Seagrass

Seagrasses are a unique group of marine angiosperms, which are flowering plants. They are the only flowering plants that grow entirely within marine environments.

These plants form vast underwater meadows that serve as highly productive ecosystems. These meadows act as vital carbon sinks, which means they store carbon from the environment. They also provide essential habitats and food for many marine organisms. In many ways, their biological importance is comparable to that of coral reefs.

To survive underwater, seagrasses have developed highly specialized biological mechanisms. They grow using a rhizome, which is an underground stem that helps anchor the plant in sand or mud.

Their leaves are adapted for life in the submerged photic zone, where sunlight can reach them. Unlike land plants, seagrass leaves have a reduced cuticle and lack stomata, which are the tiny pores used for gas exchange on land. Instead, the epidermis, or the outer layer of the leaf, serves as the primary photosynthetic tissue. The roots can survive in anoxic environments, which are areas lacking oxygen. To stay alive, the roots depend on oxygen transported down from the leaves and rhizomes.

Seagrasses are categorized into several distinct families within the order Alismatales. There are approximately 60 species of fully marine seagrasses. The family Zosteraceae, often called the seagrass family, includes 14 marine species found in temperate and subtropical waters.

The Hydrocharitaceae family, or tape-grasses, includes both fresh and marine species, though most are tropical. The Posidoniaceae family is found specifically in the Mediterranean and the southern coast of Australia.

Finally, the Cymodoceaceae family, known as manatee-grass, consists only of marine species.

Some scientists also include the genus Ruppia in these discussions, though it grows in brackish water.

The evolutionary history of seagrasses is a fascinating story of plants returning to the sea. While terrestrial plants evolved from green algae around 450 million years ago, seagrasses followed a different path. Between 70 and 100 million years ago, land plants migrated back into the ocean.

This did not happen just once. Instead, seagrasses are a paraphyletic group, meaning they evolved in parallel through three to four different lineages. This means different groups of land plants independently adapted to marine life. This transition required radical genetic changes, such as losing stomatal genes and regaining genes involved in sulfation.

One of the most interesting aspects of seagrass biology is how they reproduce. Most species undergo submarine pollination, meaning they complete their entire life cycle underwater.

While scientists once thought pollination happened only through water currents, we now know some species use animals. For example, the species Thalassia testudinum uses a mixed biotic-abiotic strategy. It produces nutritious, mucigenous clumps of pollen to attract tiny creatures. Crustaceans, such as certain crabs, and syllid polychaete worm larvae have been found carrying pollen grains. This is a different method than the nectar used by many land flowers.

The physical structure of seagrass is also unique due to its cell wall composition. Their cell walls contain cellulose, which is common in land plants. However, they also contain sulfated polysaccharides, a feature typically found in marine macroalgae like red or brown algae.

They also possess unusual pectic polysaccharides called apiogalacturonans. Recent research in 2020 also identified arabinogalactan proteins in seagrasses. These proteins may play a role in osmoregulation, which helps the plant manage salt levels. These complex cell walls combine features from both land plants and sea algae.

Despite their importance, seagrasses face significant modern threats. Many seagrass habitats are declining across the entire world. In North American coastal waters alone, there are 26 different species. Unfortunately, ten of these species are at an elevated risk of extinction. This represents 14% of all known seagrass species.

Three of these species are officially classified as endangered. The loss of these meadows could have serious repercussions for marine biodiversity and the humans who depend on them. Protecting these resources is essential for maintaining healthy coastal ecosystems.