Phytoplankton-bacteria interactions: an apparant paradox? Analysis of a model system with both competition and commensalism

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Abstract
Mineral nutrient limitation seems to stimulate phytoplankton excretion of extracellular organic carbon (EOC). Bacterial growth on EOC requires additional uptake of mineral nutrients. A paradoxical situation is created: algae stressed by lack of mineral nutrients respond in a manner whereby they stimulate their competitors for the lacking nutrients. To investigate this paradoxical ecological relation, a simple mathematical model was analyzed and compared to a biological model system. The biological model system consisted of the diatom Skeletonema costatum and a strain of a marine bacterium cultured together in chemostats with phosphate as the limiting mineral nutrient. Increasing mineral nutrient limitation by decreasing dilution rate resulted in chemostat equilibria with more bacteria and less algae, confirming the proposed paradox. Algae outcompeted themselves at low dilution rates. Predictions of the simple mathematical model were in agreement both with observed distribution of P between the 2 populations and with amount of algal C biomass in the mixed culture. Implications of results for natural aquatic ecosystems are briefly discussed.

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