The algicidal bacterium Kordia algicida shapes a natural plankton community.

Plankton communities consist of complex microbial consortia which change over time with. These fluctuations can only be partially explained by limiting resources. Biotic factors such as herbivores and pathogens also contribute to the control of algal blooms. Here we address the effect of algicidal bacteria on a natural plankton community in an indoor enclosure experiment. The algicidal bacteria, introduced into plankton taken directly from the North Sea during a diatom bloom, caused the rapid decline of the bloom forming Chaetoceros socialis within only one day. The haptophyte Phaeocystis , in contrast, is resistant against the lytic bacteria and could benefit from the removal of the competitor indicated by an onset of a bloom in the treated enclosures. This cascading effect caused by the bacterial pathogen accelerated the succession of Phaeocystis which bloomed with a mere delay of several weeks in the in situ waters at Helgoland Roads, North Sea. The algicidal bacteria can thus modulate the community within the limits of the framework of the abiotic and biotic limitations of the local environment. Implications of our findings for plankton ecosystem functioning are discussed. IMPORTANCE Plankton communities change on a seasonal basis in temperate systems with distinct succession patterns. This is mainly due to algal species that have their optimum timing relative to environmental conditions. We know that bacterial populations are also instrumental in the decay and termination of phytoplankton blooms. Here, we describe algicidal bacteria as modulators of this important species succession. Upon treatment of a natural plankton consortium with an algicidal bacterium, we observed a strong shift in the phytoplankton community structure compared to controls, which results in a succeeding bloom formation of Phaeocystis Blooms of this alga have a substantial impact on global biogeochemical and ecological cycles as they are responsible for a substantial proportion of primary production during spring in the North Sea. We propose that one of the key factors influencing such community shifts may be algicidal bacteria.