Seasonal abundance in Skagerrak-Kattegat coastal waters and host specificity of viruses infecting the marine photosynthetic flagellate Micromonas pusilla

Seawater sampled in the Skagerrak and Kattegat coastal waters during the period October 1995 to 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 was 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. In the Skagerrak and Kattegat the seasonal abundance of viruses infectious to a M, pusilla strain isolated from the Oslofjord, Norway, was at least 1 order of magnitude higher (average 2.5 x 10(5) l(-1)) than viruses infecting 2 M. pusilla strains isolated from Gull of Maine, USA (average 2.2 x 10(4) and 4.6 x 10(3) l(-1) respectively).