News Excerpt: 

In a remarkable scientific breakthrough, an international team of researchers has discovered the “nitroplast” — the first known nitrogen-fixing organelle within a eukaryotic cell.

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The nitroplast organelle discovery marks the fourth instance of primary endosymbiosis in history, a process where a prokaryotic cell is engulfed by a eukaryotic cell and evolves into an organelle.

Significance of Nitroplast discovery:

• Organelles rarely arise from these types of things.
• The first time an organelle arose was the event that gave rise to all complex life, referring to the origins of mitochondria.
• Everything more complicated than a bacterial cell owes its existence to that event, referring to the origins of the mitochondria.
• This is the second time an organelle arose around a billion years ago with the chloroplast, which gave rise to plants.

Nitroplast evolution from symbiosis to organelle:

• Initially, scientists considered UCYN-A as an endosymbiont closely associated with an alga.
• However, two recent papers suggest that UCYN-A has co-evolved with its host beyond symbiosis and now meets the criteria for an organelle.
• A study published in March 2024 in Cell showed that the size ratio between a specific group of nitrogen-fixing cyanobacteria and their marine haptophyte algal hosts remains consistent across various species of Braarudosphaera bigelowii algae.
• Their model showed that the growth of the host cell and UCYN-A are controlled by the exchange of nutrients, with their metabolisms linked.
• This synchronization in growth rates led the researchers to call UCYN-A "organelle-like."
• That's exactly what happens with organelles. Both the mitochondria and the chloroplast, scale with the cell.
• The findings indicate that UCYN-A has evolved from being a symbiotic partner to meeting the criteria for an organelle, similar to how mitochondria and chloroplasts evolved.

Confirming the organelle status:

• To classify UCYN-A as an organelle, scientists needed to confirm additional lines of evidence.
• UCYN-A imports proteins from its host cells, which is a hallmark of an endosymbiont transitioning to an organelle.
• As endosymbionts evolve into organelles, their genomes become smaller, and they start depending on the host cell for gene products or proteins to be transported into the cell.
• The proteomics work revealed that the host cell makes proteins and labels them with a specific amino acid sequence, signalling the cell to send them to the nitroplast (UCYN-A).
• The nitroplast (UCYN-A) then imports and uses these proteins from the host cell, filling gaps in certain pathways within UCYN-A.
• The process of the host cell providing proteins to UCYN-A and UCYN-A using these proteins is described as a "magical jigsaw puzzle that fits together and works." 

Implications for ocean ecosystems and agriculture:

• The discovery of the nitroplast provides new insights into ocean ecosystems. 
  • UCYN-A is globally important for its ability to fix nitrogen from the atmosphere, and researchers have found it in various locations, from the tropics to the Arctic Ocean, where it fixes a significant amount of nitrogen.
• The nitroplast also has the potential to revolutionize agriculture. 
• The Haber-Bosch process, which synthesizes ammonia fertilizers from atmospheric nitrogen, 
  • It allowed agriculture and the world population to take off in the early 20th century.
• However, it also creates enormous amounts of carbon dioxide, accounting for about 1.4% of global emissions.
  • Researchers have been trying to find a way to incorporate natural nitrogen fixation into agriculture for decades.
• This system is a new perspective on nitrogen fixation, and it might provide clues into how such an organelle could be engineered into crop plants.