The Origin of the Nitroplast
In a never-before-seen finding, scientists have recorded a rare evolutionary advancement in which an algae species has assimilated a cyanobacterium, blending into a single organism. During this process, the cyanobacterium has transformed into an organelle. This occurrence, known as primary endosymbiosis, has turned the cyanobacterium into a functioning part of the algae, similar to how ancient microbes evolved into mitochondria and chloroplasts, the generators of energy in today’s plants and animals.
This new organelle, named a 'nitroplast', is only the fourth documented case of primary endosymbiosis, where a smaller prokaryotic cell is assimilated by and becomes a crucial part of a larger eukaryotic cell. The new single organism joined together from two entities possesses the impressive ability to extract nitrogen directly from the air, a capability that its predecessors could not accomplish. The last time a primary endosymbiosis event was witnessed by evolution, as far as we know, was when plants emerged.
Nitroplasts evolved from a cyanobacterium called Atelocyanobacterium thalassa (UCYN-A) becoming absorbed by the unicellular algae Braarudosphaera bigelowii. This event took place just 100 million years ago, as concluded by the researchers recently.
What this indicates is that, unlike typical symbiotic relationships where plants depend on external bacteria for nitrogen fixation, in B. bigelowii, this capability is internalized. Previously, only some prokaryotes similar bacteria were thought to have this ability. B. bigelowii is, as of now, the only eukaryote able to convert nitrogen to ammonia independently.
The Meandering Path of Discovery
The path to this finding was not fast or direct. In 1998, Jonathan Zehr, a marine sciences professor at UC Santa Cruz, stumbled upon a DNA sequence implying the existence of a then-unknown nitrogen-fixing cyanobacterium in the Pacific Ocean. This marked the start of a journey involving numerous researchers across continents.
Kyoko Hagino, a paleontologist from Kochi University in Japan, encountered a similar challenge as she painstakingly attempted to cultivate a species of marine algae. This species would eventually turn out to be the host for the mysterious organism UCYN-A. Her success, after over 300 previous failed attempts, allowed scientists to study UCYN-A and its interactions with its host in the lab.
According to the research, the connection between UCYN-A and its algal host is characterized by a synchronized exchange of nutrients. Their metabolisms are linked.
“That’s exactly what happens with organelles,” said Zehr. “If you look at the mitochondria and the chloroplast, it’s the same thing: they scale with the cell.”