University of California Santa Cruz scientists document the first organelle that can “fix" nitrogen
Scientists confirm the speculation that a nitrogen-fixing bacterium has evolved from an endosymbiont or host cell resident into an incorporated host cell structure.
Researchers from the University of California Santa Cruz confirm the evolution of a single-celled cyanobacteria into a nitrogen-fixing organelle within the marine algae, Braarudosphaera bigelowii (B. bigelowii).
What is an endosymbiont?
An endosymbiont is an organism that lives within another organism and remains within the organism, unmolested by the host cell if the endosymbiont and host share a mutually beneficial relationship. An endosymbiont is derived from the process called endocytosis, whereby the host cell engulfs external material or another organism and packages it as it enters into its outer membrane.
A well-known endosymbiont is the mitochondria or the "powerhouse of the cell", which produces ATP (adenosine triphosphate), a molecule powering cellular activity. The Endosymbiotic Theory proposed by Lynn Margulis holds that mitochondria evolved from bacteria absorbed by early eukaryotic cells.
These bacteria created a symbiotic relationship, exchanging energy via respiration. Similarities to bacteria are evident, as is the dual membrane of mitochondria, which resembles bacterial cell walls. The Endosymbiotic theory is one of some possible theories for the origin and evolution of the mitochondria, which is still much debated. The mitochondrial innovation gave rise to cells that form the animals, fungi, and plants we see today.
The first documented nitrogen-fixing organelle
Organelles within a cell are similar to the organs within the human body. They are structures that exact specific functions that allow a cell or body to operate.
The researchers studied the cyanobacterium, Candidatus Atelocyanobacterium thalassa (UCYN-A), which has long been thought to have evolved into a nitrogen-fixing organelle (termed a nitroplast) within the single-celled algae, B. bigelowii.
The team used a laboratory technique called soft X-ray tomography, to observe algal cell division. The algal cell divided in concert with the UCYN-A. Each resulting daughter cell inherited a single UCYN-A. Although there is no standard description of an organelle, concerted division and protein importation are recognized as critical components. UCYN-A meets these criteria, earning its place as an organelle alongside mitochondria.
Rather than being produced internally, approximately half of the proteins in UCYN-A are derived from the algal host. These exogenous proteins aid UCYN-A in nitrogen fixation, potentially increasing its effectiveness. Furthermore, there is a specific mechanism for transferring proteins to UCYN-A, similar to other organelles, identified by an "address label."
The need for nitrogen-fixing crops
Nitrogen fertilisers add to global greenhouse gas emissions and pose extreme financial pressure on farmers. There is a need to develop agricultural crops that fix nitrogen for themselves, not relying on other plants like legumes.
While nitroplasts such as UCYN-A are not ideal for this purpose, the researchers propose that cyanobacteria in the early nitroplast stages (not requiring imported proteins) could be more readily integrated into crop plants. Further study of B. bigelowii may help the scientific community understand how to introduce nitrogen fixation into plant cells.
Reference of the news:
The research findings were published in the Journal, Science.