In nitrogen assimilation, plants finally consume the nitrates made by soil bacteria and use them to make nucleotides, amino acids, and other vital chemicals for life. Interestingly, high-energy environments such as lightning strikes and volcanic eruptions can convert nitrogen gas directly into nitrates – but this doesn’t happen nearly enough to keep modern ecosystems healthy on its own! Assimilation
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You may actually hear such fertilizer referred to as “nitrate fertilizer.” By pumping the soil full of nitrates, such fertilizers allow plants to grow large quickly, without being dependent on the rate at which nitrogen-fixing bacteria do their jobs! Nitrates – the end product of this vital string of bacterial reactions – can be made artificially, and are the main ingredient in many soil fertilizers. The process can be thought of as a rough (and much less efficient) analog to the cellular respiration performed by animals, which extract energy from carbon-hydrogen bonds and use oxygen as the electron acceptor, yielding carbon dioxide at the end of the process. By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes. Rather, they are “ chemotrophs” who obtain their energy from volatile chemicals. These bacteria don’t convert ammonia for plants and animals out of the goodness of their hearts. Then another type of soil bacterium, called Nitrobacter, adds a third oxygen atom to create nitrate. This requires two steps, performed by two different types of bacteria.įirst, soil bacteria such as Nitrosomonas or Nitrococcus convert ammonia into nitrogen dioxide.
In nitrification, a host of soil bacteria participate in turning ammonia into nitrate – the form of nitrogen that can be used by plants and animals. You can see the oxygen-free Rhizobium nodules, visible as big round lumps, on the roots of this cowpea plant: Rhizobia nodules on Vigna unguiculata Nitrification The hard casing of these nodules keeps oxygen out of the pockets where Rhizobium bacteria do their valuable work of converting nitrogen gas into ammonia. As a result, organisms that use it have had to develop oxygen-free compartments in which to perform their nitrogen fixation!Ĭommon examples of such nitrogen-free compartment sare the Rhizobium nodules found in the roots of nitrogen-fixing legume plants. Interestingly, the enzyme nitrogenase can only function when oxygen isn’t present. Ammonia is a nitrogen compound that can dissolve in water, and is easier for other organisms’ enzymes to interact with. These nitrogen-fixing bacteria, often called “diazotrophs,” have an enzyme called “nitrogenase” which combines nitrogen atoms with hydrogen atoms. In the process of nitrogen fixation, bacteria turn nitrogen gas from the atmosphere into ammonia. We’ll discuss each part of the process below. The basic steps of the nitrogen cycle are illustrated here: Nitrogen Cycle Steps Here we will discuss how nitrogen plays a vital role in the chemistry of life – and how it gets from the atmosphere, into living things, and back again. However, nitrogen gas is not accessible to plants and animals for use in their cells. Nitrogen, on the other hand, is inert and harmless in its gaseous form. This is an ideal balance because too much oxygen can actually be toxic to cells. Today, the Earth’s atmosphere is about 78% nitrogen, about 21% oxygen, and about 1% other gases.
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