Self-toxicity of Prymnesium parvum (Prymnesiophyceae)

The marine and brackish water prymnesiophyte Ptymnesium parvum produces highly potent exotoxins known to have detrimental effects on a variety of aquatic organisms. Culture experiments have revealed a relationship between the toxicity and the physiological state of P. parvum. In the international phycological community it is uncritically and self-evidently assumed that exotoxin-producing phytoplankton themselves remain resistant or immune to these toxins. We show that this is not true and the way of thinking about phytoplankton allelopathy and chemical warfare in the aquatic ecosystems needs to be changed. We demonstrate that cell-free culture filtrate has a self-toxic effect on the same strain of P. parvum not adapted to high exotoxin levels. The negative effect on the target population changed from the initial decline of the growth rate to complete cell lysis as the cell density of the toxin-producing test culture increased. Our findings suggest that (1) P. parvum is sensitive to the self-produced extracellular substances, (2) the sensitivity varies and depends on the adaptation to gradually increasing toxin levels in the growth environment.