obtained methanotrophic cultures were investigated in physiological tests, and their metabolism was reconstructed based on high quality genomes. The isolation of four new methanotrophic species affiliated to the genera Methyloprofundus and Methylomarinum allowed to determine specific ecophysiological preferences and key conserved and distinct features within these genera. The isolate of Methylomarinum sarcina B3 exhibited a sarcina-like cell organization, which has not been previously reported for any marine methanotroph. The unusual Embden-Meyerhof-Parnas pathway identified in the new Methyloprofundus spp. could potentially serve as an alternative to the canonical glycolytic route. Finally, the discovery of a new putative nitrate reductase in Methyloprofundus spp. could have important implications for the understanding of the diversity of bacterial nitrate reductases and anaerobic respiration. Altogether, this work has advanced the characterization of the Methyloprofundus and Methylomarinum genera and laid the foundation for future research in microbial carbon and nitrogen metabolism.