nitrogen fixation is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia.
nitrogen fixation is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia.
what is the first step in the nitrogen cycle when gaseous nitrogen is converted into ammonia?
nitrogen fixation
atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. between 5 and 10 billion kg per year are fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. these bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds. most biological nitrogen fixation occurs by the activity of mo-nitrogenase, found in a wide variety of bacteria and some archaea. mo-nitrogenase is a complex two-component enzyme that has multiple metal-containing prosthetic groups. an example of free-living bacteria is "azotobacter". symbiotic nitrogen-fixing bacteria such as "rhizobium" usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. a few non-legumes can also form such symbioses. today, about 30% of the total fixed nitrogen is produced industrially using the haber-bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.
nitrogen cycle
denitrification is the reduction of nitrate and nitrite to the gaseous form of nitrogen, including the greenhouse gas nitrous oxide, that is released back into the atmosphere. denitrification closes the nitrogen cycle by reducing the nitrogen available in organic matter fixed by phytoplankton at the surface ocean. omzs are known for the role in the global nitrogen cycle, where the different chemical species of nitrogen and different microbes play its part. under these anoxic conditions, nitrate is reduced to gaseous nitrogen or nitrite and ammonia in a process called dissimilatory nitrate reduction by the protein nitrate reductase. the ammonia and nitrite produced are further reduced by anaerobic ammonia-oxidizing bacteria (anammox) using an enzyme called hydrazine oxidoreductase. genomic studies conducted in these ecosystems reveal a growing abundance of the genes encoding for the proteins responsible for the dissimilatory nitrate reduction and anammox at the core of these omzs. such studies also provide information to map out the nitrogen cycle in the water column and provide with missing links and unexplored pathways. denitrification from omzs leads to a significant loss of inorganic nitrogen from the oceans and thus limiting growth/productivity in many regions around the world.
microbiology of oxygen minimum zones