Ingestion of bacterial cells by the marine photosynthetic ciliate Myrionecta rubra

We report the first observation of fluorescently labeled bacteria (FLB) ingestion by the cells of MR-MAL01, a temperate strain of the marine photosynthetic ciliate Myrionecta rubra Jankowski 1976 (= Mesodinium rubrum Lohmann 1908). We also investigated the time course of ingestion and digestion of bacteria as well as the ingestion rates at 3 different light intensities. In the stationary phase of growth with ambient bacterial abundance (1.4 × 10⁶ cells ml^-1) under conditions of 15°C and 60 μE m^-2 s^-1, the rates of FLB uptake and disappearance of M. rubra MR-MAL01 cells were 7.6 and 5.3 FLB grazer^-1 h^-1, respectively. The ingestion rate of M. rubra in cultures was calculated to be 53 bacteria grazer^-1 h^-1. The initial abundance of M. rubra (ca. 1.0 × 10⁴ cells ml^-1) in the ingestion-digestion experiment was comparable to the natural abundance during the M. rubra red tide in Korean coastal waters. The bacterivory rate of M. rubra increased gradually as light intensity decreased from 200 to 0 μE m ^-2 s^-1; this might enable the photosynthetic M. rubra to survive under intermittently light-limiting conditions. The present study showed a novel phenomenon of the ingestion of bacterial cells by M. rubra and the light-affected bacterivory rates of the common red tide ciliate.