Scientists find first nitrogen-fixing organelle  

Modern biology textbooks say only bacteria can transform atmospheric nitrogen into a life-sustaining form. In root nodules, legumes store symbiotic microorganisms that fix nitrogen. A recent discovery breaks that norm.  

An international team of scientists describes the first eukaryotic nitrogen-fixing organelle in two recent studies. The organelle is the fourth example of primary endosymbiosis, in which a prokaryotic cell is absorbed by a eukaryotic cell and transforms into an organelle.  

"It's very rare that organelles arise from these types of things," said UC Santa Cruz postdoctoral fellow Tyler Coale, first author on two recent articles.   

"We think it created all complex life the first time. He added the mitochondria's origins are responsible for everything more complex than a bacterial cell. "A billion years ago or so, it happened again with the chloroplast, and that gave us plants," he said.  

The third case features a chloroplast-like microorganism. Researchers discovered the first nitrogen-fixing organelle, a nitroplast.  

The organelle was discovered by chance and decades of labor. Jonathan Zehr, a UC Santa Cruz distinguished professor of marine sciences, uncovered a short DNA sequence of an unknown nitrogen-fixing cyanobacterium in Pacific Ocean saltwater in 1998. Zehr and colleagues studied UCYN-A, a mystery creature, for years.  

Paleontologist Kyoko Hagino from Kochi University in Japan was meticulously cultivate a marine alga. It was the UCYN-A host. After over 300 sample expeditions and a decade, Hagino successfully cultivated the alga in culture, allowing other researchers to explore UCYN-A and its marine alga host in the lab.  

For years, scientists regarded UCYN-A an alga-associated endosymbiont. Two recent articles reveal that UCYN-A co-evolved with its host during symbiosis and is now an organelle.  

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