High abundance of viruses found in aquatic environments

THE concentration of bacteriophages in natural unpolluted waters is in general believed to be low^1,2, and they have therefore been considered ecologically unimportant³. Using a new method for quantitative enumeration, we have found up to 2.5 x IO⁸ virus particles per millilitre in natural waters. These concentrations indicate that virus infection may be an important factor in the ecological control of planktonic micro-organisms, and that viruses might mediate genetic exchange among bacteria in natural aquatic environments.