Large viruses and infected microeukaryotes in Ross Sea summer pack ice habitats

A variety of Ross Sea summer pack ice habitats between 66 and 75degreesS were examined for viruses greater than or equal to110 nm capsid diameter. Maximum abundances of these viruses likely to infect eukaryotes were 10(6)-10(7) ml(-1) brine in surface, interior, and bottom habitats and constituted up to 18% of the total (all sizes) viruses. There is abundant ultrastructural evidence for infection of a variety of microheterotrophs and some autotrophs. One station exhibited the classical characteristics of a lower latitude algal bloom with potential viral control. The blooming alga, Pyramimonas tychotreta Daugbjerg 2000, was infected, as were two abundant heterotrophs, Cryothecomonas spp. and an unidentified flagellate, that fed on P. tychotreta. Infections were observed in only one life history stage (multiflagellate cells) of P. tychotreta, suggesting a relationship among virus-induced lysis, life-history stages, physiology, and environmental factors regulating the life cycle. There is good evidence that diatoms are not a likely source of the large viruses, and viruses in general are not a major food source for ice microheterotrophs in summer.