Ecophysiological characterization of novel Chloroflexi to probe the evolution of photosynthesis

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In FY2022, we combined field data, bioinformatics, and laboratory experiments to gain new insights into the ecology and physiology of "Candidatus Chlorohelix allophototropha" strain L227-S17, a highly novel phototrophic bacterium. In the field, we revisited Oze National Park (Fukushima-ken) and detected a second L227-17-like population in iron-rich sediment samples. These results demonstrate that L227-S17 relatives are reoccurring and diverse in a local field site and let us plan to cultivate these organisms in future. Although we did not detect L227-S17 relatives in metagenomes we sequenced from an iron-rich lake (Lake Mizugaki, Yamanshi-ken), we recovered novel genomes bins with potential for iron cycling that shed new light on nutrient cycling in iron-rich ecosystems. We are continuing to analyze our Lake Harutori dataset from FY2021. Using bioinformatics, we probed the global diversity of L227-S17 relatives by searching >160,000 publicly available DNA/RNA sequencing datasets. Genome bins recovered from this survey greatly expand the known diversity of the L227-S17 clade and provide a valuable dataset to explore the evolution of photosynthesis. In the laboratory, we obtained compelling preliminary data that the L227-S17 culture performs light-driven iron(II) oxidation, a poorly studied metabolism thought to have been key to nutrient cycling on the early Earth. Altogether, our ecophysiological study of "Ca. Chx. allophototropha" strain L227-S17 provides an important baseline dataset for probing the ancient iron cycle and history of photosynthesis on Earth.