Diversity of viruses infecting non‐extremophilic archaea has been grossly understudied. This is particularly the case for viruses infecting methanogenic archaea, key players in the global carbon biogeochemical cycle. Only a dozen of methanogenic archaeal viruses have been isolated so far. In the present study, we implemented an original coupling between stable isotope probing and complementary shotgun metagenomic analyses to identify viruses of methanogens involved in the bioconversion of formate, which was used as the sole carbon source in batch anaerobic digestion microcosms. Under our experimental conditions, the microcosms were dominated by methanogens belonging to the order Methanobacteriales (Methanobacterium and Methanobrevibacter genera). Metagenomic analyses yielded several previously uncharacterized viral genomes, including a complete genome of a head‐tailed virus (class Caudoviricetes, proposed family Speroviridae, Methanobacterium host) and several near‐complete genomes of spindle‐shaped viruses. The two groups of viruses are predicted to infect methanogens of the Methanobacterium and Methanosarcina genera and represent two new virus families. The metagenomics results are in good agreement with the electron microscopy observations, which revealed the dominance of head‐tailed virus‐like particles and the presence of spindle‐shaped particles. The present study significantly expands the knowledge on the viral diversity of viruses of methanogens.
DNA Stable Isotope Probing is emerging as a powerful tool to study host-virus interactions. Indeed, since all viruses depend on a host for virion production, a link between the isotopic compositions of hosts and the virions they produce is expected. However, stable isotope probing applied to viral DNA has never been evaluated on simple biological models. Here, this method was tested on the bacteriophage T4 and its host Escherichia coli. To validate that E. coli cells cultivated using a substrate enriched in 13C isotope were resulting on the production of 13C-labeled T4 DNA. T4 DNA buoyant density in CsCl gradient was overall higher than the values predicted by a previously established empirical model, highlighting the need to adapt this type of models when analysing modified viral DNA. Moreover, our results show a strong correlation between the proportion of 13C6-D-glucose in the substrate used for host growth and the buoyant density of T4 DNA, validating the use of DNA SIP in viral ecology, to identify viruses infecting hosts with a specific metabolism.
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