AME 41:247-260 (2005)  -  doi:10.3354/ame041247

ABSTRACT: Mono Lake is a large, alkaline, moderately hypersaline lake containing planktonic prokaryotes and viruses at concentrations that are among the highest reported for natural aquatic environments. We hypothesized that pronounced seasonality in physical and biological forcing and strong vertical gradients of chemical, physical, and biological parameters in this meromictic lake would result in dramatic temporal and spatial variability in concentrations of viruses and viral infections of prokaryotes. To test this, we investigated the temporal, vertical, and horizontal variability in hydrography, microbial concentrations, and viral infections of prokaryotes at 4 stations over a 10 mo period in Mono Lake. The infection parameters quantified included the frequency of visibly infected cells (FVIC), burst size, intracellular virus diameter, and volume of infected cells. Concentrations of chlorophyll a (chl a), prokaryotes, and viruses in individual samples ranged from 3.3 to 150 µg l^–1, 0.10 to 1.2 × 10¹¹ l^–1, and 0.14 to 1.9 × 10¹² l^–1, respectively, with the highest concentrations of each occurring in the spring. For all data combined, concentrations of viruses were significantly correlated with concentrations of prokaryotes (r = 0.68, p < 0.001, n = 68), but not with chl a. FVIC ranged from <0.1 to 3.5% for the community, but reached as high as 13% for coccoid cells in 1 sample. Averaged over the water column, the estimated fraction of prokaryote mortality due to viral lysis ranged from a low of 3.7% in September, to a high of 16% in July. Burst size, intracellular virus diameters, and volumes of infected cells were temporally variable with a trend of decreasing burst size through the spring and summer as a result of larger viruses infecting smaller cells. In contrast, these parameters did not differ systematically among stations or between the anoxic and oxic layers of the lake. The data suggested that seasonal forcing is the primary source of variability in viral infections in the lake. Overall, viral lysis appeared to make a modest contribution to the mortality of prokaryotes, but high virus–host contact rates suggested that viruses are likely to influence the clonal diversity of picoplankton in the lake.

KEY WORDS: Prokaryotes · Viruses · Hypersaline lake · Variability