It has been observed that the ciliate
Euplotes grows significantly in mass cultures of the rotifer
Brachionus plicatilis when the cultures deteriorate. To reveal the growth factors of
Euplotes sp., we carried out culture experiments with individually inoculated
Euplotes sp. by using five different microbes preferred by the rotifer for food, and observed the intracellular contents of the ciliate by microscopy.
Euplotes sp. showed high growth rates (μ
max; 1.0∼2.0) on the food microbes including the inactive microalgae,
Nannochloropsis oculata. However, active cells of
N. oculata did not promote the growth of
Euplotes sp., but significantly suppressed it compared with the control experiment of seawater alone. Furthermore, the active alga was never observed within the cells of
Euplotes sp. These findings suggest that
Euplotes sp. competes for dietal microbes preferred by
B. plicatilis except for active
N. oculata, and that active
N. oculata is an effective food for preventing the proliferation of contaminated
Euplotes sp. in a stable culture of the rotifer.
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