PHOSPHORUS LIMITATION AND NOT LIGHT CONTROLS THE EXTRACELLULAR RELEASE OF ALLELOPATHIC COMPOUNDS BY TRICHORMUS-DOLIOLUM (CYANOBACTERIA)

The planktonic freshwater cyanobacterium Trichormus doliolum. (Bharadw .) Komarek and Anagnostidis released secondary metabolites that strong ly inhibited the growth of other cyanobacteria. The major allelopathic compound inhibited PS II-mediated photosynthetic electron transport. The allelopathic compounds were released under P-limited and replete e xponential growth; the release of the major allelopathic compound, how ever, increased 30-fold under P-limited growth. Increased irradiance u nder P-limited growth led to elevated release of dissolved organic car bon, but there was no increase in release of the major allelopathic co mpound. This suggests that there are different control mechanisms for extracellular release of gross carbon and allelopathic secondary metab olices in T. doliolum. Allelopathic compounds released by T. doliolum grown under phosphorus limitation significantly reduced the growth of Anabaena variabilis even when the biomass of the latter was 20-fold gr eater than that of T. doliolum. This suggests that under P limitation less abundant species can exert allelopathic effects on more abundant taxa.