Biomass is biological matter that can be used by the zerg in order to evolve the Swarm.
Left 2 DieEdit
In the official arcade map Left 2 Die, biomass is used as a resource, and is obtained by killing special zerg units and destroying infested buildings. A science facility can be used to spend biomass in new units, structures and upgrades.
Heart of the SwarmEdit
On a island on the east side of the map "Old Soldiers", there is an outhouse labeled "Terran Biomass."