Most of what we eat is just processed CO2 from the air and water. In theory you can capture CO2 from the air, add water, add in some trace elements, run it through a very extensive process and have Soylent coming out at the other end. Not cost effective, but should be possible.
You could also start using artificially created cells to do the processing, so it's not really alive as in evolved in nature and more like bio-robots.