Datum: 19.04.2024

The nitroplast: A nitrogen-fixing organelle

Eukaryotic cells are notably complex—for example, they have various organelles, which are membrane-bound structures with specific functions. Two of these organelles, mitochondria and chloroplasts, which function in respiration and photosynthesis, evolved from the integration of endosymbiotic bacteria to the eukaryotic cell (1). In marine systems, some nitrogen-fixing bacteria are endosymbionts of microalgae, such as Candidatus Atelocyanobacterium thalassa (UCYN-A), a cyanobacterial symbiont of the unicellular algae Braarudosphaera bigelowii (2). On page 217 of this issue, Coale et al. (3) report a close integration of the endosymbiont into the architecture and function of the host cell, which is a characteristic of organelles. These findings show that UCYN-A has evolved from a symbiont to a eukaryotic organelle for nitrogen fixation—the nitroplast—thereby expanding a function that was thought to be exclusively carried out by prokaryotic cells to eukaryotes.

Biological nitrogen fixation, which reduces atmospheric dinitrogen gas (N2) into reactive ammonia (NH3), is central in the nitrogen biogeochemical cycle as the only path to incorporate the abundant dinitrogen gas into biomass. This process represents a main driver of fertilization for aquatic and terrestrial systems and is continuously studied to increase crop yields in agriculture (4). To directly benefit from the resulting ammonia, many photosynthetic organisms, from terrestrial plants to microalgae, incorporate nitrogen-fixing symbionts (5). This is the case for B. bigelowii and relatives (belonging to the algal class Prymnesiophyceae) that carry the nitrogen-fixing UCYN-A cyanobacteria. The UCYN-A symbiont lacks the genes for the oxygen-evolving photosystem II and carbon fixation, which suggests that it is unable to perform oxygenic photosynthesis and is involved in a tight partnership with the host, providing it with fixed nitrogen and receiving fixed carbon in return (6). This symbiosis is now known to be very stable, to be widespread in sunlit coastal and oceanic waters, and to play a crucial role in the nitrogen biogeochemical cycle (2). However, challenges in obtaining stable cultures of B. bigelowii and UCYN-A have limited studies on this symbiosis.
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