The correct answer is option 2: Bacteria. Bacteria play a crucial role in the nitrogen cycle, which is the circulation or recycling of nitrogen in the ecosystem. Nitrogen is an essential element for all living organisms, but most organisms cannot directly use the nitrogen gas present in the atmosphere. Bacteria have the ability to convert atmospheric nitrogen into a usable form through a process called nitrogen fixation. They convert nitrogen gas into ammonia, which can then be used by plants to make proteins. Other types of bacteria, called nitrifying bacteria, convert ammonia into nitrate, and then yet another type of bacteria, called denitrifying bacteria, convert nitrate back into nitrogen gas. This cycle of nitrogen fixation, nitrification, and denitrification is important for maintaining nitrogen balance in the ecosystem.