*Article Micractinium tetrahymenae* **(Trebouxiophyceae, Chlorophyta), a New Endosymbiont Isolated from Ciliates**

#### **Thomas Pröschold 1,\*, Gianna Pitsch 2 and Tatyana Darienko 3**


Received: 28 April 2020; Accepted: 13 May 2020; Published: 15 May 2020

**Abstract:** Endosymbiosis between coccoid green algae and ciliates are widely distributed and occur in various phylogenetic lineages among the Ciliophora. Most mixotrophic ciliates live in symbiosis with different species and genera of the so-called *Chlorella* clade (Trebouxiophyceae). The mixotrophic ciliates can be differentiated into two groups: (i) obligate, which always live in symbiosis with such green algae and are rarely algae-free and (ii) facultative, which formed under certain circumstances such as in anoxic environments an association with algae. A case of the facultative endosymbiosis is found in the recently described species of *Tetrahymena*, *T. utriculariae*, which lives in the bladder traps of the carnivorous aquatic plant *Utricularia reflexa*. The green endosymbiont of this ciliate belonged to the genus *Micractinium*. We characterized the isolated algal strain using an integrative approach and compared it to all described species of this genus. The phylogenetic analyses using complex evolutionary secondary structure-based models revealed that this endosymbiont represents a new species of *Micractinium*, *M. tetrahymenae* sp. nov., which was further confirmed by the ITS2/CBC approach.

**Keywords:** *Micractinium tetrahymenae*; Tetrahymena; *Utricularia*; facultative endosymbiosis; ciliate-algae symbiosis
