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Abstract

An axenic culture of Trichormus doliolum Bharadwaja, Komarek et Anagnostidis (Nostocales, Cyanophyta) showed growth inhibition of several axenic cyanobacteria and chlorophytes. Allelopathic activity was observed in both phosphorus-limited and -replete chemostat cultures. Phosphorus limitation led to a change in the composition of the released organic compounds and to increased allelopathic activity of the exudate. Allelopathic activity of exudates of the phosphorus-limited and -replete chemostat was confined to the same high-performance liquid chromatography fraction, indicating that the allelopathic compounds released may be similar if not identical. The exudates inhibited photosynthetic oxygen production and enhanced chlorophyll fluorescence in other cyanobacteria. Growth-inhibiting effects showed a linear dose dependency, indicating that even small concentrations of allelopathic compounds influenced the growth of other cyanobacteria and algae. A biomass of Trichormus doliolum 20 times less than that needed for growth inhibition led to an increase in chlorophyll fluorescence of other algae. This suggests that, under light-limited growth conditions, target organisms are more susceptible to allelochemical compounds that interfere with the photosynthetic electron transport. Experiments conducted under light-saturated conditions are likely to underestimate the potential of allelopathic interactions