Hello Nasmin,

Brown algae are the only seaweeds that have air bladders. The brown coloring of seaweeds in the phylum phaeophyta comes from the pigment fucoxanthin, which helps them to absorb sunlight more efficiently, allowing them to live in deeper water than other seaweed species.

All brown algae are photosynthetic, meaning they produce their own food from sunlight. In larger brown algae species, like kelp, the blades (leaves) have air bladders because they would otherwise be too heavy to float on the surface of the sea, and thus they would not be able to access the sunlight they need for photosynthesis.

Brown algae seaweeds live mainly in cold water, and the larger species are so numerous that they can sustain entire ecosystems in their own right. The air bladders of giant kelp are so buoyant that sea otters are able to use the blades as anchors to prevent themselves from floating away when they sleep.