The removal of extracellular DNA from marine environments occurs through two basic mechanisms: (1) by competent bacterial cells internalizing DNA fragments (Lorenz and Wackernagel 1994; Dubnau 1999), and (2) by cell-associated and free DNases converting DNA into nucleosides and nucleobases suitable for bacterial uptake (Paul et al. 1988; Paul and Pichard 1989; Ammerman and Azam 1991). DNases being present in all aquatic environments, this latter process is expected to be the main route for extracellular DNA cycling (Lorenz and Wackernagel 1994).