Microorganisms play a major role in driving biogeochemical cycles in virtually all environments. In the carbon cycle, methanotrophs metabolize methane, a well-known greenhouse gas, as their main carbon and energy source and are ecologically relevant as methane sink. Oxidation of methane can be performed via aerobic or anaerobic pathways. In the anaerobic oxidation of methane (AOM) different strategies have evolved. One example is the recently discovered ‘Candidatus Methylomirabilis oxyfera’. M. oxyfera performs methane oxidation coupled to nitrite reduction via an intra-aerobic pathway. M. oxyfera shows unique features also with respect to its cell biology. It is a rod- and polygonal-shaped bacterium with ridges running all along the cell length and ending in a cap-like structure at the cell poles.
Whereas the importance of microorganisms in regulating elemental cycles has been well assessed through the years, the relevance of viruses remains largely unexplored. However, by shaping microbial communities through mortality, horizontal gene transfer and metabolic reprogramming, viruses are postulated to be main drivers in most ecosystems.
Making use of advanced electron microscopy techniques, we investigated the ultrastructure of the bacteriophage and its interaction with the host. Thin sections of the bacteriophages showed that the phages have a hexagonal capsid that encloses an electron dense core, which probably contains the genetic (DNA) material. In pseudo 3D samples obtained by freeze-etching, the proteinaceous capsid appeared to have an icosahedral symmetry, displaying triangular faces and capsomeres subunits. The bacteriophages possessed neither a tail nor appendages. Electron tomography on infected M. oxyfera cells showed that, in addition to completely assembled bacteriophages, entities were observed inside the cells that contained only the central core, i.e. the capsid was not assembled yet. In addition, the bacteriophage population was isolated from the bioreactor system and used to obtain a metaviriome. Eventually, we speculate on the significance of the viral infection for the bacterial community.