What characteristic defines Cyanobacteria in ecosystems?

The defining characteristic of cyanobacteria in ecosystems is their ability to fix nitrogen, which is represented by the correct answer. Nitrogen fixation is a crucial process where certain organisms convert atmospheric nitrogen into ammonia or other nitrogenous compounds that can be utilized by plants and other organisms. This process not only aids in enriching the soil with essential nutrients but also supports plant growth, making cyanobacteria vital contributors to nutrient cycling in both terrestrial and aquatic ecosystems.

While cyanobacteria are known for producing oxygen as a byproduct of photosynthesis, this characteristic is not unique to them, as many other photosynthetic organisms, including algae and plants, also produce oxygen. They are indeed capable of thriving in low-light conditions since some species can survive in such environments; however, this trait is not the primary ecological role they are known for. Lastly, cyanobacteria are not limited to freshwater environments; they can also inhabit marine environments and extreme conditions such as hot springs and deserts, which further demonstrates their ecological versatility. Therefore, the nitrogen-fixing capability distinguishes cyanobacteria as key players in sustaining and enhancing the nutrient dynamics within ecosystems.