Question

Nitrogen fixation is a widely distributed function across the microbial world. The fact that nitrogenase, the key enzyme for nitrogen fixation, is irreversibly inactivated by oxygen, yet we see a number of aerobic and facultatively-anaerobic bacteria that fix nitrogen. Compare the mechanisms for dealing with oxygen toxicity in the symbiotic diazotroph, Bradyrhyzobium, and the free-living diazotroph, Azotobacter.

Answer 1

Nitrogen fixing bacteria creates anaerobic environments to fix atmospheric nitrogen and symbiosis with another member is one way. Bradyrhizobium which follows this way of symbiotic nitrogen fixation by handshake with legumes species like Soybean (Glycine max). Bradyrhizobium and other symbiotic diazotrophs usually dwells within roots of legume plants by forming root nodules. Root nodules are specialized structures possess thick wall that checks gas permeability; especially entry of oxygen within. In this way, bacteria create an anaerobic environment for itself and in return of it provide nitrogen nutrition to host plant. As gas permeability increases in the light, root nodules are formed on underground plant part i.e. roots. Further, nodules contains specialized protein leghaemoglobin which actively utilizes oxygen as it has higher affinity for oxygen.

Azotobacter, on the other hand is an aerobic diazotroph. It deals with oxygen toxicity through high rate of respiration. Some aerobic diazotrophs deals with oxygen toxicity by temporal management. They photosynthesize during day and fix nitrogen during night.