Microviridae, a family of bacteria-infecting ssDNA viruses, is one of the still poorly characterized bacteriophage groups, even though it includes phage PhiX174, one of the main models in virology for genomic and capsid structure studies. Recent studies suggest that they are diverse and well represented in marine and freshwater virioplankton as well as in human microbiomes. However, their diversity, abundance, and ecological role are completely unknown in soil ecosystems. Here we present the comparative analysis of 17 completely assembled Microviridae genomes from 12 viromes of a Sphagnum-dominated peatland. Phylogenetic analysis of the conserved major capsid protein sequences revealed the affiliation to Gokushovirinae and Pichovirinae as well as to two newly defined subfamilies, the Aravirinae and Stokavirinae. Additionally, two new distinct prophages were identified in the genomes of Parabacteroides merdae and Parabacteroides distasonis representing a potential new subfamily of Microviridae. The differentiation of the subfamilies was confirmed by gene order and similarity analysis. Relative abundance analysis using the affiliation of the major capsid protein (VP1) revealed that Gokushovirinae, followed by Aravirinae, are the most abundant Microviridae in 11 out of 12 peat viromes. Sequences matching the Gokushovirinae and Aravirinae VP1 matching sequences, respectively, accounted for up to 4.19 and 0.65% of the total number of sequences in the corresponding virome, respectively. In this study we provide new genome information of Microviridae and pave the way toward quantitative estimations of Microviridae subfamilies.

Viruses impact microbial activity and carbon cycling in various environments, but their diversity and ecological importance in Sphagnum-peatlands are unknown. Abundances of viral particles and prokaryotes were monitored bi-monthly at a fen and a bog at two different layers of the peat surface. Viral particle abundance ranged from 1.7 x 106 to 5.6 x 108 particles mL-1, and did not differ between fen and bog but showed seasonal fluctuations. These fluctuations were positively correlated with prokaryote abundance and dissolved organic carbon, and negatively correlated with water-table height and dissolved oxygen. Using shotgun metagenomics we observed a shift in viral diversity between winter/spring and summer/autumn, indicating a seasonal succession of viral communities, mainly driven by weather-related environmental changes. Based on the seasonal asynchrony between viral and microbial diversity, we hypothesize a seasonal shift in the active microbial communities associated with a shift from lysogenic to lytic lifestyles. Our results suggest that temporal variations of environmental conditions rather than current habitat differences control the dynamics of virus-host interactions in Sphagnum-dominated peatlands.