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 lo wland lakes. In this work, the seasonal dynamics and structure of the virio plankton 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 u nder ice (4.6 x 10(6) VLP ml(-1)) with a second maximum in autumn. After ic e-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 wa s negatively correlated with the viral abundance in the upper 2 ra of the w ater column (Spearman rank correlation, r(s) = -0.773, p < 0.01). The virio plankton 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 w ere 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 ecosyste ms suggests that FVLP are well adapted to the harsh environmental condition s or are specific to bacterial hosts found in alpine lakes. Finally, betwee n 5 and 28% of the newly produced bacteria were killed by non-filamentous v iruses, which therefore are a modest cause of bacterial mortality in this l ake.