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Cu+-chaperones are a diverse group of proteins that allocate Cu+ions to specific copper-proteins, creating different copper pools targeted to specific physiological processes. Symbiotic nitrogen fixation carried out in legume root nodules indirectly requires relatively large amounts of copper e.g. for energy delivery via respiration, for which targeted copper deliver systems would be required. MtNCC1 is a nodule-specific Cu+-chaperone encoded in theMedicago truncatulagenome, with a N-terminus Atx1-like domain that can bind Cu+with picomolar affinities. This gene is expressed primarily from the late infection zone to the early fixation zone, and is located in the cytosol, associated to plasma and symbiosome membranes, and within nuclei. Consistent with its key role in nitrogen fixation,ncc1mutants have a severe reduction of nitrogenase activity, and a 50% reduction in copper-dependent cytochromecoxidase activity. A subset of the copper-proteome is also affected in the mutant nodules. Many of these proteins can be pulled-down when using a Cu+-loaded N-terminal MtNCC1 moiety as a bait, indicating a role in nodule copper homeostasis and in copper-dependent physiological processes. Overall, these data suggest a pleiotropic role of MtNCC1 in copper delivery for symbiotic nitrogen fixation.

SUMMARYBarium (Ba) and strontium (Sr) are often used as proxies for the reconstruction of past marine productivity and global climate. The ability to accumulate Ba2+ and Sr2+ in the form of crystals is rare among eukaryotes. Here we report that unicellular heterotrophs called diplonemids (Euglenozoa), one of the most abundant groups of marine planktonic protists, accumulate conspicuous amounts of these trace elements in the form of intracellular barite (BaSO4) and celestite (SrSO4) crystals, in concentrations greater than in other known Ba/Sr-accumulating organisms. Moreover, these flagellates can uptake Sr2+ exclusively or together with Ba2+ and form (Ba,Sr)SO4. One species, Namystinia karyoxenos, is naturally capable of intracellular accumulation of Ba2+ and Sr2+ 42,000 and 10,000 times relative to the surrounding medium. Altering the amounts of both elements in the medium resulted in corresponding changes in the quantity and composition of crystals. Planktonic copepods fed with diplonemids produce celestite-rich fecal pellets, which facilitate deposition of these minerals on the seafloor. We propose that diplonemids, which emerged during the Neoproterozoic era, qualify as impactful players of Ba2+/Sr2+ cycling in the ocean that possibly contributed to sedimentary rock formation over long geological periods.