Characterization and genome analysis of a psychrophilic methanotroph representing a ubiquitous Methylobacter spp. cluster in boreal lake ecosystems

Lakes and ponds are considered as a major natural source of CH 4 emissions, particularly during the ice-free period in boreal ecosystems. Aerobic methane-oxidizing bacteria (MOB), which utilize CH 4 using oxygen as an electron acceptor, are one of the dominant microorganisms in the CH 4 -rich water columns. Metagenome-assembled genomes (MAGs) have revealed the genetic potential of MOB from boreal aquatic ecosystems for various microaerobic/anaerobic metabolic functions. However, experimental proof of these functions, i.e., organic acid production via fermentation, by lake MOB is lacking. In addition, psychrophilic (i.e., cold-loving) MOB and their CH 4 -oxidizing process have rarely been investigated. In this study, we isolated, provided a taxonomic description, and analyzed the genome of Methylobacter sp. S3L5C, a psychrophilic MOB, from a boreal lake in Finland. Based on phylogenomic comparisons to MAGs, Methylobacter sp. S3L5C represented a ubiquitous cluster of Methylobacter spp. in boreal aquatic ecosystems. At optimal temperatures (3–12 °C) and pH (6.8–8.3), the specific growth rates (µ) and CH 4 utilization rate were in the range of 0.018–0.022 h −1 and 0.66–1.52 mmol l −1 d −1 , respectively. In batch cultivation, the isolate could produce organic acids, and the concentrations were elevated after replenishing CH 4 and air into the headspace. Up to 4.1 mM acetate, 0.02 mM malate, and 0.07 mM propionate were observed at the end of the test under optimal operational conditions. The results herein highlight the key role of Methylobacter spp. in regulating CH 4 emissions and their potential to provide CH 4 -derived organic carbon compounds to surrounding heterotrophic microorganisms in cold ecosystems.