How does oxygen impact N assimilation by microbes? How do microbes protect against the detrimental impact of oxygen on N assimilation?
What will be an ideal response?
N-fixation - like the fixation of carbon - is a very energy demanding process. Therefore, it requires oxidative respiration (O2 as the terminal electron acceptor in the ETS) to generate enough ATP to run the N fixation process.
Aside from the high energy cost, a major issue regarding nitrogen fixation is that the enzyme responsible for n-fixation is highly sensitive to oxygen. O2 sensitivity occurs because of the large reducing power needed to make NH4+ (O2 wants the e-s!). Also, In the presence of oxygen the Fe protein, which is the first in the two-component system, is damaged in a way that cannot be reversed. To protect against the detrimental effect of O2 on nitrogenase activity, N-fixers can:
(1) separate phototrophy from N fixation temporally (during the day, the bacteria grow photosynthetically and generate ATP and reducing power. At night, photosynthesis ceases and nitrogenase becomes active)
(2) separate phototrophy from N fixation spatially (one example is free-living cyanobacteria with specialized cells called heterocysts. In the vegetative cells, photosynthesis goes on as normal and no nitrogenase is produced. In the heterocyst cells, which are thick-walled that oxygen can't easily diffuse in, nitrogenase is produced and can be active.)
(3) bacteriods (which are bacterial cells from the Rhizobia genus living in the roots of legumes) let plant leghemoglobins bind to O2 and transport it directly to the bacteria's ETS (rather than letting the O2 float around free in the cell).
(4) The transcription of genes coding for nitrogenase is turned off in the presence of O2 (when oxygen levels are high, NifA is blocked from activating transcription of nitrogenase genes by binding NifL)
Ammonium oxidation (i.e. nitrification) requires oxygen to be present (so that ammonium can be oxidized!), but denitrification is a strictly anaerobic process.
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