Abstract Fonsibacter (LD12 subclade) are among the most abundant bacterioplankton in freshwater ecosystems. These bacteria belong to the order Pelagibacterales (SAR11) and are related to Pelagibacter (marine SAR11) that dominate many marine habitats. Although a handful of Pelagibacter phage (Pelagiphage) have been described, no phage that infect Fonsibacter have been reported. In this study, a complete Fonsibacter genome containing a prophage was reconstructed from metagenomic data. A circularized and complete genome related to the prophage, referred to as uv-Fonsiphage-EPL, shows high similarity to marine Pelagiphage HTVC025P. Additionally, we reconstructed three complete and one draft genome of phage related to marine Pelagiphage HTVC010P, and predicted a lytic strategy. The similarity in codon usage and co-occurrence patterns of HTVC010P-related phage and Fonsibacter suggested that these phage infect Fonsibacter . Similar phage were detected in Lake Mendota, Wisconsin, where Fonsibacter is also present. A search of related phage revealed the worldwide distribution of some genotypes in freshwater ecosystems, suggesting their substantial role in shaping indigenous microbial assemblages and influence on biogeochemical cycling. However, the uv-Fonsiphage-EPL and one lineage of HTVC010P-related phage have a more limited distribution in freshwater ecosystems. Based on this, and their close phylogenetic relatedness with Pelagiphage, we predict that they transitioned from saline into freshwater ecosystems comparatively recently. Overall, the findings provide insights into the genomic features of phage that infect Fonsibacter , and expand understanding of the ecology and evolution of these important bacteria.

Wide distribution of phage that infect freshwater SAR11 bacteria