What is a Pneumatocyst?

Which part of brown algae is most important in photosynthesis?

Most macroalgae (multicellular algae) are made up of three parts. The blade is a leaf-like structure that captures light and carries on photosynthesis. The stipe is the stem of the algae. The holdfast is the anchor for the algae; it holds the algae to a solid surface such as a rock or pier. Although the holdfast may look like roots, it does not function to get nutrients or water. Some algae may have a fourth structure, the pneumatocyst; this structure is an air bladder on the stipe or blade that functions to keep the algae afloat in the water column.