Nitrogen fixation

Nitrogen fixation is the vital chemical process of converting molecular dinitrogen into ammonia. Nitrogen is a fundamental building block for life on Earth. It is a key component of organic compounds like DNA and proteins. Most organisms cannot use the nitrogen gas found in the atmosphere directly. This is because dinitrogen molecules are held together by an extremely strong triple covalent bond. To make nitrogen usable, this bond must be broken. This process, known as nitrogen fixation, occurs through biological, abiotic, and industrial means.

Biological nitrogen fixation, or diazotrophy, is driven by a specialized enzyme complex called nitrogenase. This enzyme catalyzes the reduction of nitrogen gas into ammonia. The process is quite energy-intensive. It requires the hydrolysis of 16 equivalents of ATP, which is the energy currency of cells. The actual conversion occurs at a metal cluster known as FeMoco, or the iron-molybdenum cofactor. The mechanism works through a series of protonation and reduction steps. During these steps, the FeMoco active site hydrogenates the substrate to complete the change.

Nitrogenase is a complex protein structure made of two main parts. One is a catalytic iron-dependent protein called the MoFe protein. The other is a reducing iron-only protein called the Fe protein. Scientists have identified three different types of nitrogenase based on the metals they use. The most common type is molybdenum-dependent. Other versions use vanadium or consist of iron only. These different forms are encoded by specific genes: nifH for molybdenum, vnfH for vanadium, and anfH for iron-only. These genes are often used by scientists as biomarkers to study the ecology of microbes.

One major challenge for these organisms is that nitrogenase is very sensitive to oxygen. Oxygen can rapidly degrade the enzyme. Because of this, many nitrogen-fixing bacteria must live in anaerobic conditions, where oxygen is absent. Some cyanobacteria, which are blue-green algae, solve this by using specialized cells called heterocysts. These cells provide a safe environment for the enzyme to work. Other organisms might use proteins like leghemoglobin to bind oxygen and keep it away from the nitrogenase. This delicate balance allows life to thrive even in changing environments.

History shows us how much we have learned about this process. Jean-Baptiste Boussingault discovered biological nitrogen fixation in 1838. Later, in 1880, German agronomist Hermann Hellriegel discovered how the process actually happens. Dutch microbiologist Martinus Beijerinck provided a full description of the mechanism. In 1901, Beijerinck identified Azotobacter chroococcum as the first known species capable of fixing atmospheric nitrogen. This discovery helped launch the modern era of soil science. It changed how we understand the relationship between plants and the soil.

Many plants have formed incredible symbiotic relationships with diazotrophs. Members of the legume family, such as clover, soybeans, and peanuts, are famous for this. These plants grow small structures on their roots called nodules.

Inside these nodules, symbiotic rhizobia bacteria fix nitrogen for the plant. In return, the plant provides the bacteria with a home. Other plants, called actinorhizal plants, use Frankia bacteria to form similar nodules.

This process helps fertilize the soil naturally when the plants die.

Nitrogen fixation is essential for global ecosystems and human industry. In the oceans, the marine cyanobacterium Trichodesmium is a massive contributor. It is thought to account for nearly half of all nitrogen fixation in marine systems. On land, nitrogen is a key part of the Redfield Ratio, which describes the atomic ratio of carbon, nitrogen, and phosphorus in plankton. Humans also use industrial nitrogen fixation to create essential products. This includes nitrogenous fertilizers for agriculture, as well as pharmaceuticals, textiles, dyes, and explosives. Without these processes, the modern world would look very different.