Biological complexity: introducing the nitroplast (Introduction)

by David Turell , Friday, April 12, 2024, 15:34 (225 days ago) @ David Turell

Another way to fix nitrogen:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/12_april_2024/41873...

"Coale et al. 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.

"Biological nitrogen fixation, which reduces atmospheric dinitrogen gas (N2) into reactive ammonia (NH3), is central in the nitrogen biogeochemical cycle as the only path to incorporate the abundant dinitrogen gas into biomass. This process represents a main driver of fertilization for aquatic and terrestrial systems and is continuously studied to increase crop yields in agriculture. To directly benefit from the resulting ammonia, many photosynthetic organisms, from terrestrial plants to microalgae, incorporate nitrogen-fixing symbionts. This is the case for B. bigelowii and relatives (belonging to the algal class Prymnesiophyceae) that carry the nitrogen-fixing UCYN-A cyanobacteria. The UCYN-A symbiont lacks the genes for the oxygen-evolving photosystem II and carbon fixation, which suggests that it is unable to perform oxygenic photosynthesis and is in-volved in a tight partnership with the host, providing it with fixed nitrogen and receiving fixed carbon in return. This symbiosis is now known to be very stable, to be widespread in sunlit coastal and oceanic waters, and to play a crucial role in the nitrogen biogeochemical cycle. However, challenges in obtaining stable cultures of B. bigelowii and UCYN-A have limited studies on this symbiosis.

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"The synchronized division and the import of essential eukaryotic proteins indicate that UCYN-A has evolved beyond endosymbiosis (7) and that it can instead be considered a eukaryotic organelle under the full control of the host. The organelle is called the nitroplast, taking the name proposed years ago for analogous systems and denoting its role in nitrogen fixation and its cyanobacterial origin (by analogy to plastids, which are also derived from cyanobacteria).

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"...the deep cellular integration of UCYN-A into the host and its severe genetic dependency support the view that the nitroplast of B. bigelowii can be added to the short list of endosymbiosis-derived organelles.

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"The transitions from endosymbionts to the various organelles happened independently at different times of eukaryotic evolution, and this influences their taxonomic coverage. Mitochondria acquisition (thought to have occurred around 2 billion years ago) predates the origin of the eukaryotic cell, and these organelles are found throughout the eukaryotic tree of life, with some cases of secondary loss or modification. The primary endosymbiosis that originated the chloroplast also occurred in ancient times (likely around 1.5 billion years ago) in the supergroup Archaeplastida. Chloroplasts were later transferred to other eukaryotic supergroups by secondary or tertiary endosymbiosis. The establishment of the nitroplast is more recent—about 100 million years ago—and this may explain why this organelle is taxonomically constrained to prymnesiophytes.

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"The study from Coale et al. shows that a renowned endosymbiont is actually the nitroplast organelle—an optimal adaptation of the microalgae to thrive in nitrogen-limited waters. Like in photosynthesis, a prokaryotic innovation that was incorporated by endosymbiosis into the eukaryotic cell and is now considered a eukaryotic function, these new data support the claim that nitrogen fixation is no longer an exclusive prokaryotic function and that eukaryotes can fix molecular nitrogen through the nitroplast. The nitroplast represents a textbook case of a eukaryotic organelle that complements the energy, carbon, and nitrogen needs of the algal host (see the figure) and is another example of how ecology is the theater where evolution takes place."

Comment: nitrogen is tough to fix, as we know. Here evolution uses a well-worn path to a solution. It is like convergence and can be taken as evidence for design.