Characterization and description of plant-growth-promoting methanotrophic bacteria belonging to the genus of Methylocaldum

Arid soil microbiomes present untapped resources of microbial diversity. Here, we describe twelve isolates, all belonging to the Methylocaldum genus. Based on metagenomic studies, the isolates represent the major clades of methanotrophic bacteria inhabiting the arid biomes of Southern California, comprising up to 0.03 % of the total soil microbiota. Phenotyping of isolates indicates that they are obligate methanotrophic bacteria, some capable of methanol utilization. All strains can fix nitrogen, use nitrate and ammonia as a N-source, and have key genetic signatures of autotrophy, methylotrophy, and N₂O assimilation. Based on the 16S rRNA phylogeny and whole -genome analyses, all strains are assigned to the species M. gracile. Three isolates from the rhizosphere of native Californian plants (Strains 0917, YM2 and S3V3) and GT1B-W are set apart from the other M. gracile strains, despite sharing <98 % of average nucleotide identity. Microbes isolated from plant rhizosphere display 150 unique genetic features and a series of tandem gene duplications predicted to contribute to their interactions with plants, including the 20-gene polyketide biosynthesis cluster and the TRAP C4-dicarboxylate transport system. Consistent with the genetic properties that may indicate an enhancement of plant-cooperation functions, the rhizosphere isolates support the survival of plants, Boechera depauperata and Arabidopsis thaliana, under drought conditions. Based on genetic and phenotypic characteristics, we propose to designate strains 0917, YM2, S3V3, and GT1B-W as a new subspecies of Methylocaldum gracile - Methylocaldum gracile subspecies dēsertum, L.n. dēsertum - a desert, to represent the native habitat of the species. The amended description of the M.gracile species is provided.