Seagrasses are the only flowering plants that live in ocean environments. There are about 60 species of seagrasses that grow completely in the ocean. These species belong to four families: Posidoniaceae, Zosteraceae, Hydrocharitaceae, and Cymodoceaceae. All of these families are part of the order Alismatales, which is a group of plants with one seed part. Seagrasses developed from plants that originally lived on land and later returned to the ocean between 70 and 100 million years ago.

The name "seagrass" comes from the long, narrow leaves that many species have. These plants grow by extending underground stems called rhizomes, which help them spread across large areas, forming underwater meadows that look like grasslands. Many seagrass species resemble grasses from the Poaceae family that live on land.

Like all plants that make their own food, seagrasses use photosynthesis to grow. They do this in the underwater areas where there is enough sunlight, usually in shallow, protected coastal waters where they are rooted in sandy or muddy ocean floors. Most seagrass species pollinate underwater and complete their life cycle entirely in the ocean. Earlier, scientists thought that pollination happened only through ocean currents, but this is not true for at least one species, Thalassia testudinum. This species uses both natural water movement and small animals, such as crabs and certain worm larvae, to help with pollination. The plant produces sticky pollen clusters that attach to these animals, unlike flowers on land, which use nectar to attract pollinators.

Seagrass meadows are dense underwater areas that are among the most productive ecosystems on Earth. They help store carbon and provide homes and food for many types of marine life, similar to the diversity found in coral reef ecosystems.

Overview

Seagrasses are a group of flowering plants that live in the ocean and developed separately three to four times from land plants. These plants have the following features:

• They only grow in coastal or ocean areas and nowhere else.
• Their pollination happens underwater using special pollen.
• Their seeds, which are carried by living things (like animals) and non-living things (like water currents), are produced underwater.
• Their leaves have a thinner waxy layer and an outer layer without tiny holes (called stomata) that help with water loss. The leaves are the main part used for making food through sunlight.
• Their underground stems (called rhizomes) help the plant stay rooted.
• Their roots can survive in areas with little oxygen and get oxygen from the leaves and rhizomes. The roots also help move nutrients to the rest of the plant.

Seagrasses greatly affect the physical, chemical, and living conditions of coastal waters. They provide important benefits to the environment, such as serving as places where many animals reproduce and grow, and helping support fishing industries. However, scientists know less about how seagrasses function. There are 26 seagrass species in North American coastal waters. Studies show that seagrass habitats are declining worldwide. Ten seagrass species are more likely to go extinct (14% of all seagrass species), and three are classified as endangered. Losing seagrass and reducing the variety of seagrass species will harm marine life and the people who depend on the resources and benefits seagrasses provide.

Seagrasses create important coastal ecosystems. The worldwide decline of these underwater grasslands, which provide food and homes for many ocean animals, shows the need to protect and understand these valuable resources.

Evolution

About 140 million years ago, seagrasses developed from early monocots, which were able to live in the ocean. Monocots are types of grasses and grass-like flowering plants (angiosperms) whose seeds usually have only one embryonic leaf, called a cotyledon.

Terrestrial plants may have first appeared around 450 million years ago, evolving from green algae. Later, seagrasses developed from land plants that returned to the ocean. Between 70 million and 100 million years ago, three separate groups of seagrasses (Hydrocharitaceae, Cymodoceaceae complex, and Zosteraceae) evolved from a single group of monocot flowering plants.

Other plants that live in the ocean, such as salt marsh plants, mangroves, and marine algae, have more varied evolutionary histories. Despite having fewer species, seagrasses have spread to the continental shelves of all continents except Antarctica.

Recent studies of the genomes of Zostera marina and Zostera muelleri have helped scientists better understand how flowering plants adapted to life in the sea. As plants returned to the ocean, some genes were lost (for example, genes related to stomata) or became less active (such as genes involved in making terpenoids). Other genes were gained, like those involved in sulfation.

Genome research also shows that adapting to the ocean required major changes in the structure of cell walls. However, scientists still do not fully understand the composition of seagrass cell walls. In addition to traits inherited from land plants, seagrasses likely adapted to their new environment, which includes challenges like high salt levels, grazing by animals, and bacteria. Their cell walls appear to combine features found in both land angiosperms and marine algae, along with new structural elements.

Taxonomy

Seagrasses are a diverse group of marine plants with about 60 species spread across five families: Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae. These families are part of the order Alismatales, as classified by the Angiosperm Phylogeny Group IV System. The genus Ruppia, which grows in brackish water, is not always considered a true seagrass by some scientists. Some researchers have placed Ruppia in the Cymodoceaceae family, but the APG IV system and The Plant List Webpage do not support this classification.

Cell walls

Seagrass cell walls contain the same complex sugars found in land plants called angiosperms, such as cellulose. However, some seagrass cell walls have special sugars called sulfated polysaccharides, which are also found in red, brown, and green algae. In 2005, scientists suggested that marine angiosperms, like seagrasses, may have redeveloped the ability to make sulfated polysaccharides. Another unique feature of seagrass cell walls is the presence of unusual pectic polysaccharides called apiogalacturonans.

