Diel remodeling and cellular integration of the nitroplast

Nitrogen-fixing eukaryotes were not believed to exist in nature until the recent discovery of a N₂-fixing organelle, or nitroplast, in the marine microalga Braarudosphaera bigelowii. This nitroplast (formerly known as UCYN-A2) has long been recognized as key cyanobacterial contributor to global oceanic N₂ fixation. However, how this novel organelle is integrated and regulated within the architecture of a eukaryotic cell remains unclear. Here, we combine multiscale volumetric imaging with cryo-electron tomography to resolve the native architecture, cellular integration, and diel remodeling of the nitroplast in cultured and environmental cells. We find that the nitroplast occupies up to 10% of the cell volume and exhibits close interfaces with multiple host organelles through membrane contact sites, while integration of this metabolically demanding compartment does not disrupt global scaling of host organelles. Interestingly, the chloroplast-to-nitroplast volume ratio is conserved across distinct life stages. Cryo-electron tomography reveals that the nitroplast retains a reinforced four-layer cyanobacterial envelope and is additionally surrounded by two host-derived layers that remodel across the day-night cycle. During daytime N₂ fixation, these host-derived barriers become locally discontinuous and the organelle interface becomes enriched with two distinct vesicle populations. Our findings suggest that dynamic control of organelle accessibility through transient membrane gating represents a fundamental strategy by which eukaryotic cells could domesticate new endosymbiotic functions during early organellogenesis.