Nitrogen is a nutrient essential for all life on Earth. Although nitrogen gas (N2<\/sub>) is plentiful, it is largely unavailable to most organisms without a process known as nitrogen fixation, which converts dinitrogen to ammonium\u2014a major inorganic nitrogen source.<\/p>\n\n\n\n While there are bacteria that are able to reduce dinitrogen to ammonium, researchers at the University of Rhode Island, Institut de Ci\u00e8ncies del Mar in Barcelona, University of California at Santa Cruz and the Massachusetts Institute of Technology have discovered nitrogen-fixing symbiotic organisms exhibiting behaviors similar to organelles. In fact, researchers posit these symbiotic organisms \u2013 UCYN-A, a species of cyanobacteria \u2013 may be evolving organelle-like characteristics. Their study was recently published in the journal Cell<\/a>.<\/p>\n\n\n\n UCYN-A live in a symbiotic relationship with a closely related group of marine algae, B. bigelowii, <\/em>in areas of the open ocean that are often low in nutrients. Most nitrogen-fixing bacteria have mechanisms to regulate dinitrogen use when fixed sources of nitrogen are available, alleviating the high energetic cost of this process. However, UCYN-A have lost the genes allowing this and are able to fix nitrogen gas into ammonium even in nutrient-rich environments. The host, in-turn, provides it with carbon fixed photosynthetically by its chloroplasts. <\/p>\n\n\n\n The study details how researchers found a size relationship between UCYN-A and their symbiotic partner cells \u2013 consistent with the size relationships between other organelles and their hosts. As organelles get larger, so do their host cells \u00ad\u2013 eventually dividing and replicating. Mathematical modeling revealed the metabolic trade-offs which regulate the relative cell size through nutrient acquisition and exchange.<\/p>\n\n\n\n