Methanosarcina Spherical Virus, a Novel Archaeal Lytic Virus Targeting Methanosarcina Strains
Authors: Weidenbach Katrin , Nickel Lisa , Neve Horst , Alkhnbashi Omer S. , Künzel Sven , Kupczok Anne , Bauersachs Thorsten , Cassidy Liam , Tholey Andreas , Backofen Rolf , Schmitz Ruth A. 
Published: September 2017

A novel archaeal lytic virus targeting species of the genus
Methanosarcina
was isolated using
Methanosarcina mazei
strain Gö1 as the host. Due to its spherical morphology, the virus was designated
Met
hanosarcina
s
pherical
v
irus (MetSV). Molecular analysis demonstrated that MetSV contains double-stranded linear DNA with a genome size of 10,567 bp containing 22 open reading frames (ORFs), all oriented in the same direction. Functions were predicted for some of these ORFs, i.e., such as DNA polymerase, ATPase, and DNA-binding protein as well as envelope (structural) protein. MetSV-derived spacers in CRISPR loci were detected in several published
Methanosarcina
draft genomes using bioinformatic tools, revealing a potential protospacer-adjacent motif (PAM) motif (TTA/T). Transcription and expression of several predicted viral ORFs were validated by reverse transcription-PCR (RT-PCR), PAGE analysis, and liquid chromatography-mass spectrometry (LC-MS)-based proteomics. Analysis of core lipids by atmospheric pressure chemical ionization (APCI) mass spectrometry showed that MetSV and
Methanosarcina mazei
both contain archaeol and glycerol dialkyl glycerol tetraether without a cyclopentane moiety (GDGT-0). The MetSV host range is limited to
Methanosarcina
strains growing as single cells (
M. mazei
,
Methanosarcina barkeri
and
Methanosarcina soligelidi
). In contrast, strains growing as sarcina-like aggregates were apparently protected from infection. Heterogeneity related to morphology phases in
M. mazei
cultures allowed acquisition of resistance to MetSV after challenge by growing cultures as sarcina-like aggregates. CRISPR/Cas-mediated resistance was excluded since neither of the two CRISPR arrays showed MetSV-derived spacer acquisition. Based on these findings, we propose that changing the morphology from single cells to sarcina-like aggregates upon rearrangement of the envelope structure prevents infection and subsequent lysis by MetSV.

IMPORTANCE
Methanoarchaea are among the most abundant organisms on the planet since they are present in high numbers in major anaerobic environments. They convert various carbon sources, e.g., acetate, methylamines, or methanol, to methane and carbon dioxide; thus, they have a significant impact on the emission of major greenhouse gases. Today, very little is known about viruses specifically infecting methanoarchaea that most probably impact the abundance of methanoarchaea in microbial consortia. Here, we characterize the first identified
Methanosarcina
-infecting virus (MetSV) and show a mechanism for acquiring resistance against MetSV. Based on our results, we propose that growth as sarcina-like aggregates prevents infection and subsequent lysis. These findings allow new insights into the virus-host relationship in methanogenic community structures, their dynamics, and their phase heterogeneity. Moreover, the availability of a specific virus provides new possibilities to deepen our knowledge of the defense mechanisms of potential hosts and offers tools for genetic manipulation.