Sergio A. Muñoz-Gómez1, Martin Kreutz2, Thilo Bauer3, Sebastian Hess4


Symbioses between eukaryotes and anoxygenic photosynthesizers are exceedingly rare in nature; only two examples have ever been reported. One of these, the ciliate Pseudoblepharisma tenue, provides an intracellular niche to both purple bacteria and green algae. Because of such an unusual combination of disparate physiologies, P. tenue is physiologically complex. However, P. tenue also appears to be ecologically specialized as it combines anoxygenic photosynthesis and phagotrophic predation in organic-rich and hypoxic sediments. To gain further insights into the origin of the P. tenue symbiotic consortium, we focused next on its closest relatives. Alongside the purple-green P. tenue in Simmelried pond samples, we found a purely green ciliate similar to P. tenue, also known as P. tenue var. chlorelligera. The green ciliate has its cytoplasm packed with green algae and, in contrast to P. tenue, also harbors two different kinds of colorless intracellular bacteria. Phylogenetic analyses reveal that the purple-green and the purely green ciliates are sisters, and together are tentatively sister to all known Spirostomum species. Moreover, they both share the same green algal symbiont related to Chlorella sp. K10, which suggests that this symbiosis predates their evolutionary divergence. By contrast, the bacterial symbionts of P. tenue var. chlorelligera are distantly related to the purple bacterial symbionts of P. tenue, but are closely related to each other, and possibly belong to the sister genera Ca. Accumulibacter and Propionivibrio in the Gammaproteobacteria. To explain the origin of a unique purple-green symbiosis, we argue based on these findings, that a facultatively anaerobic lifestyle predisposed the ancestor of P. tenue to acquire sediment-dwelling green algae and purple bacteria. Because the green algal symbionts were acquired first, they may have paved the way for the acquisition of purple bacteria that used their fermentative byproducts as a source of electrons. Such a series of events may help explain why purple symbioses are so rare in nature.

International Society of Protistologists, Conference Meeting, 26–30 July 2021

Status: Accepted talk

1 Institute for Zoology, Cologne Biocenter, University of Cologne, Cologne, Germany
2 Private Laboratory, Constance, Germany
3 Private Laboratory, Bornheim, Germany
4 Institute for Zoology, Cologne Biocenter, University of Cologne, Cologne, Germany


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