Mesodinium rubrum exhibits genus-level but not species-level cryptophyte prey selection

The marine ciliate Mesodinium rubrum is known to form large non-toxic red water blooms in estuarine and coastal upwelling regions worldwide. This ciliate relies predominantly upon photosynthesis by using plastids and other organelles it acquires from cryptophyte prey. Although M. rubrum is capable of ingesting different species of cryptophytes, mainly Teleaulax amphioxeia plastids have been detected from wild M. rubrum populations. These observations suggest that either M. rubrum is a selective feeder, or T. amphioxeia are taken up because of higher availability. To test these hypotheses, we determined whether the ciliate showed different grazing rates, growth responses, or plastid retention dynamics when offered Storeatula major, T. amphioxeia, T. acuta, or a mix. When M. rubrum was offered the cryptophyte S. major as prey, no evidence was found for ingestion. In contrast, M. rubrum grazed both Teleaulax spp. equally, was able to easily switch plastid type between them, and the ratio of each in the ciliate reflected the abundance of free-living prey in the culture. M. rubrum grew equally well when acclimated to each plastid type or when having mixed plastids. However, when offered single prey, T. amphioxeia could sustain higher M. rubrum growth rates (mu) over longer periods. Compared to other M. rubrum strains, this culture had higher grazing rates, greater ingestion requirements for reaching mu(max), and appeared to rely more on plastid sequestration than de novo division of cryptophyte organelles. Our results suggest that while M. rubrum may prefer Teleaulax-like cryptophytes, they do not select among the species used here.