Many people think of seaweed as a marine plant, but in fact, all seaweeds are actually colonies of algae. There are three different phyla of seaweed: red algae (rhodophyta), green algae (chlorophyta) and brown algae (phaeophyta). 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. Of the approximately 1,800 species of brown algae, around 99 percent are marine. This group contains the largest and most complex seaweed species, giant kelp.
Function of Air Bladders
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.
Structure of Air Bladders
The air bladders of brown algae seaweeds, known as pneumatocysts, are small, balloon-like structures located at the bases of the blades. They are filled with a mixture of oxygen, nitrogen and carbon dioxide resulting both from the metabolic activity of the surrounding cells and from equilibration between the gases in the bladder and gases in the surrounding water.
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.