Lecture 36: Microbes and the N CycleMMG 301 Fall 2002 Page 1 Lecture 36: Microbes and the N Cycle (unless noted otherwise, all figures & tables are from: Microbial Life by Perry, Staley & Lory (2002) Sinauer Associates, Inc.) 14.01 N a group VA (15) element; electronic configuration: 1s2 2s2 2p3; stable; 99.6% 7 15.01 13.01 N stable; 0.4% N unstable; radioactive; ½ life ≈ 10 min!! 7 7 Oxidation state of key nitrogen compounds Compound Oxidation State of N Organic N: R-NH2 (amino N) -3 Ammonia: NH3 -3 Dinitrogen: N2 (N≡N) 0 Nitrous oxide N2O +1 Nitric Oxide NO +2 Nitrite: NO2- +3 Nitrate: NO3- +5 Largest reserve of nitrogen is atmospheric Major constituents of dry air 78% nitrogen (very stable) 21 % oxygen 1% argon 0.03 % carbon dioxideMMG 301 Fall 2002 Page 2 Nitrogen Fixation - chemical (industrial) N2 fixation: Haber- Bosch process Al, Fe N2 + 3H2 -----------> 2NH3 400°C 400 atm - biological N2 fixation: uniquely prokaryotic process by which N2 (N≡N) is fixed into cellular material (energy-demanding; reducing power-demanding) Nitrogenase (an O2-sensitive, Fe-Mo-enzyme) N≡N + 8H+ + 8e- + 16 ATP ------> 2NH3 + H2 + 20-30 ADP + 16 Pi Assays for N2 fixation: 15N2 fixation into cell material (mass spec analysis) : Acetylene Reduction to Ethylene: HC≡CH↑ + 2 H+ + 2 e- → C2H4↑ (gas chromatography)MMG 301 Fall 2002 Page 3 Many different N2-fixers (diazotrophs) among Bacteria (and some Archaea). Azotobacter sp. (Fig. 19.10) Cyanobacteria: Big-Time Nitrogen Fixers, e.g. Anabaena sp. (Fig. 21.21). - “Simple” growth requirements: Light, CO2, N2, SO4, PO4, trace elements: Fe, Mo, Mg, others How do cells protect nitrogenase from inactivation by O2??MMG 301 Fall 2002 Page 4 Nitrification Chemolithotrophic growth on reduced nitrogen compounds (cell C from CO2). NH3 →→→→ NO2- →→→→ NO3- (not balanced) Common genera of nitrifying bacteria: Nitrosomonas europea (Fig. 19.34b) “Nitroso”-group: ammonia oxidizers, e.g. Nitrosomonas NH3 + 1.5 O2 →→→→ NO2- + H2O + H+ (a 6-electron oxidation) ∆∆∆∆Go’ = -275 kJ “Nitro”-group: nitrite oxidizers, e.g. Nitrobacter, Nitrococcus NO2- + 0.5 O2 -------> NO3- (a 2-electron oxidation) Nitrococcus mobilis (Fig. 19.34h) ∆∆∆∆Go’ = -76 kJ Denitrification NO3- + 5e- + 6 H+ →→→→ 0.5 N2↑↑↑↑ (+ N2O↑↑↑↑) + 3 H2O Pseudomonas sp. - nitrate used as terminal electron acceptor in anaerobic respiration. - phylogenetically widespread trait - Pseudomonas : common genus of denitrifying bacteria in
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