An RNA-Seq library prepared from a Mesodinium major bloom off the coast of Chile.

Bloom-forming marine phytoplankton are major contributors to primary production and key regulators of local biogeochemistry. While many phototrophs have permanently integrated photosystems, some, like the cosmopolitan ciliate Mesodinium rubrum and its sister species Mesodinium major, are acquired phototrophs. That is, they steal and retain functional prey organelles (chloroplasts, mitochondria, and nuclei) to maintain their photosynthetic lifestyle. By functioning as both a predator and a primary producer, Mesodinium has complex influences on coastal ecosystems. While field and laboratory studies have explored the biogeography, cellular architecture, and photophysiology of the M. major/rubrum species complex, the genetic adaptations that allow M. major/rubrum to achieve ecological success while relying on stolen cellular machinery remain unclear. Because the transient integration of prey plastids is thought to be a step along the endosymbiosis pathway to permanent acquisition of phototrophy in eukaryotes, M. major/rubrum and the closely related but entirely heterotrophic congener M. pulex comprise an ideal system to study these adaptations.