So what do seaweeds have in common with terrestrial plants? Both require sunlight to photosynthesize. The image above shows oxygen bubbles forming around a photosynthesizing seaweed, in a quiet tidepool.

How do seaweeds photosynthesize under water? Seaweeds have a variety of adaptations for carrying out photosynthesis under the water. Since algae live underwater, they are surrounded by water and nutrients. The algae absorb water and nutrients directly from seawater. Seaweeds have flat blades and the individual cells that make up the alga can absorb the water and nutrients that they need directly. The blades also have air bladders that keep them floating at the surface, within reach of plenty of sunlight.

The main materials required for photosynthesis are carbon dioxide, water and sunlight. Seaweeds absorb carbon dioxide (CO₂) that is dissolved in seawater. CO₂is sometimes limiting in areas where species are exposed to air during low tides. Water is not a limiting factor because they are immersed in it. Light is perhaps the most important limiting factor because as depth increases, light decreases.