The Nitroplast: A New Organelle on the Block

Figure 1: The nitroplast organelle located in the B. bigelowii bacterium [1]

Written by Karon Johnson ‘27 

Edited by Andrew Ni ‘26

Remember that one phrase we all repeated in middle school biology, almost like a chant? “The mitochondria is the powerhouse of the cell.” It was simple, catchy, and let’s be honest, probably the only thing most of us took away from the class. But behind that phrase lies a world of intricate cellular machinery. In the world of biology, organelles are the stage where the drama of life unfolds. Mitochondria power our cells. Chloroplasts capture sunlight to fuel plants. For decades, though, scientists wondered: could there ever be a eukaryotic organelle that fixes nitrogen, the element at the heart of every protein, every leaf, and every living thing?

That question has finally been answered. In 2024, researchers of the microscopic world unveiled the nitroplast, the first known nitrogen-fixing organelle, nestled inside a marine alga [1–3]. This discovery doesn’t just rewrite cell biology textbooks; it opens the door to possibilities ranging from more sustainable agriculture to futuristic biotechnology [4–6]. 

The nitroplast, more formally known as the cyanobacteria Candidatus Atelocyanobacterium thalassa (UCYN-A), likely began as a symbiotic bacterium living inside the eukaryotic alga Braarudosphaera bigelowii [1]. Over time, it became a permanent resident and thus an organelle. This kind of relationship is called endosymbiosis and is defined as a beneficial relationship between two organisms, with one organism typically living inside the other [3]; it’s also the same exact process that gave rise to mitochondria and chloroplasts millions of years ago. Understanding exactly how the nitroplast is integrated into B. bigelowii and its nitrogen-fixing are key to understanding how exactly the world can optimize it for the betterment of society.  

In order to confirm the nature of the relationship between the nitroplast and B. bigelowii, microbiologists first had to learn how to culture B. bigelowii, or in other words, grow it in a stable laboratory environment. Thanks to evolutionary biologist Kyoko Hagino-Tomioka, scientists were finally able to culture B. bigelowii cells, and further studies were conducted to study the nitroplast at the molecular level [2]. Utilizing soft X-ray tomography, an imaging technique used to visualize the internal architecture of cells, researchers found mitochondria were wrapped around the nitroplast, as if it were directly feeding it energy [2]. Moreover, the nitroplast was also receiving proteins and other key cellular tools from neighboring organelles, making it clear it was no longer a symbiotic bacterium but a fully integrated organelle 

By confirming the nitroplasts’ role in nitrogen fixation, researchers opened up an entirely new field for biological innovation: the capacity for eukaryotic cells to directly harness atmospheric nitrogen [1,4]. This makes the nitroplast far more than just a new entry in the organelle catalog; it is a proof of principle that challenges long-held assumptions about cellular metabolism [4,5]. The implications are enormous: nitrogen, the ultimate nutrient bottleneck in ecosystems, could one day be fixed by engineered crops themselves [7–9]. Imagine a future where crop plants host nitroplasts, pulling nitrogen directly from the air. Fertilizer factories could become obsolete. Food production could soar while environmental costs plummet, and further efforts towards environmental remediation could be pursued [7–9]. And agriculture isn’t the only field that stands to benefit. Synthetic biology may one day co-opt organelles like the nitroplast to enhance cellular metabolism. In tissue engineering, for instance, adding the capacity to fix nitrogen could boost cell growth and repair [8]. 

Yet the road from discovery to application is not without its obstacles. Incorporating a complex organelle like the nitroplast into other species will require untangling how it coordinates with host metabolism, imports proteins, and exchanges energy with mitochondria [1,2]. Evolutionary history shows that organelles such as mitochondria and chloroplasts took millions of years to fully integrate into cells, suggesting that synthetic biology faces a daunting challenge [3–5]. Still, the very existence of the nitroplast proves that such integration is possible, offering researchers a rare natural model for how eukaryotes and nitrogen-fixing partners can successfully merge [1–3]. 

Fortunately, the story of the nitroplast is still being written. As researchers continue to investigate its biology, the organelle serves as both a window into the functionality of endosymbiosis and a blueprint for the future. Just as the discovery of mitochondria and chloroplasts reshaped our understanding of organelle complexity, the nitroplast demonstrates that evolution is still capable of producing exciting innovations [1–3]. It is a reminder that the natural world remains one of humanity’s greatest educators. Whether the nitroplast eventually fuels agricultural mechanisms, advances biotechnology, or simply strengthens our knowledge of endosymbiosis, its discovery marks a milestone in cell and molecular biology that will resonate for decades to come [4–6].

It turns out that the chant we all learned in middle school, “the mitochondria is the powerhouse of the cell”, may soon need a sequel. The nitroplast shows that life’s cellular machinery still holds surprises waiting to be uncovered. From mitochondria to nitroplasts, evolution continues to change the world around us, revealing that even the tiniest of organelles can change how we power our planet.

References:  

1. Coale TH, Loconte V, Turk-Kubo KA, Vanslembrouck B, Mak WKE, Cheung S, et al. Nitrogen-fixing organelle in a marine alga. Science. 2024 Apr 12;384(6692):217–22.

2. Perkins KK. The Scientist. [cited 2025 Sep 21]. The First Nitrogen-Fixing Organelle Found in Marine Alga. Available from: https://www.the-scientist.com/the-first-nitrogen- fixing-eukaryotic-marine-alga-discovered-72225 

3. Haavisto V. American Society for Microbiology. [cited 2025 Sep 21]. Beyond Endosymbiosis: Discovering the First Nitroplast. Available from: https://asm.org:44 3/articles/2024 /june/beyond-endosymbiosis-discovering-first-nitroplast 

4. York A. Nitroplast organelle unveiled. Nature Reviews Microbiology. 2024 Jun;22(6):323–323.  

5. Cowing K. The Nitroplast Revealed: A Nitrogen-fixing Organelle In A Marine Alga [Internet]. Astrobiology. 2024 [cited 2025 Sep 20]. Available from: https://astrobiology.com/2024/04/the-nitroplast-revealed-a-nitrogen-fixing-organelle-in-a-marine-alga.html

6. Massana R. The Nitroplast: A Nitrogen-Fixing Organelle. Institute of Parasitology, Czech Academy of Sciences. 2024 [cited 2025 Sep 17]. Available from: https://www.paru.cas.cz/en/news-events/news-detail/7229-the-nitroplast-a-nitrogen-fixing-organelle/ 

7. Vitousek PM, Hättenschwiler S, Olander L, Allison S. Nitrogen and Nature. ambi. 2002 Mar;31(2):97–101.

8. Oldroyd GE, Dixon R. Biotechnological solutions to the nitrogen problem. Current Opinion in Biotechnology. 2014 Apr 1;26:19–24.

9. Fowler D, et al. The global nitrogen cycle in the twenty-first century. Philosophical Transactions of the Royal Society B: Biological Sciences [Internet]. [cited 2025 Sep 19]. Available from: https://royalsocietypublishing.org/doi/10.1098/rstb.2013.0164