What will happen to the yeasts if the vessel carrying out yeast fermentation is tightly sealed? Will the rising level of CO2 inside the vessel be toxic to the growing yeasts or the yeasts have a strategy to overcome this situation?
Major problems derived from alteration of medium pH (1) and over pressure (2) should be considered.
(1) pH: Dissolved CO2 would acidify the growth medium:
CO2 + H2O HCO3- + H+
(2) The amount of CO2 formed is high: remember that 2 mol CO2 are produced per mol glucose, which would mean a high increase in vessel inner pressure. This could lead to an accident!
Yes, the fermenting yeast has strategies to overcome virtually any hard situation. However, the resistance to hazardous conditions is of course finite. So I will complete the reply given by Vicente Bernal.
(3) Although yeast can easily grow in anaerobic conditions, the excess of dissolved CO2 caused by the high pressure will mainly hinder yeast growth by unbalancing decarboxylation reactions.