Scientists unearthed a variety of organelle, a fundamental unit within cells, capable of transforming nitrogen gas into a usable form for cellular development.
The identification of this structure, termed a nitroplast, within algae may enhance endeavors to genetically modify plants for self-nitrogen conversion, thereby potentially increasing agricultural output while lessening reliance on fertilizers. The findings were detailed in the journal Science on April 11th.
"The conventional wisdom suggests that nitrogen fixation exclusively occurs within bacteria and archaea," states ocean ecologist Jonathan Zehr from the University of California, Santa Cruz, a collaborator on the research. This particular strain of algae represents the "primary nitrogen-fixing eukaryote," he elaborates, alluding to the category of organisms encompassing plants and animals.
Back in 2012, Zehr and his team documented a close interaction between the marine algae Braarudosphaera bigelowii and a bacterium known as UCYN-A, which appeared to inhabit the algal cells or their vicinity. The researchers postulated that UCYN-A converted nitrogen gas into compounds, like ammonia, utilized by the algae for growth. Conversely, it was believed that the bacteria obtained a carbon-based energy source from the algae.
However, in their most recent investigation, Zehr and colleagues assert that UCYN-A should be classified as organelles within the algae rather than distinct organisms. As per genetic analysis conducted in a previous study, Zehr explains, the ancestors of both the algae and bacteria established a symbiotic relationship approximately 100 million years ago. This symbiosis ultimately led to the emergence of the nitroplast organelle, now observed in B. bigelowii.
Defining organelles
Scientists employ two primary criteria to determine whether a bacterial cell has transitioned into an organelle within a host cell. Initially, the structure under scrutiny must be inherited across successive generations of the host cell. Secondly, the structure must depend on proteins supplied by the host cell.
Through the examination of numerous algae cells undergoing different phases of cell division, the research team observed that the nitroplast undergoes division just prior to the complete division of the entire algae cell. Consequently, one nitroplast is transmitted from the parental cell to its progeny, mirroring the inheritance pattern of other cellular structures.
Subsequently, the scientists observed that the nitroplast acquires the necessary proteins for its growth from the surrounding algae cell. Jonathan Zehr explains that the nitroplast itself, comprising over 8% of the volume of each host cell, lacks essential proteins vital for photosynthesis and genetic material synthesis. He notes, "Many of these proteins from the algae essentially fill the metabolic gaps within the nitroplast."
The breakthrough was facilitated by the efforts of Kyoko Hagino, a co-author of the study from Kochi University in Japan, who dedicated approximately a decade to refining a method for cultivating the algae in laboratory conditions. This advancement enabled more detailed scrutiny of the algae, as highlighted by Zehr.
Siv Andersson, a researcher specializing in organelle evolution at Uppsala University in Sweden, expresses admiration, stating, "It's truly remarkable." Andersson emphasizes the meticulous observation of hallmark characteristics typically associated with organelles.
Upgraded plants
Jonathan Zehr suggests that comprehending the interaction between the nitroplast and its host cell could bolster endeavors to genetically modify crops capable of nitrogen fixation. This innovation could diminish the reliance on nitrogen-based fertilizers and mitigate some of the associated environmental harm. "The strategies employed in facilitating this system could be applied to the engineering of terrestrial plants," he asserts.
Eva Nowack, a researcher specializing in symbiotic bacteria at Heinrich Heine University Düsseldorf in Germany, emphasizes the significance of nitrogen availability in determining crop yields. "The presence of a nitrogen-fixing organelle in crop plants would undoubtedly be remarkable," she remarks. Nonetheless, Nowack cautions that implementing this capability in plants will pose significant challenges. For instance, ensuring stable gene transfer across generations within plant cells containing the genetic blueprint for the nitroplast would be particularly challenging. "That would represent the most formidable obstacle," she cautions.
Jeffrey Elhai, a cell biologist at Virginia Commonwealth University in Richmond, Virginia, commends the progression of this research towards a significant milestone in understanding. He remarks, "It's truly gratifying and impressive to witness this research culminate in what undoubtedly constitutes a significant advancement."
