Lecture 26: N-cycle Nitrogen Fixation 2024 PDF

Lecture 26 1. Overview of the N-Cycle The Nitrogen Cycle 2. Nitrogen Fixation 3. Nitrification 4. Denitrification 5. Examples: Fish Tanks, Agriculture Overview Features of Biogeochemical...

Lecture 26 1. Overview of the N-Cycle The Nitrogen Cycle 2. Nitrogen Fixation 3. Nitrification 4. Denitrification 5. Examples: Fish Tanks, Agriculture Overview Features of Biogeochemical Cycles Each step is mediated by sets of organisms Cycling replenishes reactants over time Cycling couples the oxidation and reduction of reactants, and this typically requires coupling of aerobic and anaerobic Discuss with your neighbor: How do the groups of bacteria involved in the nitrogen cycle probably make their ATP?  We focus on the following 3 processes today: 1. Nitrogen fixation: turning N2 gas into amino acids 2. Nitrification: production of nitrate (NO3) from ammonia (i.e. from biomass, nitrogen fixation) via aerobic respiration 3. Denitrification: Turning nitrate into N2 gas via anaerobic respiration Oxidized Oxidation State NO3- (+5) Nitrate Highly mobile in soil NO2- (+3) Nitrite Reactive NO (+2) Nitric Oxide Reactive gas N2O (+1) Nitrous Oxide Greenhouse gas (anesthetic) N2 (0) Dinitrogen Inert Gas NH3 (-3) Ammonia Gas, (NH3 + H2O  NH4+ + OH-, pH 7) NH Reduced 4 + (-3) Ammonium Low mobility in soil Overview The Nitrogen Cycle Animals Organic N (protein)  Organic N, (e.g. protein, urea) Organic N (protein)  NH4+ Plants NO3  Organic N (protein) NH4  Organic N (protein) Overview Plants Microbes Animals N2 Fixation N2 Fixation Nitrogen Fixation Nitrogenase (NifH, NifK, NifD) is the key enzyme Assimilatory, requires lots of energy Inhibited by O2, downregulated by NH4+ N2 fixed into NH4+ (in glutamine) Performed by diverse diazotroph microorganisms e.g. Rhizobia (aerobic chemo-organo-heterotrophs) N2 Fixation Rhizobia Symbiotic N2-fixation Symbionts of Legumes: e.g. bean, clover, alfalfa, etc. Microbes provide N to plant Plant provides - C and Energy source - A nice safe place to live - Leghemoglobin N2 Fixation Cyanobacterial heterocysts for nitrogen fixation Krithika Kumar et al. Cold Spring Harb Perspect Biol 2010;2:a000315 Nitrification Nitrification (Bacterial Phyla: Proteobacteria, Nitrospira) (Archaeal Phlya: Thaumarchaeota) Metabolism: Chemo-litho-autotrophs, obligate aerobes Key enzyme: Ammonia monooxygenase (AmoA) Electron Donors: NH4+ or NO2- Electron Acceptors: O2 Carbon Sources: CO2 (typically) Habitat: widespread in oxic soil and aquatic Nitrification Ammonia oxidizers: 2 NH3 + 3 O2  2 NO2- + 2 H2O + 2H+ e.g. Nitrosomonas (Bacteria) NO3-/NH4+ e.g. Nitrosopumilus (Archaea) Nitrite oxidizers: 2 NO2- + O2  2 NO3- e.g. Nitrobacter Nitrification Nitrous Oxide e will then be further oxidized to Nitrate by e.g. Nitrobacter Sewage, full of organic N, enters the Ithaca waste water plant every day. The most important step in biological treatment involves bubbling air though the waste stream. What is the major form of N in the discharge from the plant? A. N2 B. NO3 C. NH3 D. N2O E. Microbial biomass Denitrification Denitrification (Phylogenetically diverse) A form of anaerobic respiration Metabolism: Chemo-organo-heterotrophs, facultative anaerobes (typically) Electron Donors: Organic carbon (typically) Electron Acceptors: NO3-, NO2- Carbon Sources: Organic carbon Habitat: widespread anoxic soil and aquatic habitats Return N to the atmosphere as N2 or N2O gas Denitrification (nar/nap) (nirK/nirS) NO3-/NH4+ (norB) (nosZ) Denitrification Denitrification mate electron donor is organic carbon (e.g. glucose) xamples: Fish Tank xamples: Fish Tank Removes Nitrate! xamples: Agriculture N-cycle Examples: Agriculture Humans have more than doubled the input of NH3 into the biosphere though the Haber-Bosch process and the widespread use of legume crops. xamples: Agriculture N-cycle Examples: Agriculture N in fertilizer is mostly in NH4+ Nitrifiers convert NH4+ to NO3- NO3- moves with water in runoff, a cause of eutrophication Denitrifiers convert NO3- into N2O and N2 gasses N2O has ~310x more powerful Global Warming Potential than CO2 N2O promotes acid rain (becomes nitric acid in atmosphere)  N-cycle Examples: Agriculture Eutrophication: nutrient enrichment induced water pollution Caused by N and P in runoff (from fertilizers) NY Times, 10/3/17: https://www.nytimes.com/interactive/2017/10/03/science/earth/lake-erie.html xamples: Agriculture Ocean Dead Zones (anoxia)