Ingestion of cryptophyte cells by the marine photosynthetic ciliate Mesodinium rubrum

Abstract

We investigated the mechanism of capturing and ingesting cryptophyte cells by a labo- ratory strain of the marine photosynthetic ciliate Mesodinium rubrum Lohmann 1908 (= Myrionecta rubra Jankowski 1976), a cosmopolitan red tide species. When offered cryptophytes as food, M. rubrum, originally grown photosynthetically for 2 wk, used its bifurcated oral tentacles to instantly seize prey cells when encountered. Immediately after capturing a prey cell, M. rubrum swam in a zigzag pattern (30 to 60 µm long linear paths) for >4 s, without showing the large jumps (with ca. 2000 µm long linear paths) that were usually observed when the predator was not feeding. M. rubrum with a cryptophyte attached to its tentacles became motionless while the prey cell was moved to the oral surface of the predator, a process that took <10 s. Engulfment of a captured prey cell by M. rubrum occurred through a cytostome-like structure and took ca. 15 s. Once engulfed, the prey was slowly delivered to the posterior end of the ciliate over a period of ca. 63 s. The whole feeding process lasted approximately 92 s. With increasing mean prey concentration, specific growth rates of M. rubrum feeding on the cryptophyte increased, with saturation at a mean prey concentration of 44 cells ml -1 . The maximum specific growth rate (mixotrophic growth) of M. rubrum feeding on the cryptophyte was 0.521 d -1 , under continuous illumination of 60 µE m -2 s -1 , while its growth rate (phototrophic growth) under the same light conditions without added prey was 0.357 d -1 . The inges- tion rate of M. rubrum feeding on cryptophytes increased continuously with increasing prey concen- tration. The maximum ingestion rate was 8.9 cryptophytes ciliate -l d -1 . M. rubrum may sometimes