Sahlsten, E.
General and Marine Microbiology, Göteborg University, Box 462, SE-405 30 Göteborg, SWEDEN. E-mail: Sahlsten@gmm.gu.se
Seawater sampled in the Skagerrak and Kattegat coastal waters, during the period October 1995 - September 1996 were screened for the occurrence of viruses lytic to marine microalgae. Viruses lytic to the photosynthetic marine picoflagellate Micromonas pusilla (Butcher) Manton & Parke (Prasinophyceae) were detected in all seawater samples screened. Evidence for viral lysis of any other of the 11 algal species tested were not obtained. Several viruses infecting different strains of M. pusilla were isolated. Ten isolated viruses which were tested for host-specificity were found to be species-specific to M. pusilla and even strain-specific to 1-3 of the 6 strains of M. pusilla used in the experiment. The abundance of viruses infectious to a M. pusilla strain isolated from the Oslofjord, Norway, was at least one order of magnitude higher (average 2.5 * 105 l-1) than viruses infecting two M. pusilla strains isolated from Gulf of Maine, USA (average 2.2 * 104 and 4.6 * 103 l-1, respectively). This is the first time that strain specificity has been reported in MpV. The negative results in detecting lytic infection in the algal species other than M. pusilla, might have been due to the possibility of these algae already being latent infected.