The genetic diversity of plastids associated with mixotrophic oligotrich ciliates

Mixotrophic oligotrich ciliates use plastids sequestered from algal prey to acquire energy and metabolic products from photosynthesis. We isolated mixotrophic oligotrichs from coastal waters off Massachusetts (MA) and California (CA), as well as from two freshwater ponds in MA, and identified associated plastid 23S rRNA genes. Ciliates were identified using a combination of microscopy and 18S rRNA phylogeny, and included Laboea strobila and various Pseudotontonia, Spirotontonia, and Strombidium species from marine waters, and Limnostrombidium viride from freshwater. Overall, nearly half of all plastid sequences recovered from ciliates were haptophytes, followed by 15-20% for stramenopiles and chlorophytes, and < 10% originating from cryptophyte algae. No plastid sequences were from dinoflagellates. Cells of Strombidium 'biarmatum' collected from coastal MA in spring and fall possessed mostly Micromonas plastids, but during spring also possessed cryptophyte sequences. During spring, L. strobila were found to have a mix of chlorophyte, cryptophyte, haptophyte, and stramenopile sequences, while a co-occurring Strombidium sp. had mostly chlorophyte and haptophyte sequences. Mixotrophic Pseudotontonia and Spirotontonia spp. were sampled during summer from coastal MA and in Monterey Bay, CA, and cells from both populations were dominated (> 70%) by haptophyte plastids of similar phylogenetic origin. L. viride were also evaluated from two freshwater ponds and possessed mostly Chrysochromulina sp. (haptophyte) and synurid (stramenopile) sequences. These results are the first survey of the genetic diversity of plastids associated with pelagic mixotrophic oligotrich ciliates and suggest that some species may selectively retain plastids from certain algal groups, while others appear to be generalists.