Antagonistic Interactions between Microalga Nannochloropsis oculata (Eustigmatophyceae) and Ciliate Euplotes sp.

Abstract

To understand the functional roles of microorganisms in aquaculture, the interaction between the microalga Nannochloropsis oculata and the ciliate Euplotes sp., which occurs in mass culturing of the rotifer Brachionus plicatilis, was examined. We carried out culture experiments of Euplotes sp. initially inoculated at densities of 1 cell/drop 0.15ml, 30 cells ml^-1, and 400 cells ml^-1. Euplotes sp. were fed dietary bacteria together with N. oculata (initial density; 1×10⁷ cells ml^-1), the algal culture filtrate, or no algae (control experiment). N. oculata at a density of 1×10⁷ cells ml^-1 or more clearly suppressed the growth of Euplotes sp. in all experiments. The final yields were 20∼49% of the controls. However, the culture filtrate of N. oculata did not suppress the growth of Euplotes sp. On the contrary, the co-existence of increased Euplotes sp. of ca. 1000 cells ml^-1 or more distinctly caused the algae to aggregate, and inhibited the algal growth by 80% of the final densities. Consequently, antagonistic interactions, appearing to be density-dependent, were found to occur between these microbes. To our knowledge, this is the first report on the anti-ciliate activity of the algae.