Growth responses of the marine photosynthetic ciliate Myrionecta rubra to different cryptomonad strains

The photosynthetic Myrionecta rubra (=Mesodinium rubrum) is a cosmopolitan ciliate that freguently feeds on free-living cryptomonad species in diverse marine environments. Hence, M. rubra might feed on a wide range of cryptomonad species. To explore this possibility, we investigated the growth responses of M. rubra strain MR-MAL01, which was starved for 20 d, to diverse cryptomonad strains CR-MAL01, CR-MAL02, CR-MAL03, CR-MAL04, CR-MAL05, and CR-MAL06. Phylogenetic analysis of the aligned 18S rRNA sequences from cryptomonads suggested that the strains CR-MAL01, CR-MAL02, CR-MAL04, and CR-MAL05 were included in cryptomonad Clade 2, while the strains CR-MAL06 and CR-MAL03 were included in cryptomonad Clade 3, with high bootstrap value (98%) and posterior probability of 1. Intriguingly, M. rubra strain MR-MAL01 grew better in the presence of cryptomonad strains CR-MAL01, CR-MAL02, CR-MAL03, CR-MAL04, and CR-MAL05 (0.16 to 0.40 d^-1) than in the absence of a cryptomonad strain (0.011 d^-1), indicating that M. rubra could grow in the presence of diverse cryptomonad prey. However, M. rubra (MR-MAL01) did not grow better in the presence of cryptomonad strain CR-MAL06. Further comparisons of the growth rates of MR-MAL01 showed that cryptomonads in Clade 3 might be less favored than those in Clade 2 by the starved M. rubra. Here, we demonstrated that MR-MAL01 has preferences with regard to potential cryptomonad prey strains, suggesting that the growth of MR-MAL01 may be partially affected by the availability of preferred cryptomonad prey types.