As proposed by Charles Darwin in the nineteenth century, natural selection, the engine that drives evolution, is how species adapt to their environments. Unlike the Neo-Darwinist consensus, the American evolutionary biologist Lynn Margulis did not believe that random genetic mutations were the sole cause of inherited variation. She came up with a new theory called symbiogenesis. The endosymbiotic theory states that organelles like mitochondria and chloroplasts, the sites of cellular respiration and photosynthesis, were once free-living bacteria that were later ingested by the recipient cells. The theory of symbiogenesis was fiercely challenged, including Margulis’s manuscript, which was rejected by 15 academic journals before finally being published in The Journal of Theoretical Biology in 1967. It was not until many years later that mitochondria and chloroplasts were accepted as once being free-living bacteria before becoming endosymbionts inside eukaryotic cells.
Two papers published recently, one in the journal Science and another in the Cell, have generated new interest in the endosymbiotic theory. The discovery concerns nitrogen fixation. Nitrogen is a key component in proteins and DNA of all living organisms. Although nitrogen gas makes up about 78% of the Earth’s atmosphere by volume, plants and animals lack a system that can utilise atmospheric nitrogen. Bacteria and archaea help convert atmospheric nitrogen gas to ammonia by nitrogen fixation (or ammonification) to make nitrogen usable for plants. Unlike many free-living nitrogen-fixing bacteria, legumes, a class of plants in the family Fabaceae, bear the nitrogen-fixing bacteria in their root nodules. Ammonia is converted to nitrites and nitrates (nitrification) and then back into atmospheric nitrogen (denitrification) with the help of bacteria to complete the cycle. In marine environments, like on Earth, bacteria and archaea are also involved in ammonification, nitrification, and denitrification. Beyond mitochondria and chloroplasts, the current discovery extends the earlier reports of a nitrogen-fixing cyanobacterium in marine algae and establishes it as a new organelle. The new organelle that the authors call nitroplast co-evolved with its host cell.
In 1998, Jonathan Zehr, at the University of California, Santa Cruz, U.S. discovered a cyanobacterium Candidatus Atelocyanobacterium thalassa or UCYN-A in the water of the Pacific Ocean capable of fixing nitrogen. Later, Kyoko Hagino at Kochi University, Japan, found the marine algae Braarudosphaera bigelowii as the host for UCYN-A and could successfully culture the host cells. Both teams had established UCYN-A as a symbiotic cyanobacterium for marine single-cell eukaryotic algae.
Bonafide organelles need to satisfy several criteria. First, the organelle must be integrated into the function and overall architecture of the host cell. Second, proteins must be imported to the organelle from the host cell to carry out some of its functions. Third, organelles must be in sync with the host cell’s growth. Last, organelles must be inherited in the newly dividing cells during host cell division. All these above criteria were satisfied by nitroplast, as presented by several lines of evidence by the authors. During a symbiont’s transformation into an organelle within a eukaryotic cell, its genome becomes frugal, encoding fewer proteins and utilizing the host cell’s proteins to perform some of its essential functions. In line with expectations, nearly half of the nitroplasts’ proteins are from the host cell. Although the reports present evidence of establishing nitroplasts as organelles, the loss of some of nitroplasts’ genetic material and migration to the host cell nucleus still needs to be established. Unlike mitochondria and chloroplast endosymbiosis, which happened nearly two billion years back, nitroplast’s evolution as an organelle is relatively recent (about 100 million years).
The discovery has revolutionary implications, especially in agriculture. Agriculture was transformed in the last century by the discovery of a method for synthesizing ammonia from nitrogen and hydrogen in the laboratory. Although the Haber-Bosch method of industrial-scale production revolutionised agriculture by introducing ammonia as a fertilizer that helped increase crop yield manifold, industrial ammonia production contributes to water and air pollution and climate change with its carbon dioxide emissions. The current discovery has the potential to play a vital role in getting rid of the harmful side effects of industrial ammonia production. Several novel biotechnological applications may use the result of the current discovery of nitroplasts as independent nitrogen-fixing organelles. They are: engineering host cells and their nitroplasts with minimal genomes sufficient to grow efficiently and fix nitrogen, making plant cells fix nitrogen by engineering them to include nitroplasts and organelle transformation in plant cells to introduce nitroplast and its host genes to fix nitrogen. Although promising and futuristic, all these are highly challenging and far from reality.
(Binay Panda is a Professor at JNU, New Delhi)
