Seagrass

Seagrasses are special flowering plants that grow in the ocean. Unlike seaweed or beachgrass, they are true plants with flowers, and there are about 60 different kinds. They belong to four plant families and evolved from land plants that moved into the sea around 70 to 100 million years ago.

These plants have long, narrow leaves that spread out in large underwater areas called meadows, looking a bit like grass on land. They live in shallow coastal waters where sunlight can reach them, anchored in sandy or muddy bottoms. Like other plants, seagrasses make their own food using sunlight through a process called photosynthesis.

Seagrass meadows are very important for the ocean. They help store carbon, giving them a role in fighting climate change, and they provide homes and food for many sea animals. These meadows are among the most productive places in the ocean, supporting life in ways similar to coral reefs.

Overview

Seagrasses are special plants that grow in the sea and came from land plants that returned to the ocean many millions of years ago. They can only live in places where the water meets the land or in the ocean itself. Their pollination and seed making all happen underwater, and their leaves are made for capturing sunlight even though they don’t have parts that let air in like land plants do.

These plants help keep coastal areas healthy by giving food and shelter to many sea animals. Sadly, many seagrass areas are disappearing, which can hurt both sea life and the people who depend on the ocean. Protecting these important plants is very important for keeping the sea healthy.

Evolution

Seagrasses are special flowering plants that grow in the ocean. They evolved from land plants that moved back into the sea between 70 and 100 million years ago. These plants are part of a group called monocots, which includes grasses and similar plants.

Unlike other plants that live in the sea, such as salt marsh plants or mangroves, seagrasses all come from the same family of plants. Even though there aren't many different kinds of seagrasses, they have managed to grow on the shallow parts of every continent except Antarctica. Scientists have studied their genes to learn how they changed to live in the ocean, which helps us understand how plants adapt to new environments.

Taxonomy

Seagrasses are special flowering plants that grow in the ocean. There are about 60 different kinds of seagrasses, grouped into five families: Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae. All of these families belong to a group called Alismatales.

The plant group called Ruppia grows in slightly salty water and some scientists do not consider it a true seagrass. The way scientists group these plants can vary depending on the system they use.

Cell walls

Seagrass cell walls have similar parts to those of land plants, like cellulose. Some seagrass cell walls also have special kinds of polysaccharides, which are more common in sea algae such as red, brown, and green algae.

Besides polysaccharides, seagrass cell walls include important proteins. One type, called arabinogalactan proteins, helps build the cell wall and control processes inside cells. These proteins were studied in seagrass for the first time in 2020 and show unique features that might help seagrass manage water balance.

Seagrass cell walls also contain lignin, which makes walls stronger, though usually less than in land plants. Understanding these cell wall parts can be useful for making paper or insulating materials.

Sexual recruitment

See also: Seagrass meadow § Using propagules, and Seagrass meadow § Movement ecology

Seagrasses face many challenges from human activities. To survive and adapt, they need to create new plants, which helps them stay strong and flexible. This process, called sexual recruitment, allows seagrasses to grow in new places and adjust to changes in their environment.

Some seagrasses produce small, tough seeds that stay dormant for a while before growing. These plants usually live for a short time but can bounce back quickly after disturbances. Other seagrasses create seeds that can travel far, helping them grow in areas with better conditions. One special seagrass, Posidonia oceanica, can live for thousands of years and forms huge underwater meadows. Its seeds store extra food to help new plants grow, even when they can't make much energy from sunlight right away. These seedlings also grow special root hairs to stick to the ocean floor until their roots are strong.

Intertidal and subtidal

Seagrasses that grow in the intertidal and subtidal zones face very different conditions because of tides. Subtidal seagrasses often get less light because of things in the water that block the light. These seagrasses can be studied quickly using smart computer programs. Intertidal seagrasses are often out of the water and face big temperature changes, very bright sunlight, and drying out. These challenges can sometimes cause the seagrasses to die when the tide is low. Intertidal seagrasses are usually smaller to handle these tough conditions better.

In contrast, subtidal seagrasses live under water and adapt to low light by having longer leaves and wider blades, which helps them grow more. They also change how they use light to stay healthy. Because their environments are so different, intertidal and subtidal seagrasses act in very different ways to use light best for their growth.

Seagrasses need carbon to grow, and they can use both carbon from the air and from water. Even when intertidal seagrasses are out of the water, they can still use carbon from the air to survive. This difference means that seagrasses from these two zones might have different chemical signatures in their tissues.

Seagrass meadows

Seagrass beds, also called meadows, can be made of just one type of plant or many mixed together. In cooler areas, usually just one or a few kinds of seagrass grow, like eelgrass in the North Atlantic. But in warmer places, like the Philippines, there can be many different types growing together.

Seagrass meadows are important homes for many sea creatures. They give shelter and food to fish, turtles, manatees, and many smaller animals. The plants help keep water clean and clear by trapping dirt and slowing down the water. They also help protect shorelines from big waves and storms. Because seagrasses live underwater, they make a lot of oxygen, which helps all the sea life. These meadows store a lot of carbon, helping to keep our planet healthy. Scientists even found one of the largest seagrass areas near the Bahamas by following tiger sharks with cameras.

Seagrass microbiome

Further information: marine microbiomes

See also: plant holobiont

The idea of a holobiont looks at how a plant and its tiny living partners, like bacteria and viruses, work together as one team. This teamwork helps the plant get important nutrients, stay healthy, and grow. Scientists study this in many plants, including seagrasses.

Most studies focus on a type of seagrass called Zostera marina. These tiny living partners change depending on where they are on the plant. Some help protect the plant from harmful germs, while others help it grow. Scientists are also interested in finding a "core" group of these tiny partners that stay with the plant, but this is still being studied.

Research on these tiny partners has grown with better tools. Scientists now look at different parts of the plant and different places to see how these partners help seagrasses stay healthy and grow. They also study how changes in the environment affect these tiny partners.

Threats and conservation

Seagrasses are very important plants that live in the ocean, even though they only cover a tiny part of it. Sadly, these plants have been disappearing more and more over time. Since the late 1800s, more than one-fifth of all seagrass areas have been lost, and this loss continues to happen each year. Some types of seagrass are now at risk of disappearing completely.

People and nature both harm seagrasses. Human activities like building along coastlines, driving motorboats, and certain fishing methods can damage seagrass areas. Changes in the climate, such as rising sea levels and warmer temperatures, also hurt seagrasses. Too many nutrients in the water, from things like sewage, can cause other plants to grow too fast and block sunlight that seagrasses need to survive.

Conservation is very important to protect seagrasses. Learning more about these plants and their value is a big step. It’s also hard to keep track of all seagrass areas around the world. Balancing people’s needs with protecting nature is another challenge. Scientists are working hard to study seagrasses and find ways to save them. Some countries are starting to make plans to protect seagrass areas and raise awareness about their importance.

Each year on March 1, World Seagrass Day is celebrated to remind people how important seagrasses are to our oceans.