The novel application of violacein produced by a marine Duganella strain as a promising agent for controlling Heterosigma akashiwo bloom: Algicidal mechanism, fermentation optimization and agent formulation

Controlling harmful algal blooms with algicidal bacteria is thought to be an efficient and eco-friendly way but lack of comprehensive studies from theory to practice limited the field application. Here we presented a purple bacterial strain Duganella sp. A3 capable of killing several harmful algae, including Heterosigma akashiwo, a world-wide fish-killing microalga. A bioactivity-guided purification and identification approach revealed the major algicidal compound of A3 as the pigment violacein, which was never reported for its algicidal potential before. Violacein rapidly disrupted cell permeability, caused long-term oxidative stress, but mildly affected algal photosystem, which might explain its highly species-specific activity against unarmored H. akashiwo. To explore the application potential of violacein, a fermentation optimization approach combing single-factor and multi-factor experiments was conducted to increase the violacein yield, which finally reached 0.4199 g/L just using a simple medium formula beneficial for compound purification. Finally, taking advantages of the physical and chemical stabilities, we successfully developed the novel application of violacein as a sustained-releasing and easy-to-preserve algicidal agent using alginate-acacia-gum-chitosan encapsulation, which paved the path for its future application in controlling H. akashiwo bloom.