High and differential viral infection rates within bacterial 'morphopopulations' in a shallow sand pit lake (Lac de Créteil, France).

The ecology of viruses in shallow artificial freshwaters is poorly documented and there is no reference for sand pit lakes. We examined the seasonal abundances and infection rates of viruses in the sand pit Lake Créteil (France). Bacteria were the best predictor of viral abundance (4.0-7.8 × 10(10) viruses L(-1) ), with an average virus-to-bacteria ratio of 13.5 ± 1.9. Virus-induced bacterial mortality (range 37-86%, mean 65%) was higher than that in typical pelagic situations. This was related to high specific contact rates between viruses and bacterial hosts and high burst size (BS) estimates. Seasonal fluctuations in viruses and bacteria were rather homeostatic, although temperature was a major driver of microbial activities. Different bacterial morphotypes, i.e. 'morphopopulations', were analysed. Rod cells dominated the total (90%) and infected (89%) communities. Elongated rods were the most infected (45% of infected cells), whereas fat rods exhibited the highest BS estimates (mean=72 viruses per bacterium) due to a larger specific cell volume. We conclude that the lytic activity of viruses is high and heterogeneous for different bacterial-host phenotypes in the sand pit Lake Créteil. A theoretical exercise shows that this can exert a strong influence on the processes occurring in plankton food webs.