The impacts of two strains of the dinoflagellate Alexandrium tamarense, differing in lytic activity, on the abundance and the composition of microbial communities (<150μm) were studied in North Sea water during spring with Phaeocystis globosa as a dominant species. Cell-free suspensions (supernatant) of exponentially growing lytic and non-lytic Alexandrium culture were added at different concentrations to natural microbial communities under nutrient rich conditions. The non-lytic strain had a positive impact on diatoms whereas the lytic strain suppressed phytoplankton growth in comparison to the control. P. globosa, present as single cells in the initial community, increased in abundance and formed colonies in all treatments. However, total abundance and number of colonies was low with lytic Alexandrium additions, whereas shape of the colonies, but not abundance of cells, was affected by non-lytic Alexandrium additions. During the 4-day experiment, bacterial abundance was constantly higher with high lytic additions (highest concentration equivalent to 1000cellsml-1) whereas nanoflagellate abundance in the same treatments was found to be lower at the end of the experiment. Initial bacterial community composition differed significantly among lytic Alexandrium, non-lytic Alexandrium and North Sea water. However, neither bacterial activity nor composition was significantly affected by the supernatants after 96h. Our results indicated that Alexandrium allelochemicals do not inhibit growth and production of bacteria in seawater collected during spring in the North Sea. © 2010 Elsevier B.V.