Amazing stuff!
"... Known as ‘primary endosymbiosis,’ this process has been foundational for complex organisms (like humans). It describes how a single-celled organism is engulfed by a larger, more complex eukaryotic cell and evolves into a functional organelle. ...
This new, fourth example of primary endosymbiosis is being called a ‘nitroplast’ and has been found within an algal species called Braarudosphaera bigelowii. It appears to have evolved just 100 million years ago. ...
At the same time, UCYN-A appears to have discarded parts of its genome in order to take on proteins supplied by the algal host.
“That’s one of the hallmarks of something moving from an endosymbiont to an organelle,” ..."
"... The organelle also provides insight into ocean ecosystems. All organisms need nitrogen in a biologically usable form, and UCYN-A is globally important for its ability to fix nitrogen from the atmosphere. Researchers have found it everywhere from the tropics to the Arctic Ocean, and it fixes a significant amount of nitrogen. ..."
"Eukaryotic cells are notably complex—for example, they have various organelles, which are membrane-bound structures with specific functions. Two of these organelles, mitochondria and chloroplasts, which function in respiration and photosynthesis, evolved from the integration of endosymbiotic bacteria to the eukaryotic cell. In marine systems, some nitrogen-fixing bacteria are endosymbionts of microalgae, such as Candidatus Atelocyanobacterium thalassa (UCYN-A), a cyanobacterial symbiont of the unicellular algae Braarudosphaera bigelowii. ... report a close integration of the endosymbiont into the architecture and function of the host cell, which is a characteristic of organelles. These findings show that UCYN-A has evolved from a symbiont to a eukaryotic organelle for nitrogen fixation—the nitroplast—thereby expanding a function that was thought to be exclusively carried out by prokaryotic cells to eukaryotes. ..."
From the editor's summary and abstract:
"Editor’s summary
Many partnerships have been formed between nitrogen-fixing microbes and carbon-fixing eukaryotes that need nitrogen to grow. The possibility of a eukaryote with a nitrogen-fixing organelle derived from endosymbiosis, which is called a nitroplast, has been speculated. Studying a marine alga with a cyanobacterial endosymbiont, Coale et al. used soft x-ray tomography to visualize cell morphology and division of the alga, revealing a coordinated cell cycle in which the endosymbiont divides and is split evenly, similar to the situation for plastids and mitochondria in these cells ... Proteomics revealed that a sizable fraction of the proteins in this structure are encoded by and imported from the alga, including many that are essential for biosynthesis, cell growth, and division. These results offer a fascinating view into the transition from an endosymbiont into a bona fide organelle. ...
Abstract
Symbiotic interactions were key to the evolution of chloroplast and mitochondria organelles, which mediate carbon and energy metabolism in eukaryotes. Biological nitrogen fixation, the reduction of abundant atmospheric nitrogen gas (N2) to biologically available ammonia, is a key metabolic process performed exclusively by prokaryotes. Candidatus Atelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N2-fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga. Here we show that UCYN-A has been tightly integrated into algal cell architecture and organellar division and that it imports proteins encoded by the algal genome. These are characteristics of organelles and show that UCYN-A has evolved beyond endosymbiosis and functions as an early evolutionary stage N2-fixing organelle, or “nitroplast.”"
Scientists discover first nitrogen-fixing organelle (original news release)
The nitroplast: A nitrogen-fixing organelle (no public access) A bacterial endosymbiont of marine algae evolved to an organelle
Nitrogen-fixing organelle in a marine alga (no public access)
The new organelle, indicated by an arrow
A soft x-ray tomography image shows B. bigelowii cell division, with the nitroplasts (UCYN-A) in cyan.
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