BIOL 2004 Lecture 16: Diversity of Bacteria II
17 Mar 2017
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Nitrifiers: two kinds, ammonia oxidisers: nh3 --> no2, nitrite oxidisers: no2 --> no3, highly positive reduction potentials, aerobes, nitrogen cycle in soil (very simplified) Nitrogen fixation = very important: carried out only by certain bacteria and archaea, many (not all) nitrogen fixers are autotrophs, ocean: 40-80% of world total, where nitrogen fixation occurs; mostly cyanobacteria. In symbiotic association with certain plants: especially: rhizobia (alphaproteobacteria again, reduction of atmospheric nitrogen (n2) to ammonia. For biosynthesis: catalyzed by nitrogenase enzyme complex. Large energy expenditure (high activation energy of n2: 16+ atp per n2 --> 2nh3. Includes 6 high-energy electrons: part of nitrogenase complex inactivated by oxygen: problem for aerobic nitrogen fixers, some coping strategies: Facultative aerobes often fix nitrogen only under anaerobic conditions. Some free-living alphaproteobacteria: metabolic activity + thick capsule keeps cellular. O2 low: cyanobacteria - "heterocysts": non-photosynthetic cells with thick walls. Just remember main points, no need to remember specific details.
