Cell-associated viral community composition and its functional potential in a dimictic lake on the Canadian Shield

The Turkey Lake Watershed (TLW), located in Northern Ontario, forms a cascading lake system that ultimately drains into Lake Superior. Historically, the TLW has served as a key research site for studies on acid rain and climate change. Despite its long-standing ecological importance, investigations into its microbial communities remain limited, and to date, no studies have explicitly examined the viral component of this watershed. A major obstacle to such research is the challenge of obtaining sufficient viral biomass from remote locations like the TLW for downstream sequencing and analysis. To address this, we focused on viral signatures associated with microbial cells, which require less biomass at the time of collection. Using this approach, we conducted a seasonal metagenomic survey of Big Turkey Lake (BTL)--one of the primary lakes within the TLW--from summer 2018 to winter 2020. Our study characterized the diversity and composition of cell-associated viral communities, predicted potential host relationships, and identified auxiliary metabolic genes (AMGs) to better understand the functional roles of these viruses. The results revealed a dynamic viral community that varied across seasons. Most viral contigs were classified within the class Caudoviricetes, while members of the Phycodnaviridae emerged as important non-bacteriophage contributors to the viral assemblage. We also reconstructed six draft viral metagenome-assembled genomes (vMAGs), ranging in size from 11 to 45 kbp, all identified as Caudoviricetes, with two being potential temperate phages, and a third containing ribosomal genes. Notably, AMGs associated with lipid metabolism were detected exclusively in a winter sample, suggesting a potential season- or niche-specific role for these genes in BTL, while AMGs related to phosphorus acquisition were present in all samples. Together, these findings provide the first insight into the viral ecology of the TLW and establish a foundation for future studies investigating the roles of viruses in shaping the microbial dynamics in this important habitat.