Population growth and plastid type of Myrionecta rubra depend on the kinds of available cryptomonad prey

Myrionecta rubra, a cosmopolitan red tide ciliate, is not only a predator on cryptomonads but also is a main prey known so far for survival of harmful algae Dinophysis species. This common mixotrophic ciliate functionally seems to have plastid preference, but this possibility has been rarely studied using genetic analysis combined with growth rates of M. rubra. We investigated dynamics of plastids of starved M. rubra (strain MR-MAL01) with the addition of cryptomonads having different types of plastids as prey. Population growth of '2-month starved' M. rubra strain MR-MAL01 was 0.09±0.014d^-1 when supplied with cryptomonad prey strain CR-MAL03, in sharp contrast to the growth rate of the predator strain MR-MAL01, 0.48±0.059d^-1 with prey strain CR-MAL01. Within 30min after the addition of cryptomonad prey strain CR-MAL03 to a '3-week starved' M. rubra population, both the 'CR-MAL03 type' 1192-bp PCR product and the 'CR-MAL01 type' 802-bp PCR product from diagnostic primers, which are specific to plastid 16S rRNA gene, were amplified from the ciliates. Only the 802-bp PCR product was detected in the ciliates supplied only with prey strain CR-MAL01. Both kinds of PCR product from M. rubra were continuously detected for 2 weeks after being washed free of prey in the treatment supplied with cryptomonad prey strain CR-MAL03. The M. rubra strain MR-MAL01 thus can ingest both prey strains, CR-MAL01 and CR-MAL03, and can retain plastids of strain CR-MAL03 for an extended period of time. The differential phototrophic growth of M. rubra after the inoculation of the two different cryptomonad prey species implies that M. rubra sometimes can retain plastids with which it is not normally found in a symbiotic relationship, in spite of a resultant lower growth rate.