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Phytoplankton-bacteria-protozoan interactions: a qualitative model portraying indirect effects

A mathematical model is described that allows study of interactions between microbial organisms and phytoplankton when considered from a community context. In particular, the model is used to understand the paradoxical behaviour of phytoplankton which, when under nutrient stress, stimulates bacteria the very o r g a ~ s m s it competes with for inorganic nutrients. The model demonstrates that such behaviour, although at first sight seemingly detrimental, can be highly stable and provides phytoplankton with definite indirect advantages. Protozoa are found to be important in forming and instrumental in distributing these advantages. A general method is put forward that interprets interactions within their community context and clarifies the role of indirect effects. Such a framework makes it possible to design testable hypotheses to study further the role of microbial communities.