A microbial eukaryote with a unique combination of purple bacteria and green algae as endosymbionts

Guest post by Sergio Muñoz-Gómez (@Sergiophyceae).

Our paper on the only purple-green eukaryote know is out! This is a unique eukaryote that harbours two distinct photosynthetic endosymbionts: purple bacteria and green algae. Pseudoblepharisma tenue was first described by A. Kahl in 1926 but was virtually forgotten since then. I teamed up with Sebastian Hess at the University of Cologne (@lab_hess) and light microscopist Martin Kreutz to re-discover P. tenue which had been seen in ponds of the Simmelried moorland (Constance, Germany).

P. tenue is a large purple-green ciliate that prefers to dwell in hypoxic and fluffy organic-rich sediments of shallow forest ponds. Despite its name, Pseudoblepahrisma tenue is more closely related to Spirostomum species, and can either respire aerobically or ferment inside its mitochondrion.

The purple bacterial endosymbionts, 'Candidatus Thiodyction intracellulare', belong to the Chromatiaceae (Gammaproteobacteria). They photosynthesize anoxygenically but lost all known genes for the oxidation of sulfide, thiosulfate and sulfur (sulfur dissmilation)! The green algal endosymbionts are closely related the the Hydra viridissima endosymbiont Chlorella sp. K10. They have a versatile metabolism that includes oxygenic photosynthesis, obviously, but also diverse fermentation pathways.

We believe that the purple symbionts supply photosynthate to the ciliate host, and the green symbionts might produce hydrogen by fermentation for the purple symbionts. We also conclude that P. tenue is a mixotroph with dual lifestyle that combines anoxygenic photosynthesis in the light and (micro-)aerobic phagotrophy.

Check out our paper for more details about how this physiologically complex eukaryote might be making a living! Science writer Abby Olena @abbyolena wrote a beautiful piece about our work in @TheScientistLLC. If you want to know some of the back-story of our paper, go check out her article!

Paper: A microbial eukaryote with a unique combination of purple bacteria and green algae as endosymbionts. https://advances.sciencemag.org/content/7/24/eabg4102

SciComm article: A Protist Hosts Both Green Algae and Purple Bacteria Symbionts. https://www.the-scientist.com/news-opinion/a-protist-hosts-both-green-algae-and-purple-bacteria-symbionts-68875