The gas primarily responsible for ocean acidification is carbon dioxide (CO2). When carbon dioxide is absorbed by seawater, it undergoes a series of chemical reactions that decrease the pH of the water, making it more acidic. This acidification poses significant threats to marine ecosystems, particularly organisms with calcium carbonate shells or skeletons, such as corals and some shellfish. Increased ocean acidity can hinder their ability to grow and maintain their structures, impacting biodiversity and marine food webs.