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Nitroplast: The Fourth Primary Endosymbiosis

Priscilla Leung
Oct 25, 20244 min read

Over billions of years, eukaryotic cells have become incredibly complex, with characteristic organelle structures performing various functions for the growth and maintenance of living organisms. However, the formation of such organelles is largely unknown; that is, until the fourth primary endosymbiosis occurred where scientists were able to witness the appearance of a novel organelle – nitroplast (Figure 1), thus sparking further discussions on previous theories on the origins of organelles in eukaryotic cells, as well as biotechnological applications of nitroplasts. 


The Endosymbiotic Theory: Origins & Evidence


Primary endosymbiosis was a pivotal event in the evolution of eukaryotic cells. It describes the process by which a cell engulfs another through phagocytosis,  but instead of digesting it, the smaller, engulfed cell develops a symbiotic relationship with the host cell. In exchange for protection, the smaller cell would be able to carry out specific tasks for the host’s development. This theory suggests that billions of years ago, the smaller cells aforementioned were individual prokaryotic microbes, and after it was engulfed, it is known as an endosymbiont. The difference between that and an organelle is its production of proteins; typically, the endosymbiont encodes all necessary proteins in its genome whereas the proteins of an organelle are stored in the host cell’s nucleus and are transported to the protein after translation. Over time, however, the endosymbiont may lose its DNA to the host, become increasingly integrated and dependent on the host, evolving into an organelle. 


It is likely through this process that eukaryotic cells formed, which are characterized by the presence of membrane-bounded organelles. 


For instance, according to the endosymbiotic theory, mitochondria was once a prokaryotic cell, i.e. bacteria that were specialized in producing chemical energy in the form of adenotriphosphate (ATP). By a random event, mitochondria was integrated into another prokaryotic cell, and where the host cell provided shelter and safety from the external environment, mitochondria supplied ATP for the host cell, allowing for higher biological complexity. Eventually, eukaryotic cells evolved to utilize mitochondria as an organelle, and the mitochondria lost its cell wall and nugatory DNA, making it impossible for it to survive outside the cell.


Some pieces of evidence that supports this (in regards to the mitochondria):

  • Mitochondria divides by binary fission as do prokaryotic cells

    • The mitochondria of protists, including algae have FtsZ homologues (Filamenting temperature-sensitive mutant Z) which are typically present in bacterial cells and encode protein responsible for recruiting cells in the assembly of new cell wall during cell division, like actin in most eukaryotic cells

  • Mitochondria also has its own circular DNA and their ribosome subunits are smaller, i.e. 30S and 50S


A similar hypothesis is regarded for the formation of chloroplasts in plant cells. 


What are Nitroplasts?


As aforementioned, the basis of endosymbiosis is a symbiotic relationship between endosymbiont and host cell. 


Nitrogen is essential for the survival and growth of plants as it is used in the production of vital compounds like amino acids. However, this element is inaccessible to most organisms, and plants must rely on nitrogen-fixing  bacteria in the root microbiome. That is, until recently, a novel organelle has been found in Braarudosphaera bigelowii, a eukaryotic marine alga which allows them the ability to fixate nitrogen. 


Fun Fact: The Braarudosphaera bigelowii is a coccolithophore (See FIgure 2), unicellular marine organism that heavily rely on oceans, existing everywhere since over a 100 million years ago, and have shield-like structures on its surface for reasons unknown. The B. bigelowii specifically has 12 pentaliths, which are calcareous scales (meaning made of calcium carbonate) with five-fold symmetry. It has twelve sides, forming a dodecahedron shape. Their coccosphere allows them to capture a lot of carbon and calcium, playing a crucial role in the ocean’s carbon cycle.


The nitroplast in Braarudosphaera bigelowii is believed to have originated from an endosymbiont, an unknown nitrogen-fixing cyanobacterium, or more specifically Candidatus Atelocyanobacterium thalassa (UCYN-A). It has a reduced genome and is closely linked to B. bigelowii cells, i.e. scaled cell-organelle sizes and synchronized growth rates. 



In addition, scientists noticed that UCYN-A lacks key metabolic pathways like tricarboxylic acid cycle (used for energy production) and RuBisCO (ribulose bisphosphate carboxylase, an enzyme involved in the fixation of carbon during photosynthesis), but are present in the B. bigelowii cell. UCYN-A would also receive proteins, specifically enzymes, from the B. bigelowii cell for production of certain amino acids and nucleotides which are used in pathways incomplete in UCYN-A. Therefore, it must not be able to function independently and scientists posit that the UCYN-A is an organelle, naming it nitroplast.  


The discovery of this organelle could have immense impact in agriculture; currently, fertilizers are used to aid plants in nitrogen intake. Unfortunately, such fertilizers are often detrimental or even toxic to the environment, resulting in nutrient or biodiversity loss. If plants could be genetically modified to have nitroplasts in their cells, this could reduce the need for such fertilizers and boost agricultural productivity. 





Works Cited


AH Documentary (2024). Two Lifeforms Merging To Create A New Cell - Nitroplast. [online] YouTube. Available at: https://www.youtube.com/watch?v=hFpps4hmHXE [Accessed 1 Oct. 2024].


Anton Petrov (2024). Incredible Discovery of an Entirely New Organelle That Fixes Nitrogen. [online] YouTube. Available at: https://www.youtube.com/watch?v=eGkV_k8IcQ0 [Accessed 1 Oct. 2024].



Clark, D.P., Pazdernik, N.J. and McGehee, M.R. (2019). Regulation of Protein Synthesis. Molecular Biology (Third Edition). doi:https://doi.org/10.1016/b978-0-12-813288-3.00018-5.


Coale, T.H., Loconte, V., Turk-Kubo, K.A., Vanslembrouck, B., Mak, E., Cheung, S., Ekman, A., Chen, J.-H., Hagino, K., Takano, Y., Nishimura, T., Adachi, M., Mark Le Gros, Larabell, C. and Zehr, J.P. (2024). Nitrogen-fixing organelle in a marine alga. Science, 384(6692), pp.217–222. doi:https://doi.org/10.1126/science.adk1075.


Haavisto, V. (2024). Beyond Endosymbiosis: Discovering the First Nitroplast. [online] ASM.org. Available at: https://asm.org/Articles/2024/June/Beyond-Endosymbiosis-Discovering-First-Nitroplast.



Kiefel, B.R., Gilson, P.R. and Beech, P.L. (2004). Diverse Eukaryotes have Retained Mitochondrial Homologues of the Bacterial Division Protein FtsZ. Protist, 155(1), pp.105–115. doi:https://doi.org/10.1078/1434461000168.


Massana, R. (2024). The nitroplast: A nitrogen-fixing organelle. Science, 384(6692), pp.160–161. doi:https://doi.org/10.1126/science.ado8571.


Wikipedia. (2024). Braarudosphaera bigelowii. [online] Available at: https://en.wikipedia.org/wiki/Braarudosphaera_bigelowii.

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