Hi-C assembled genomes of estuarine populations reveal virus-microbe associations and a broad interaction range of a cyanophage

Abstract

Aquatic microbes play key roles in global biogeochemical cycles and their viral-induced mortality influences the flow of carbon and nutrients between the dissolved and particulate pools. However, many microbes remain uncultivated, hindering understanding of their metabolic capabilities and preventing isolation of viruses that infect them. Here we augment metagenomic sequencing with Hi-C, a proximity-linkage method whereby DNA within a cell is physically bound and then sequenced to link contigs within a metagenome that originated from the same cell. In a size-fractioned water sample from beneath the euphotic zone in a hypoxic estuarine fjord in Puget Sound, WA we resolved 49 proximity-linked bins above 50% complete, including 21 Hi-C Assembled Genomes (HAGs) over 90% complete and a nearly complete genome of the eukaryotic green alga Picochlorum. Viral and microbial sequence within the same HAG identified 18 virus-microbe interactions. A myovirus and a siphovirus were associated with 2 different genera within the Saltatorellus clade of Planctomycetes, a phylum for which no virus has been identified. A partial Phycodnaviridae genome linked to Haptophyte sequence is consistent with contemporaneous observations of a dissipating coccolithophore bloom. A cyanophage S-CAM7-like sequence had a broad interaction range. It was associated with a partial Synechococcus genome in the >3.0 {micro}m size fraction and with a Gammaproteobacteria related to Alcanivorax in the 0.2{micro}m-3.0{micro}m fraction. We suggest that viruses produced in surface waters that are shuttled to depth on sinking aggregates may interact with different hosts in deeper waters, providing an important avenue for gene transfer across broad taxonomic ranges. ImportanceAquatic microbes are important in global elemental cycling. Knowing which viruses infect them in the environment remains a challenge. Using Hi-C, a molecular technique to physically link DNA within a cell, we assembled nearly complete genomes of both prokaryotes and eukaryotes from a hypoxic estuary. Hi-C links captured virus-host interactions for known virus-host pairs and for hosts with no previously known viruses. The same virus was linked to two distinct microbes in different size fractions of water, suggesting it has a broad host range. Viral lysis in surface waters generates sinking particles that deliver newly produced viruses to deeper waters where they interact with different potential hosts, providing an opportunity for gene exchange between unrelated microbes.