The primary gas in the bubbles produced by fermentation is carbon dioxide (CO2). During fermentation, microorganisms such as yeast convert sugars into energy, producing CO2 and ethanol as byproducts. This process is not only significant in biological systems but is also widely used in food and beverage production, such as in the making of bread, beer, and wine. The release of CO2 is what causes dough to rise and contributes to the characteristic effervescence in fermented beverages.