Abundance, morphology and distribution of planktonic virus-like particles in two high-mountain lakes

Direct counts of virus-like particles (VLP) by transmission electron micros copy revealed abundances of up to 3 x 10(7) ml(-1) in the plankton of two r emote high-mountain lakes in the Alps and the Pyrenees. Most VLP were icosa hedric without a tail, and with diameters between 40 and 90 nm, but very la rge ones with diameters of up to 325 nm were also observed. VLP outnumbered bacteria by a factor of 4.2-42.8 and bacterial cells were infected with la rge numbers (>50) of viral particles. This study constitutes the first repo rt on aquatic viruses for alpine lakes and it suggests that they may be an important additional source of bacterial mortality in these systems.