Life on this planet is incredibly diverse, but, paradoxically, the rules that govern life as we know it are simple. A certain harmony is maintained across all life, from bacteria and amoeba to complex plants and animals. For example, all life shares fundamental elements such as genetic material in the form of DNA and cellular structures. Discoveries that fundamentally alter how we perceive life are rare.
A recent discovery of a microscopic nitrogen-capturing factory – an organelle named the "nitroplast" – in a kind of marine algae alters what we know about the boundaries of life.
But let’s take a step back first.
At the root of Earth's biological diversity is the last universal common ancestor, or “LUCA”, the shared ancient microscopic ancestor from which all life on this planet diverged. From this primordial form, three distinct domains of life evolved: bacteria, archaea, and eukarya. Every form of life that we know of today falls into these three groups.
Bacteria and archaea are single-celled organisms without a defined nucleus or other complex organ-like structures. These microbes have thrived on Earth for billions of years due to their simplicity and adaptability.
Eukarya, the domain of life that includes plants, animals, and fungi, is characterised by more complex cells with a nucleus and organised into intricate structures known as organelles, which are similar to tiny organs inside the body. These organelles perform specialised functions within cells, an evolutionary leap that allowed these organisms to undertake a wide range of life processes, including photosynthesis in plants, and energy conversion in animals.
You may remember mitochondria as the “powerhouses of the cell” from your school textbooks. You might also recall that chloroplasts are responsible for capturing sunlight to produce energy in plants. These organelles are vital to the evolution of life we see around us.
A third kind of organelle, not as widespread in nature, is the chromatophore, which is found in certain microbes that can make their food through photosynthesis.
Organelles developed from bizarre events in which one cell engulfs a smaller one, not as prey but as a partner, leading to a permanent and beneficial merger. This event, known as primary endosymbiosis, is exceedingly rare. Up until the recent discovery, they were thought to have occurred only three times in 3.5 billion years of life on this planet.
The evolution of the smaller cell involved losing many of its own genes, which were transferred to the larger cell's nucleus. There has to be a benefit to both the larger engulfing cell and the smaller one, a match in complicated factors.
Now, in a study published in the prestigious journal, Science, researchers report the discovery of the nitroplast in the marine algae, Braarudosphaera bigelowii. The nitroplast evolved from a nitrogen-fixing blue-green bacterium (cyanobacterium) which, through a series of evolutionary adjustments, has become an integral part of the algae’s cellular machinery. This marks only the fourth known instance of such a transition, where a free-living organism has evolved into a true organelle.
The journey to discovering the nitroplast involved an international team of scientists, spearheaded by Jonathan Zehr, a professor of marine sciences at the University of California, Santa Cruz. Zehr first detected genetic traces of the nitrogen-fixer in Pacific seawater samples back in 1998. Tyler Coale, a postdoctoral scholar specialising in the physiology of marine eukaryotes, played a crucial role in detailing the cellular processes that integrate the nitrogen-fixer into its algal host's system.
The breakthrough came from sophisticated imaging techniques that allowed the scientists to figure out that the nitrogen-fixer wasn’t just residing within the algae but was an active participant in the cellular processes, dividing in unison with the host cell — a hallmark of organelles. The researchers also found a seamless flow of proteins, cells, and genetic materials that showed the nitrogen-fixer was actually a part of the algal cell.
The relationship between the algae and its nitroplast is mutually beneficial. In exchange for essential nitrogen from the nitroplast, the algae provide “food” in the form of carbon from photosynthesis. It’s a win-win situation for both.
Speaking on the special nature of organelle formation, author Tyler Coale notes that “the first time we think it happened, it gave rise to all complex life. Everything more complicated than a bacterial cell owes its existence to that event,” in reference to the mitochondria. He adds that “a billion years ago or so, it happened again with the chloroplast, and that gave us plants.”
So, what’s the importance of the nitroplast?
Nitrogen is vital for all life forms since it is a key component of amino acids and nucleic acids. While most complex organisms cannot directly utilise atmospheric nitrogen, they depend on processes like nitrogen fixation by microbes to convert inert nitrogen into a usable form. For example, plants have depended on relationships with nitrogen-fixing bacteria to obtain this essential nutrient. The nitroplast upends this wisdom by showing that some complex organisms can fix their nitrogen.
We invest heavily in nitrogen-based fertilisers to enhance agricultural productivity. The discovery of nitroplasts hints at bioengineering applications. By studying how the cooperative existence of nitroplasts and algae occurs, scientists might be able to engineer similar nitrogen-fixing capabilities into crop plants — a development that could greatly enhance agricultural productivity without heavy reliance on chemical fertilisers.
There are exciting times ahead. What’s certain already is that the discovery of nitroplasts is one for the school textbooks.
Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal.
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