I found the answer written almost verbatim, as if scientists have been puzzled by this for generations. It seems these two scientists were as fascinated with yeast flatulence as you are. If these spheres are transferred into an aqueous solution of sugar they develop into small animals. They are endowed with a sort of suction trunk with which they gulp up the sugar from the solution.
Digestion is immediately and clearly recognizable because of the discharge of excrements.
These animals evacuate ethyl alcohol from their bowels and carbon dioxide from their urinary organs. Thus one can observe how a specifically lighter fluid is exuded from the anus and rises vertically whereas a stream of carbon dioxide is ejected at very short intervals from their enormously large genitals. Translated into cleaner, less potty-oriented language, yes, yeast does emit CO2 in gas phase. I shudder to consider how you sing to yeast. If in fact you are communicating in like manner with our tiny little friends, the pioneering work of Jane Goodall and her chimpanzee research will pale in comparison.