The Nitroplast, a Nitrogen-Fixing Organelle

by Mechas
 

Nitrogen is the most abundant element in the atmosphere as well as an essential element in all of life. As nicely explained in previous posts, nitrogen is needed to synthesize nucleic acids and proteins. Yet despite making up approximately 78% of our atmosphere, we humans (like the vast majority of living forms) are unable to utilize this nitrogen gas (N₂). We rely on a few microbes that fix nitrogen by reducing N₂ to ammonia (NH₃), which is the form utilized by the remaining organisms on planet Earth. This capacity to fix nitrogen was thought to be a unique property of a limited number of diazotrophic bacteria and archaea. Until now.

 

The relevance of biological nitrogen fixation to our biosphere is underscored by the various symbiotic interactions that enable eukaryotes - including animals, plants, fungi, and protists – to benefit from the ammonia produced by nitrogen-fixing microbes. Many terrestrial plants associate with rhizobia bacteria to form nodules where the bacteria fix and transfer nitrogen to the host in an environment that protects the nitrogenaseenzyme from the damaging effects of oxygen. Lichens, themselves a symbiosis between a fungus and a photosynthetic alga or bacterium, also obtain nitrogen from associated nitrogen-fixing bacteria. Other examples include associations between protists, such as algae, and nitrogen-fixing cyanobacteria. A recent study by Coale and collaborators analyzed one such association and demonstrated that intracellular "cyanobacteria" (which in fact, are no longer complete cyanobacteria) constitute a novel nitrogen-fixing organelle: the nitroplast.

 

Coale and colleagues focused on the alga Braarudosphaera bigelowii, which carries out oxygenic photosynthesis during daylight hours. This alga plays an important role in marine nitrogen fixation through the action of its endosymbiont UCYN-A, aka Candidatus Atelocyanobacterium thalassa. The endosymbiotic UCYN-A is related to cyanobacteria but, unlike all known cyanobacteria, it lacks the genes for carbon fixation (i.e., photosynthesis). However, UCYN-A does have the nif genes for nitrogen fixation. At the outset, it was unclear whether the association between B. bigelowii and UCYN-A represented a symbiosis or a new case of evolution that goes beyond symbiosis and towards the formation of an organelle. To solve this puzzle, the authors probed this system for biological features that are indicative of organelles, like coordinated cell division, import of proteins by the endosymbiont, and possible gene exchange between partner cells.

 

Fig. 1 B. bigelowii/UCYN-A organization. Structural characterization during the light cycle: (Left) Differential interference contrast (DIC) micrograph displays the whole alga and showcases UCYN-A spheroid body (SB) and plastids (Pls) in the FR21 culture; the correspondent 4′,6-diamidino-2-phenylindole (DAPI) staining reveals the cell nucleus (Nu). In both images, the UCYN-A spheroid body (SB) is identified above the nucleus. Scale bars, 5 mm. (Right) Orthoslice from soft X-ray tomography (SXT) data; color-coded 3D renderings of subcellular components. Adapted from Source. Frontispiece:A nitroplast inside B. bigelowii (coccoliths removed), marked by a black arrow. Like other haptophytes, the cell has 2 unequal flagella. Source 

By using soft X-ray tomography to image individual specimens, the authors generated 3D maps of cells and quantified organelle changes throughout the cell cycle. The images show that UCYN-A is distinctly positioned between two energy-providing chloroplasts. UCYN-A also follows a precise sequence of division events, coordinated with algal cell growth and division, which implies control of its division by the B. bigelowii host. Such control over division ensures the vertical transmission of UCYN-A to daughter cells and represents an important step along the trajectory from an endosymbiont to an organelle.

 

The authors also carried out differential genomic/proteomic analyses to determine which proteins were encoded in the B. bigelowii  genome yet localized in isolated UCYN-A bodies. At peak hours of nitrogen fixation (during daylight), nearly 30% of the proteins present in UCYN-A came from genes encoded in the B. bigelowiigenome. These proteins are presumably imported onto UCYN-A from the B. bigelowii host cell cytoplasm via a transit peptide found in most of them, analogous to what occurs in chloroplasts and mitochondria.

 

Fig. 2. Evolution and function of the nitroplast. B. bigelowii chloroplast fixes inorganic carbon to produce glucose, which feeds the respiratory chain in mitochondria that produces adenosine triphosphate (ATP), which fuels nitrogen fixation in the nitroplast. Glucose, ammonia, and ATP generated by the organelles, together with externally incorporated compounds (phosphorous, mineral nutrients, and vitamins), are the building blocks for cell metabolism, resulting in cell growth and division. Adapted from Source.

As with many endosymbionts and organelles, the genome of UCYN-A has undergone substantial reduction when compared to closely related cyanobacteria. This gene loss results in incomplete metabolic pathways. Nicely enough, proteins imported from B. bigelowii complement some of these incomplete pathways, such as the synthesis of amino acids like threonine, proline and serine, nucleotides, vitamins, and cofactors. Other proteins were predicted to participate in cellular activities by repairing oxidative damage, regulating UCYN-A transcription and translation, and coordinating circadian cycles by cryptochromes. Thus, UCYN-A depends on its host for numerous metabolic processes.

 

Distinguishing between endosymbiont and organelle is no easy task. There are only a few known cases of organelles that have evolved from primary symbionts, the prime examples being the mitochondrion and chloroplast which carry out respiration and photosynthesis in eukaryotes, respectively. Another case involves the independent acquisition of a photosynthetic chromatophore resulting from the endosymbiosis between the amoeba Paulinella and a cyanobacterium. The study by Coale and collaborators expands the list to include the nitroplast, which has evolved beyond an endosymbiont and is in an early stage of organellogenesis.

 

With its reduced genome size, metabolic dependency, and synchronous division, the nitroplast is now an intrinsic part of the B. bigelowii cell architecture. It also offers another beautiful example of how time and evolution provide the fabric for continuous biological innovations.

 

 

 

From the archive: STC had two posts dealing with Braarudosphaera bigelowii and its UCYN-A symbiont earlier, here and here.
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