Virioplankton in the lake Sevan
In 2009 quantitative distribution of viruses and virus-induced mortality of heterotrophic bacterioplankton were studied in the water column of alpine lake Sevan during spring-summer period. Virioplankton abundance measured using epifluorescence microscopy ranged from 21,4x10 <sup>6</sup> to 75,9x10 <sup>6</sup> particles/ml reaching maximum numbers in July. Virus-to-bacterium numbers ranged from 2,8 to 11,2. There was moderate correlation between viral and bacterial numbers in the littoral of the lake Sevan. Viral infection of bacterioplankton was measured by transmission electron microscopy (TEM). Bacteriophages infect from 1,8 to 14,6% of bacteria with up to 36 viruses released upon host cell lysis. A peak of infection occurs at the beginning of May and the end of August. Despite the high numbers of viruses their impact on heterotrophic bacterioplankton in the lake Sevan was not significant: viruses lysed less than 14,6% of bacterial production and release of dissolved organic carbon during viral-mediated lysis of bacteria may compensate only a small fraction (average 2,0-3,4%) of bacterioplankton ration.