I doubt it's the Lacto that carbonated it. I looked through some literature and could only find papers that say Lacto produces small amounts of carbon dioxide. Anyone has info to support or contradict?
If you have a truly homofermentative strain of Lacto, it won't produce any CO2. If it's heterofermentative, it will definitely produce CO2. It will produce half as much CO2 and ethanol per glucose molecule as yeast would, which is still quite a lot of CO2.
I hate to quote wikipedia, but here's what I found: "During fermentation, pyruvate is metabolised to various compounds. Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols."
"The chemical equation below shows the alcoholic fermentation of glucose, whose chemical formula is C6H12O6. One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules: C6H12O6 → 2 C2H5OH + 2 CO2"
"In heterolactic fermentation, the reaction proceeds as follows, with one molecule of glucose being converted to one molecule of lactic acid, one molecule of ethanol, and one molecule of carbon dioxide:
C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2"