Solidified Air Bubbles were vital for the survival of the Matoran in Mahri Nui. They were locatated in underwater plants known as airweed and harvested by Hydruka. Air Bubbles would trap a Matoran inside, so he/she would be able to breath underwater while they were away from the city.
Weapons
The Matoran of Mahri Nui later developed a way to use the air bubbles as weapons. Based off the natural weapons of Hydruka, these Air Launchers fired bubbles of solidified air. On impact, the bubbles changed back into a gas-like form, toxic to any species adapted or naturally breathing underwater.