Virus morphological diversity and relationship to bacteria and chlorophyll across a freshwater trophic gradient in the Lake Michigan watershed

The critical roles of viruses in nutrient cycling are well established for marine ecosystems, but less is known for freshwater viruses about diversity, seasonality, and relationships with productivity and ecosystem trophic status. This study aimed to examine virus morphotypes and abundance in relation to prokaryotic cell abundance and chlorophyll a concentrations in three seasonal temperate freshwater sites representing eutrophic, mesotrophic, and oligotrophic status in Lake Michigan watershed. We found strong correlations between virus-like particle abundance (VLPA) and prokaryote abundance (PA) and chlorophyll concentration across the trophic gradient (r = 0.744, P < 0.001, n = 123), but without consistent seasonal patterns. TEM showed Myoviridae, Podoviridae, Siphoviridae, and non-tailed virus morphotypes. The eutrophic site was distinct with one- to two-fold higher PA and VLPA than mesotrophic and oligotrophic sites. But the eutrophic and mesotrophic sites had more similar VLPA:PA ratios (eutrophic 26, mesotrophic 25.5, oligotrophic 4.1). The oligotrophic site also had a lower proportion of Podoviridae and more Siphoviridae than the other sites. The similarity between eutrophic and mesotrophic sites, but a distinctive oligotrophic site, suggests that the effects of terrestrial runoff as sources of viruses and hosts may be important, along with P limitation and trophic status, in determining virus-host relationships in freshwaters.