Pneumatocyst

Phytology
A pneumatocyst is a large float containing gas found in brown algae. An organism may have more than one. They provide buoyancy to lift the blades toward the surface, allowing them to receive more sunlight for photosynthesis.

The proportion of gases in the pneumatocysts adjusts by equilibrating with the surrounding air or water. The pneumatocyst can hold O2, CO2 and CO.

Medicine
A pneumatocyst can also be an air bladder in the lungs which generally appears after a pulmonary trauma. These also go by the name of traumatic pneumatoceles.