Melissa Steele-Ogus grew up in Berkeley, California. She received a BS in Environmental Sciences and BA in Molecular Biology from the University of California, Berkeley, in 2012. She earned a PhD in Biology from the University of Washington in 2021, studying the actin cytoskeleton of Giardia lamblia. In her free time, she enjoys dancing, baking, and birdwatching. She may be secretly some sort of weird bug, but probably isn't.

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  • The endosymbiont ofEpithemia clementinais specialized for nitrogen fixation within a photosynthetic eukaryote. ISME communications Moulin, S. L., Frail, S., Braukmann, T., Doenier, J., Steele-Ogus, M., Marks, J. C., Mills, M. M., Yeh, E. 2024; 4 (1): ycae055


    Epithemia spp. diatoms contain obligate, nitrogen-fixing endosymbionts, or diazoplasts, derived from cyanobacteria. These algae are a rare example of photosynthetic eukaryotes that have successfully coupled oxygenic photosynthesis with oxygen-sensitive nitrogenase activity. Here, we report a newly-isolated species, E. clementina, as a model to investigate endosymbiotic acquisition of nitrogen fixation. We demonstrate that the diazoplast, which has lost photosynthesis, provides fixed nitrogen to the diatom host in exchange for fixed carbon. To identify the metabolic changes associated with this endosymbiotic specialization, we compared the Epithemia diazoplast with its close, free-living cyanobacterial relative, Crocosphaera subtropica. Unlike C. subtropica, in which nitrogenase activity is temporally separated from photosynthesis, we show that nitrogenase activity in the diazoplast is continuous through the day (concurrent with host photosynthesis) and night. Host and diazoplast metabolism are tightly coupled to support nitrogenase activity: Inhibition of photosynthesis abolishes daytime nitrogenase activity, while nighttime nitrogenase activity no longer requires cyanobacterial glycogen storage pathways. Instead, import of host-derived carbohydrates supports nitrogenase activity throughout the day-night cycle. Carbohydrate metabolism is streamlined in the diazoplast compared to C. subtropica with retention of the oxidative pentose phosphate pathway and oxidative phosphorylation. Similar to heterocysts, these pathways may be optimized to support nitrogenase activity, providing reducing equivalents and ATP and consuming oxygen. Our results demonstrate that the diazoplast is specialized for endosymbiotic nitrogen fixation. Altogether, we establish a new model for studying endosymbiosis, perform a functional characterization of this diazotroph endosymbiosis, and identify metabolic adaptations for endosymbiotic acquisition of a critical biological function.

    View details for DOI 10.1093/ismeco/ycae055

    View details for PubMedID 38707843