Microalgae in space
One issue with long-term space travel is creating enough oxygen for astronauts to breathe. Astronauts typically rely on machinery (requiring parts and maintenance) for this purpose. Microalgae can 1) convert carbon dioxide to oxygen, 2) be easily transported with astronauts, 3) use the natural "fertilizer" found in astronaut waste, 4) manufacture biofuel in space, and 5) allows food production in space.
Microalgae on submarines
A similar set of issues encountered by astronauts are encountered by submariners on long missions. Submarines have two separate processes to 1) scrub the carbon dioxide from the air and 2) create breathable oxygen from water. Microalgae can decrease reliance on these systems while using human waste onboard submarines.