Nitrogen is a crucial element for all living organisms, playing a key role in the growth of crops on land and microscopic oceanic plants that produce half the planet’s oxygen. While atmospheric nitrogen gas is abundant, plants cannot utilize it without the help of nitrogen-fixing bacteria. Legumes like soybeans, peas, and alfalfa have formed partnerships with Rhizobial bacteria to convert atmospheric nitrogen into ammonium, making them important sources of protein in food production. Scientists at the Max Planck Institute for Marine Microbiology in Bremen, Germany, have made a groundbreaking discovery that Rhizobial bacteria can also form partnerships with tiny marine plants called diatoms, solving a long-standing marine mystery with potential applications in agriculture.
For many years, it was assumed that most nitrogen fixation in the oceans was carried out by cyanobacteria. However, in vast regions of the ocean, there were not enough cyanobacteria to account for measured nitrogen fixation, sparking a controversy among scientists. Researchers identified gene fragments encoding the nitrogen-fixing enzyme in a non-cyanobacterial nitrogen fixer, but could not determine the specific organism responsible. After collecting seawater samples from the tropical North Atlantic, the scientists spent three years analyzing the genome of the mysterious nitrogen fixer. They discovered that it was closely related to Rhizobia, which form symbiotic relationships with legumes, suggesting that the marine Rhizobia might also be a symbiont.
The researchers developed a genetic probe to fluorescently label the Rhizobia in the diatoms and confirmed their suspicions that it was a symbiont. This discovery marked the first known symbiosis between a diatom and a non-cyanobacterial nitrogen fixer, leading to the naming of the newly discovered symbiont Candidatus Tectiglobus diatomicola. The team used nanoSIMS technology to show that the Rhizobia exchange fixed nitrogen with the diatom in exchange for carbon, supporting the diatom’s growth significantly by fixing 100-fold more nitrogen than it needs.
Further research showed that the new symbiosis is widespread in the world’s oceans, especially in regions where cyanobacterial nitrogen fixers are rare. These tiny organisms play a crucial role in sustaining marine productivity and the global oceanic uptake of carbon dioxide. The findings also hint at exciting opportunities for future agricultural engineering, as the symbiosis shows similarities to endosymbiotic cyanobacteria that function as nitrogen-fixing organelles. The organelle-like nature of the marine Rhizobia opens up the possibility of engineering nitrogen-fixing plants in the future. The scientists will continue to study the newly discovered symbiosis and explore if similar partnerships exist in other regions of the oceans.
