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 which flows and eventually connects into Lake Superior. Historically, the TLW has been used extensively for studying the effects of acid rain and other aspects related to climate change. While microbes are integral components of all aquatic habitats, investigations into TLWs microbial community are still relatively limited, and there are no studies that have specifically investigated the viral fraction of that community. One reason for this oversight is the difficulty in obtaining enough virus biomass from a remote location such as the TLW, which could then be used effectively in downstream sequencing and analysis. To overcome this, we investigated viral signatures associated with microbial cells, which requires less biomass at the time of sample collection. We performed a seasonal metagenomics study of Big Turkey Lake (BTL), one of the main lakes in the TLW, to examine cell-associated viral communities in the summer of 2018, spring-autumn of 2019, and winter of 2020. We evaluated viral diversity, predicted potential host identity, and performed an analysis of auxiliary metabolic genes (AMGs) to gain additional functional insights into the detected viruses and their potential impacts. Through our analysis, we reveal a dynamic viral community across seasons. The majority of viral contigs (56-70%) belonged to unclassified viruses. Of those that were classified, [~]2-25% were assigned to the recently abolished Caudovirales order (now Caudoviricetes class), and[~] 1.5-19% were in the viral family Phycodnaviridae; i.e., viruses which normally infect eukaryotic algae. Six draft viral metagenome-assembled genomes (vMAGs) with a genome size range of 11-45 kbp were also reconstructed and identified as Caudovirales. Interestingly, AMGs for lipid metabolism were only detected in the winter, suggestive of a potential niche specific role for these AMGs in Big Turkey Lake. Collectively, these insights provide an essential footing for further work on the role of viruses in this important freshwater habitat.