From a sci-fi perspective, algae has been proposed as a food source for spacecraft life support systems. Going by the most optimistic estimates, 6 liters of algae produces enough oxygen and food to sustain a human indefinitely.

http://www.projectrho.com/public_html/rocket/celss.php#id--G...

I for one do not welcome a future where most people live on green slime and only the wealthy have grains/vegetables/meat. At least not without lots of investment into artificial flavoring.