Growth and Chloroplast Replacement of the Benthic Mixotrophic Ciliate Mesodinium coatsi.

While the ecophysiology of planktonic Mesodinium rubrum species complex has been relatively well studied, very little is known about that of benthic Mesodinium species. In this study, we examined the growth response of the benthic ciliate Mesodinium coatsi to different cryptophyte prey using an established culture of this species. M. coatsi was able to ingest all of the offered cryptophyte prey types, but not all cryptophytes supported its positive, sustained growth. While M. coatsi achieved sustained growth on all of the phycocyanin-containing Chroomonas spp. it was offered, it showed different growth responses to the phycoerythrin-containing cryptophytes Rhodomonas spp., Storeatula sp., and Teleaulax amphioxeia. M. coatsi was able to easily replace previously ingested prey chloroplasts with newly ingested ones within 4 d, irrespective of prey type, if cryptophyte prey were available. Once retained, the ingested prey chloroplasts seemed to be photosynthetically active. When fed, M. coatsi was capable of heterotrophic growth in darkness, but its growth was enhanced significantly in the light (14:10 h light:dark cycle), suggesting that photosynthesis by ingested prey chloroplast leads to a significant increase in the growth of M. coatsi. Our results expand the knowledge of autecology and ecophysiology of the benthic M. coatsi.