Were the submariners fresh recruits or long-time personnel practiced at (acclimated to) operating in a high-CO2 environment? Is a submarine’s ratio of O2 to CO

Were the submariners fresh recruits or long-time personnel practiced at (acclimated to) operating in a high-CO2 environment? Is a submarine’s ratio of O2 to CO2 maintained at a normal fresh air ratio at its higher absolute CO2 level? etc.