Recently Pat Brown introduced the Impossible Burger with the "magic" ingredient of heme, a naturally occurring bio-chemical in animals, that makes meat smell, taste and bleed like meat. What are the naturally occurring non-animal sources of heme?
Heme is a chemical compound most widely known because of it being a part of hemoglobin - the oxygen carrier in our bodies. It is, however, found in other hemoproteins such as myoglobin (muscle tissues) etc.
It is usually found only in animals - that is also part of the reason why the absorption rate of iron from these sources is estimated to be 7%-35% as opposed to 2%-20% absorption rate of iron from plant sources (which do not bind iron to heme). It can, however, be found and extracted from root nodules of some plants, notably of soy. The way folks over at Impossible Foods get their heme is by fermentation of engineered yeast - not unlike other "fake meat" companies like Quorn which use engineered and lab-grown mycoproteins.
TL;DR: It is most likely grown from engineered yeast, or, alternatively, harvested from root nodules of certain plants.