Seasonal variation in viral-induced mortality of bacterioplankton in the water column of a large mesotrophic lake (Lake Biwa, Japan)

Viruses are ubiquitous and abundant in aquatic systems, yet knowledge of virus-bacteria interactions in thermally stratified water columns of large lakes is limited. We explored the possible factors that affect viral abundance, infection rate, and the relative importance of viral lysis to heterotrophic nanoflagellate (HNF) grazing as a mortality factor of bacterioplankton in the upper (euphotic zone, 5 m) and deeper (aphotic zone, 50 m) layers of the large mesotrophic Lake Biwa, Japan. Data obtained for a full seasonal cycle indicated that bacterial abundance was the best predictor (r(2) = 0.85) of viral abundance (range 1.0 x 10(10) to 4.1 x 10(10) viruses l(-1)), yielding an average virus:bacteria ratio of 8.2 +/- 1.3 (SD). Variation in the frequency of visibly infected cells (range 1.8 to 4.1 %) was largely accounted for by the linear combination of bacterial production and HNF abundance (r(2) = 0.81, p < 0.001, n = 24). The percentage of daily bacterial production destroyed by viruses was estimated to be high (52.7 +/- 16.2 %) in the upper layer during the stratification period, which was on average 3.0-fold greater than the percentage of bacterial production consumed by HNF in that layer. In contrast, the corresponding value in the deeper layer was moderate (13.6 +/- 5.2 %), being 0.6-fold lower than the percentage of bacterial production consumed by HNF. Our data suggest that carbon and nutrient flux patterns controlled by viruses and HNF vary with depth in thermally stratified water columns of Lake Biwa.