Seasonal dynamics of viruses in an alpine lake: importance of filamentous forms

Viruses are an important component of the planktonic food web in freshwater and marine systems, but most studies have been done in the ocean and in lowland lakes. In this work, the seasonal dynamics and structure of the virioplankton as well as their impact on bacteria during a day/night cycle were studied in an alpine lake located 2417 m above sea level. The abundance of virus-like particles (VLP) was determined at 5 discrete depths (0.5 to 8 m) by direct counts with a TEM in samples collected from May to November 1998 at weekly to bi-weekly intervals. Viruses reached the highest abundances under ice (4.6 x 10(6) VLP ml(-1)) with a second maximum in autumn. After ice-break, the VLP abundance decreased to undetectable values (<2 x 10(4) VLP ml(-1)) probably because of the negative effect of solar radiation that was negatively correlated with the viral abundance in the upper 2 ra of the water column (Spearman rank correlation, r(s) = -0.773, p < 0.01). The virioplankton was morphologically diverse, consisting of forms commonly found in other aquatic systems, but unlike other studies, we found filamentous VLP (FVLP) 450 to 730 run long that attained abundances of up to 1.3 x 10(6) ml(-1) and accounted for 7 to 100% of the total viral abundance. These FVLP were found occasionally inside filamentous heterotrophic bacteria (> 10 mum) and their respective abundances were positively correlated (r(s) = 0.728, p < 0.01). The absence of these conspicuous forms in other aquatic ecosystems suggests that FVLP are well adapted to the harsh environmental conditions or are specific to bacterial hosts found in alpine lakes. Finally, between 5 and 28% of the newly produced bacteria were killed by non-filamentous viruses, which therefore are a modest cause of bacterial mortality in this lake.