TEMPERATE VIRUSES AND LYSOGENY IN LAKE-SUPERIOR BACTERIOPLANKTON

The morphology and abundance of free viruses were measured in spring, summer, and fall at one site in Lake Superior. Free viral head sizes r anged from 10 to 70 nn and tail length ranged from 10 to 110 nm. The v ast majority (98%) of free viral head sizes were less than or equal to 60 nm, smaller than reported in most freshwater habitats. Most of the se free viruses (70%) had polyhedral heads and tails, indicative of ba cteriophage. Free viral abundance only ranged from 0.1 to 9 x 10(6) vi ruses ml(-1) in the surface microlayer (top 20 mu m) and subsurface wa ter (20 m) in Lake Superior, but viruses were 2-15 times more abundant in the surface microlayer. This difference may be due to the enrichme nt of bacterial hosts, higher levels of UV light that induce temperate phage, or differences in viral burst sizes in the surface microlayer relative to subsurface water. Bacterioplankton were always more abunda nt than free viruses in both the surface microlayer and subsurface wat er, which resulted in some of the lowest virus-to-bacterium ratios rep orted for marine or freshwater environments. Temperate viruses from bo th habitats responded equally to mitomycin-C and UV light treatments u sed to induce prophage into lytic cycles. An estimated 0.1-7.4% of the bacterioplankton from this site in Lake Superior contained temperate prophage depending on viral burst sizes that were assumed. Three times more bacteria in the surface microlayer may contain temperate viruses compared to bacterioplankton in subsurface waters. In the western arm of Lake Superior, bacterioplankton infected by temperate phage may be more important for the survival of bacteriophage populations than as future carbon sources for new microbial production.