Final answer:
Cyanobacteria can perform nitrogen fixation to convert atmospheric nitrogen N₂ into ammonia NH₃, allowing them to grow in environments with limited nitrogen availability. Therefore correct option is A
Step-by-step explanation:
The organism that can get its nitrogen from nitrogen fixation and grow even if easily used forms of nitrogen are not available in its environment is a. Cyanobacteria. These single-celled organisms have the capability to fix atmospheric nitrogen, N₂, into ammonia, NH₃, which can then be utilized for their growth. Cyanobacteria can live in aquatic environments where they play a crucial role in the nitrogen cycle and are well-equipped to perform nitrogen fixation thanks to specialized cells called heterocysts.
The heterocysts protect nitrogenase, the enzyme responsible for the fixation of nitrogen, from oxygen which inhibits its function. Hence, cyanobacteria are vital in ecosystems, especially in areas with nitrogen-limited conditions where they can convert an otherwise inaccessible form of nitrogen in the atmosphere into a form that can be used by themselves and other organisms.