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Abstract

Algicidal bacteria are important players regulating the dynamic changes of plankton
assemblages. Most studies on these bacteria have focused on the effect on single
algal species in simple incubation experiments. Considering the complexity of species
assemblages in the natural plankton, such incubations represent an oversimplification
and do not allow making further reaching conclusions on ecological interactions.
Here, we describe a series of co‐incubation experiments with different level of complexity
to elucidate the effect of the algicidal bacterium Kordia algicida on mixed cultures
of a resistant and a susceptible diatom. The growth of the resistant diatom
Chaetoceros didymus is nearly unaffected by K. algicida in monoculture, while cells of
the susceptible diatom Skeletonema costatum are lysed within few hours. Growth of
C. didymus is inhibited if mixed cultures of the two diatoms are infected with the
bacterium. Incubations with filtrates of the infected cultures show that the effects
are chemically mediated. In non‐contact co‐culturing we show that low concentrations
of the lysed algae support the growth of C. didymus, while higher concentrations
trigger population decline. Complex cascading effects of algicidal bacteria have
thus to be taken into account if their ecological role is concerned.