Question

What kinds of organisms occur in nitrogen-fixing symbioses with plants? a. cyanobacteria b. actinobacteria c. rhizobia d. all of the above e. none of the above

Short answer

The correct answer is 'd. all of the above'. Cyanobacteria, actinobacteria, and rhizobia can all occur in nitrogen-fixing symbioses with plants.

Step-by-step solution

Understand Symbiosis and Nitrogen Fixing

Symbiosis is a close and long-term biological interaction between two different biological organisms. Nitrogen fixation is a process by which nitrogen in the atmosphere is converted into ammonium. Certain kinds of bacteria are known to fix atmospheric nitrogen, a process much of life on Earth depends on.

Analyze Each Option

Analyze each given option one by one. Cyanobacteria, also known as blue-green algae, have the ability to fix nitrogen. Actinobacteria, a type of gram-positive bacteria, also contains species capable of nitrogen fixation. Rhizobia are nitrogen-fixing bacteria that form symbiotic relationships with leguminous plants.

Choose the Right Answer

Based on the analyses, all of the mentioned organisms: cyanobacteria, actinobacteria, and rhizobia can form nitrogen-fixing relationships with plants. Thus, the correct option would be 'd. all of the above'.

Key concepts

Symbiosis

Symbiosis refers to the interaction between two different organisms living in close proximity, often to the mutual benefit of both. This relationship can take several forms, including mutualism, commensalism, and parasitism. In the context of nitrogen fixation, symbiosis usually describes a mutualistic relationship. For instance, certain bacteria and plants work together to sustain ecosystems through nitrogen fixation. The plant provides carbohydrates and a suitable environment for the bacteria, and in return, the bacteria convert atmospheric nitrogen into forms that the plant can use for growth, like ammonium or nitrate. This reciprocal relationship is vital for nutrient cycling in ecosystems, ensuring plants get the nitrogen they need to thrive.

Cyanobacteria

Cyanobacteria, often referred to as blue-green algae, are a phylum of bacteria that obtain their energy through photosynthesis. These microorganisms play a critical role in nitrogen fixation, particularly in aquatic environments. Cyanobacteria can convert atmospheric nitrogen ( N_2 ) into ammonium ( NH_4^+ ), which is accessible to plants and other organisms. This process is especially prominent in bodies of water where cyanobacteria grow extensively. Some species of cyanobacteria form symbiotic relationships with plants like the water fern Azolla, aiding in their nitrogen requirements. Besides aiding in nitrogen fixation, they are also responsible for oxygen production that supports aquatic life.

Actinobacteria

Actinobacteria are a group of Gram-positive bacteria characterized by high-GC content in their DNA. This phylum includes a variety of species that play a role in soil health. Some actinobacteria, like those in the genus Frankia, engage in symbiotic nitrogen fixation. They form associations with the roots of actinorhizal plants, such as alder trees, converting atmospheric nitrogen into a form the plant can uptake. This process helps improve soil fertility and allows plants to thrive in nutrient-poor environments. Actinobacteria are also crucial in decomposing organic matter, thus playing a significant role in carbon cycling.

Rhizobia

Rhizobia are a group of bacteria known for their ability to form symbiotic relationships with leguminous plants, such as soybeans, peas, and lentils. These bacteria invade plant root hairs to form nodes where nitrogen fixation takes place. In these nodes, rhizobia convert atmospheric nitrogen into ammonia, which is then assimilated into amino acids by the plant. This symbiosis is essential for sustainable agriculture, as it reduces dependency on synthetic fertilizers. Moreover, rhizobia can improve soil health, making them integral to crop rotation practices. Farmers often rely on rhizobia-based inoculants to enhance plant growth and yield through this natural process. By improving soil nitrogen content, rhizobia contribute significantly to sustainable farming and ecosystem health.