Bacteria do not replenish oxygen in the air. Plants are primarily responsible for releasing oxygen into the atmosphere through the process of photosynthesis, which converts sunlight into chemical energy while producing oxygen as a byproduct. Bacteria, on the other hand, play different roles in the environment, such as decomposing organic matter, participating in nutrient cycling, and some bacteria, known as cyanobacteria or blue-green algae, can perform photosynthesis and contribute to oxygen production in certain habitats like aquatic ecosystems.