Methanotrophy by a Mycobacterium species that dominates a cave microbial ecosystem.

So far, only members of the bacterial phyla Proteobacteria and Verrucomicrobia are known to grow methanotrophically under aerobic conditions. Here we report that this metabolic trait is also observed within the Actinobacteria. We enriched and cultivated a methanotrophic Mycobacterium from an extremely acidic biofilm growing on a cave wall at a gaseous chemocline interface between volcanic gases and the Earth's atmosphere. This Mycobacterium, for which we propose the name Candidatus Mycobacterium methanotrophicum, is closely related to well-known obligate pathogens such as M. tuberculosis and M. leprae. Genomic and proteomic analyses revealed that Candidatus M. methanotrophicum expresses a full suite of enzymes required for aerobic growth on methane, including a soluble methane monooxygenase that catalyses the hydroxylation of methane to methanol and enzymes involved in formaldehyde fixation via the ribulose monophosphate pathway. Growth experiments combined with stable isotope probing using 13C-labelled methane confirmed that Candidatus M. methanotrophicum can grow on methane as a sole carbon and energy source. A broader survey based on 16S metabarcoding suggests that species closely related to Candidatus M. methanotrophicum may be abundant in low-pH, high-methane environments.

Terminal Bacteroid Differentiation Is Associated With Variable Morphological Changes in Legume Species Belonging to the Inverted Repeat-Lacking Clade.

Medicago and closely related legume species from the inverted repeat-lacking clade (IRLC) impose terminal differentiation onto their bacterial endosymbionts, manifested in genome endoreduplication, cell enlargement, and loss of cell-division capacity. Nodule-specific cysteine-rich (NCR) secreted host peptides are plant effectors of this process. As bacteroids in other IRLC legumes, such as Cicer arietinum and Glycyrrhiza lepidota, were reported not to display features of terminal differentiation, we investigated the fate of bacteroids in species from these genera as well as in four other species representing distinct genera of the phylogenetic tree for this clade. Bacteroids in all tested legumes proved to be larger in size and DNA content than cultured cells; however, the degree of cell elongation was rather variable in the different species. In addition, the reproductive ability of the bacteroids isolated from these legumes was remarkably reduced. In all IRLC species with available sequence data, the existence of NCR genes was found. These results indicate that IRLC legumes provoke terminal differentiation of their endosymbionts with different morphotypes, probably with the help of NCR peptides.