Inhibition of growth and photosynthesis of the dinoflagellate Peridinium gatunense by Microcystis sp. (cyanobacteria): A novel allelopathic mechanism

We describe a novel allelopathic interaction whereby the cyanobacterium Microcystis sp. inhibits photosynthesis in the freshwater dinoflagellate Peridinium gatunense by abolishing its internal carbonic anhydrase activity. Our analysis indicated a positive correlation between the winter presence of Microcystis and the timing of the annual spring bloom of Peridinium in Lake Kinneret (Sea of Galilee, Israel). Microcystis severely inhibited the growth of Peridinium in mixed laboratory cultures. This was attributed to the excretion of allelopathic substances rather than to successful competition for nutrients. Microcystis-free spent medium (MFSM) inhibited steady-state photosynthesis of Peridinium, as indicated by the reduced rate of oxygen evolution and by a suppressed fluorescence signal. Boosting the inorganic carbon concentration to 25 mmol L^-1, i.e., 10-fold higher than essential to saturate photosynthesis in the absence of MFSM, partially restored the photosynthetic capacity. MFSM-treated Peridinium cells accumulated a larger internal inorganic carbon pool concomitantly with reduced accumulation of photosynthetic products. The MFSM slowed the hydration rate of CO₂ (assessed from the rate of ¹⁸O exchange between ¹³C¹⁸O₂ and water), indicating that the activity of an internal carbonic anhydrase was severely depressed. Although the nature of the active component in the MFSM that inhibits CA activity has yet to be identified, the ecological consequences of its excretion to the water body are emerging as a potent allelopathic substance that may control the development of phytoplankton competitors. We propose that its allelopathic activity could control the fate and succession of the P. gatunense bloom in Lake Kinneret.