The nitrogen cycle

Animals can not fix N2. They get their nitrogen by eating plants or by eating something that eats plants. Nitrogen Fixation is very expensive process In the biosphere, the nitrogen cycle is a vast collection of metabolic processes of different species function interdependently to salvage and reuse biologically available nitrogen.

Key terms of The Nitrogen Cycle Nitrogen Fixation: Conversion of N 2 to ammonia (NH3) By any bacteria in soil/water having the nitrogenase complex, e.g. Rhizobiumin root nodules of legumes. Nitrification: Conversion of ammonia to nitrite (NO2-) and then nitrate (NO3-). Both reactions carried out by bacteria

Assimilation: Conversion of NH3, NO2-,, NO3-( inorganic) into organic compounds (proteins, DNA, & other forms) All living cells (plants, animals, & bacteria). Ammonification: Conversion of the amine groups of organic compounds into simpler compounds (often, ammonia NH3). Mostly via decay processes carried out by decomposer bacteria

Denitrification: Conversion of NH3, NO2-,, NO3-to N2 Mostly by anaerobic bacteria in water logged soil, bottom sediments of lakes, swamps, bogs and oceans.

Overview of the N-cycle The first product of biological fixation is ammonia (NH3or +NH4). In principle: this ammonia can be used by most living organisms,However, soil bacteria and plant are in fierce competition for NH3 Bacteria are more abundant and active, but plants have their ways. In either case, Nitrification proceeds: NH3 -NO2 -NO3

Plants and many bacteria can also reduce nitrate and nitrite ammonia (reductases). -NO3 NO2 NH3 The new ammonia is incorporated into organic molecules by plants& bacteria. (Assimilation). When organisms die, microbial degradation of their proteins returns ammonia to restart the cycle. Some bacteria can convert nitrate to N2 under anaerobic conditions (denitrification)

Nitrification Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil.

Nitrifying bacteria Nitrifiers: (heterotrophs& autotroph) are delicate organisms and extremely susceptible to a variety of inhibitors. They are extremely slow growing Nitrifying bacteria need a relatively clean environment with a continuous supply of ammonia and oxygen.

Two bacterial species are required for nitrification: 1) Ammonia-Oxidizing Bacteria: Nitrosomonas Present in large numbers They require ammonia and CO2, and found in a great variety of soils, oceans, rivers, lakes, and sewage disposal systems

2) Nitrite Oxidation Microorganism: Nitrobacter Aerobic, but occasionally also anaerobic They are widely distributed in soils, fresh water, seawater, mud layers, sewage disposal systems, and inside stones of buildings, rocks, and inside concrete surfaces

Nitrate Assimilation NO3 NO2 NH4+ amino acids Requires large input of energy Forms toxic intermediates Mediated by enzymes (Reductases) that are closely regulated Nitrate levels, light intensity, and concentration of carbohydrates all influence the activity of nitrate reductases at the transcription and translation levels These factors stimulate a protein, phosphatase, that dephosphorylates several serine residues on the nitrate reductase protein thereby activating the enzyme

Ammonium Assimilation Ammonium is highly toxic, yet essential to both animals and plants. Animal & Plant cells rapidly assimilate into amino acids. In plants: this requires the action of two enzymes: Glutamine synthetase and Glutamate synthase

Denitrification Denitrification converts nitrates (NO3)in the soil to atmospheric nitrogen (N2) Denitrifying bacteria live deep in soil and in aquatic sediments where conditions make it difficult for them to get oxygen. The denitrifying bacteria use Nitrates as an alternative to oxygen, leaving free nitrogen gas as a byproduct. They close the nitrogen cycle!