Nitroplast

A nitroplast is a nitrogen-fixing organelle found in the unicellular marine alga Braarudosphaera bigelowii. It is thought to have originated from a cyanobacterium (blue-green alga) and is still in the early stages of its evolution. It is the first known organelle capable of fixing atmospheric nitrogen, thereby potentially overcoming the inability of eukaryotes to fix nitrogen.

Enabling nitrogen fixation through endosymbiosis: The birth of the nitroplast

Nitrogen is an essential component of important biomolecules, such as DNA and amino acids (the building blocks of proteins). Atmospheric nitrogen, abundantly occurring as dinitrogen gas (N₂), is inaccessible to eukaryotic plants and animals. Consequently, before eukaryotes can use it, it must be converted into a bioavailable form (a form that a living system can use): ammonia (NH₃). This process of converting nitrogen into ammonia, called nitrogen fixation, is carried out in nature by several species of bacteria, some of which are symbiotic. These bacteria live in close association with eukaryotic host plants and receive shelter and photosynthetic products in exchange for fixing nitrogen. Despite this close association, the bacteria largely retain their structural and functional integrity and remain distinct from the host.

However, the marine cyanobacterium Candidatus Atelocyanobacterium thalassa, commonly called UCYN-A (short for unicellular cyanobacteria nitrogen-fixing group A), has integrated almost completely with its unicellular algal host, B. bigelowii, via endosymbiosis. Through this integration, it has formed a nitrogen-fixing subcellular structure called a nitroplast. This nitroplast is an organelle that enables its eukaryotic host to fix nitrogen.

Evidence for endosymbiotic origin

The endosymbiotic origin of the nitroplast was established in phases as a result of research on UCYN-A and B. bigelowii that spanned decades. Following the cyanobacterium’s discovery in the late 1990s, metagenomic studies established that UCYN-A cannot live independently outside the cell body of its host—i.e., it is an obligate endosymbiont—because it lacks essential genes for vital metabolic pathways, including photosystem II, RuBisCO, and the tricarboxylic acid cycle. Once scientists could grow the host alga in the laboratory, they discovered that UCYN-A and B. bigelowii maintain a distinct and consistent size ratio: the host-to-symbiont proportions remain the same across different sublineages of the host alga. This means that as a B. bigelowii cell grows, the UCYN-A inside it grows proportionately. Mathematical modeling later showed that this precise size ratio allows the most efficient exchange of nitrogen and photosynthetically fixed carbon between the host and UCYN-A, thereby optimizing their synchronized growth. The discovery of synchronized growth—similar to the way eukaryotic organelles such as mitochondria and chloroplasts grow in tandem with their host cells—led to the characterization of UCYN-A as “organelle-like.”

To qualify as an organelle, however, an “organelle-like” endosymbiont must also divide in sync with its host cell and import host proteins. In 2024, through soft X-ray tomography, a technique for real-time visualization of intact eukaryotic cells, the replication and division of UCYN-A and its host were shown to be tightly coupled. In effect, the algal host strongly controls the fate of its UCYN-A, and each daughter cell arising from the dividing algal parent cell receives a copy of UCYN-A. This process loosely mirrors the transmission of mitochondria and chloroplasts during eukaryotic cell division.

Furthermore, proteomics—a branch of molecular biology that looks at the structures, functions, and interactions of all the proteins expressed in a cell or a tissue—revealed that almost half of all proteins found in UCYN-A are derived from its algal host. These proteins, vital for the functioning of UCYN-A, are tagged with specific amino acid sequences for transport from the host cytoplasm into UCYN-A. Mitochondria and chloroplasts in eukaryotes, thought to have evolved from endosymbiotic bacteria, likewise depend on host-derived proteins for their growth and functioning, which supports the analogy between UCYN-A and these organelles.

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In 2024, scientists classified UCYN-A as an early evolutionary stage nitrogen-fixing organelle, or, more specifically, a nitroplast, because it checks the boxes of host protein importation and coordinated division with the host cell.

Evolutionary significance and ecological impact

The length of the amino acid tags on proteins that are imported into an organelle from the host cell cytoplasm has given scientists an idea of how long ago the organelle evolved from an endosymbiont. The host proteins imported into UCYN-A suggest an evolution of the nitroplast more recent than that of mitochondria and chloroplasts. The nitroplast is thought to be no older than 100 million years, and thus at an early stage of evolution. This offers the opportunity to study the way organelles evolve from such events as endosymbiosis.

The widespread distribution of UCYN-A and B. bigelowii in open oceans, combined with UCYN-A’s ability to fix nitrogen in nutrient-poor environments, indicates that the nitroplast plays a significant role in marine protein production. Experts suggest introducing nitroplast genes or the organelle itself into crop plants through genetic engineering to impart to the plants the ability to fix nitrogen on their own. This could reduce dependence on nitrogen fertilizers, which are a major source of agricultural carbon dioxide emissions, while boosting terrestrial plant productivity and promoting sustainable agriculture.