Utilization of interferometric light microscopy for the rapid analysis of virus abundance in a river

There is a constant need for direct counting of biotic nanoparticles such as viruses to unravel river functioning. We used, for the first time in freshwater, a new method based on interferometry differentiating viruses from other particles such as membrane vesicles. In the French Marne River, viruses represented between 42 and 72% of the particles. A spring monitoring in 2014 revealed their increase (2.1 × 10⁷ to 2.1 × 10⁸ mL^-1) linked to an increase in algal biomass and diversity of bacterial plankton. Predicted virus size distributions were in agreement with transmission electron microscopy analysis suggesting a dominance of large viruses (≥60 nm).