The functional importance of bacteriophages in the microbial loopof an oligomesotrophic lake over a diel cycle

Abstract

The abundances of the different compartments of the microbial loop (i.e., viruses, heterotrophic bacteria, heterotrophic nanoflagellates, and pigmented nanoflagellates), total (TPP) and excreted (EPP) primary production, bacterial production (BP), viral lytic activity (LA), and bacterivory by nanoflagellates (FG) were measured on June 15 and 16, 1998, in a moderate-altitude oligomesotrophic lake (Lac Pavin, France), at 5 and 10 m depths. At both depths, losses of the bacterial community by viral lysis (LA^{5 m} = 1.7 x 10⁶ cells.1^-1.h^-1, LA^{10 m} = 2.0 x 10⁶ cells.1^-1.h^-1) were, on average, lower than those due to the grazing activity of flagellates (FG^{5 m} = 10.3 x 10⁶ cells.1^-1.h^-1 , FG^{10 m} = 8.4 x 10⁶ cells.1^-1.h^-1). A carbon budget exercise indicated that, for the sampling period and depths, 17.8 % of C from TPP (= 38.1 % of EPP) was used by bacteria. On the other hand, 52.7 % of BP (= 2.15 % of TPP) was grazed by nanoflagellates, while 11.0 % of BP (= 0.45 % of TPP) was lysed by viruses.