Amazing stuff!
"Move over, chloroplast—there’s a new organelle in algae. Dubbed the nitroplast, this intracellular compartment converts nitrogen into a more palatable form for the coastal coccolithophore Braarudosphaera bigelowii, similar to the way symbiotic bacteria that live on or in the roots of land plants do the same. But because it’s an organelle instead of a symbiote, it makes this species the only eukaryote known to have this ability.
Much like the light-energy-harvesting compartments that give plants the ability to photosynthesize, the new organelle appears to have started as an endosymbiont. Some 100 million years ago, it partnered with the single-celled Braarudosphaera, receiving biomolecules in exchange for converting nitrogen gas to ammonia, a form of the nutrient more bioavailable to the marine alga. But over time, the microbe became more and more dependent on its host, and its genome lost the code for producing proteins that the alga now provided. According to new intracellular imaging and proteomic evidence, the once-independent organism has become fully “domesticated”—simply part of the alga rather than a separate entity living within.
Such an organelle is “an optimal adaptation of the microalgae to thrive in nitrogen-limited waters,” ... and serves as “another example of how ecology is the theater where evolution takes place.”"
"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 (1). 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 (2). ... 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 note 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, ... 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.”"
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