Elevated carbon dioxide levels dramatically change how reishi mushrooms (Ganoderma lucidum) grow and what compounds they produce. High CO₂ suppresses normal cap formation, pushes the mushroom into elongated “antler” shapes, and rewires metabolism by reducing amino acids while shifting triterpenoid and steroid profiles. The findings explain why poor ventilation during cultivation leads to misshapen fruiting bodies and altered medicinal chemistry, and why CO₂ control directly affects reishi quality.