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An unusual symbiosis of a ciliate, green alga, and purple bacterium

Phys.org
 2021-06-14

Cover picture for the articleDr. Sebastian Hess and his team at the University of Cologne's Institute of Zoology have studied a very rare and puzzling tripartite symbiosis. This consortium consists of a ciliate as host and two types of endosymbionts: A green alga and a previously unknown purple bacterium. Through genetic analyses of the pink-green ciliate, the researchers discovered that the endosymbiotic bacterium belongs to the so-called "purple sulfur bacteria" (family Chromatiaceae), but has lost the ability to oxidize reduced sulfur compounds, a hallmark of the other members of the Chromatiaceae. The genome of the purple bacterium is greatly reduced, suggesting that the bacterium became mainly specialized in carbon fixation through photosynthesis. It is probably no longer able to live outside of the host cell. Thus, the new bacterial species, Candidatus Thiodictyon intracellulare, is a notable exception among the known purple sulfur bacteria.

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