A non-photosynthetic, aerobic, nitrogen fixing soil bacterium is

Explanation

Let's explain each option individually: a) Rhizobium: Rhizobium is a genus of soil bacteria that are known for their ability to form a symbiotic relationship with leguminous plants. They are aerobic and can fix atmospheric nitrogen into ammonia within specialized structures called nodules on the roots of legume plants. This nitrogen fixation benefits both the bacteria and the plants in a mutualistic association. b) Clostridium: Clostridium is a genus of bacteria that includes various species, some of which are involved in nitrogen fixation. However, the nitrogen-fixing species within the Clostridium genus are anaerobic, meaning they carry out their activities in the absence of oxygen. Clostridium species are well-known for their diverse metabolic capabilities, including the production of spores and their role in various biological processes. c) Azotobacter: This option is correct. Azotobacter is a genus of non-photosynthetic, free-living aerobic nitrogen-fixing bacteria. These bacteria are commonly found in soil environments and can fix atmospheric nitrogen into ammonia, making it available for plants and other organisms. Azotobacter plays a significant role in the nitrogen cycle, contributing to soil fertility. d) Klebsiella: Klebsiella is a genus of bacteria that includes both nitrogen-fixing and non-nitrogen-fixing species. Some species of Klebsiella are capable of nitrogen fixation, while others are not involved in this process. Like Azotobacter, some nitrogen-fixing Klebsiella species are also free-living aerobic bacteria that contribute to nitrogen availability in the soil. In summary, the correct answer is c) Azotobacter. Azotobacter is a non-photosynthetic, free-living, aerobic soil bacterium that can fix atmospheric nitrogen into ammonia, playing an essential role in nitrogen cycling and soil fertility. The other options (Rhizobium, Clostridium, and Klebsiella) are also relevant to nitrogen fixation but differ in their specific characteristics and associations with plants or other organisms.