In addition to polysaccharides, a group of proteins called hydroxyproline-rich glycoproteins are important parts of land plant cell walls. A type of these proteins, called arabinogalactan proteins, is especially interesting because they help shape the cell wall and control cell processes. These proteins are found in many seed plants, as well as in ferns, lycophytes, and mosses. Their structure includes large sugar parts made of arabinogalactans (usually more than 90% of the molecule) that are chemically linked to smaller protein parts (usually less than 10% of the molecule). Different species and tissues have unique sugar changes, which may affect how the proteins work. In 2020, scientists first isolated and studied arabinogalactan proteins from a seagrass. While the basic structure of these proteins is similar in land plants, their sugar parts show unique features that may help seagrasses manage water balance.

Another part of plant cell walls is a group of cross-linked phenolic polymers called lignin, which help strengthen the walls. Lignin is also found in seagrass, but it is usually present in smaller amounts compared to land angiosperms. Therefore, seagrass cell walls seem to have a mix of features from both land plants and marine algae, along with new structures. Dried seagrass leaves could be useful for making paper or as insulation, so understanding their cell wall composition has practical value.

Sexual recruitment

Seagrass populations are currently in danger because of human activities. The ability of seagrasses to survive changes in their environment depends partly on genetic differences, which come from sexual reproduction. When seagrasses create new plants, they increase their genetic variety, helping them grow in new areas and adapt to changes in their surroundings.

Seagrasses use different methods to spread to new areas. Some species store small seeds with hard outer coverings in the soil. These seeds can stay inactive for months. These seagrasses usually live for a short time and can quickly recover from damage by growing near the original plants (e.g., Halophila sp., Halodule sp., Cymodocea sp., Zostera sp., and Heterozostera sp.). Other seagrasses produce floating fruits that contain large seeds that can grow right away. These are common in long-lived seagrasses, such as Posidonia sp., Enhalus sp., and Thalassia sp. The seeds of long-lived seagrasses can travel farther than those of short-lived species, allowing their offspring to grow in areas with less light, even if the parent plants are far away.

Posidonia oceanica (L.) Delile is one of the oldest and largest seagrass species on Earth. A single plant can form meadows up to 15 kilometers wide and may be hundreds or thousands of years old. These meadows help shape the landforms of Mediterranean coasts, making this seagrass a key habitat for conservation. Recently, Posidonia oceanica has been flowering and producing new plants more often than in the past. This seagrass has special traits to help its young plants survive. Its seeds store large amounts of nutrients, which support the growth of new shoots and roots for up to one year. During the early stages of germination, when leaves are not yet developed, the seeds can perform photosynthesis, increasing their chances of growing successfully. The young plants also have flexible root systems that can form sticky hairs to help them attach to rocky surfaces. However, scientists still do not fully understand some aspects of Posidonia oceanica’s reproduction, such as when its seeds begin photosynthesis or how they stay attached to surfaces until their roots are fully developed.

Intertidal and subtidal

Seagrasses that grow in the intertidal and subtidal zones face changing environmental conditions because of the rising and falling of tides. Subtidal seagrasses often experience lower light levels because of natural and human activities that increase the amount of particles in the water, which block light. Scientists can use artificial intelligence to estimate light conditions in subtidal areas accurately, which helps solve problems faster than older methods.

Seagrasses in the intertidal zone are regularly exposed to air, which causes them to face extreme temperature changes, very strong light, and dryness. These conditions can harm seagrass when they are above water during low tide. Dryness during low tide is the main reason seagrass does not grow in the upper parts of the intertidal zone. Seagrasses in the intertidal zone are usually smaller than those in the subtidal zone to reduce the stress of being out of water. They also change how they use light, such as reducing their ability to make food from sunlight and increasing ways to protect themselves from too much light when exposed to air.

In contrast, subtidal seagrasses adapt to less light caused by water and particles above them. Seagrasses in deeper subtidal areas usually have longer and wider leaves than those in shallower areas or the intertidal zone. This helps them make more food, which supports their growth. To handle low light, these seagrasses increase their chlorophyll levels and change the balance of two types of chlorophyll to use available light more efficiently. Because seagrasses in the intertidal and subtidal zones face very different light conditions, they adjust their features to maximize food production and protect themselves from too much light.

Seagrasses take in large amounts of inorganic carbon to create food. Seagrass and other underwater plants use both carbon dioxide (CO₂) and bicarbonate (HCO₃⁻) for making food. Even when exposed to air during low tide, intertidal seagrasses can still use CO₂ from the air to photosynthesize. This means the sources of carbon used for food may differ between intertidal and subtidal seagrass. Since the carbon isotope levels in plant tissues depend on the type of carbon used for photosynthesis, seagrasses in the intertidal and subtidal zones may have different carbon isotope ranges.

Seagrass meadows

Seagrass beds can be made up of only one type of plant or a mix of several species. In temperate areas, one or a few species often dominate, such as eelgrass (Zostera marina) in the North Atlantic. In tropical areas, seagrass beds are usually more diverse, with as many as thirteen species found in the Philippines.

Seagrass beds are rich in life and support many species from different groups, such as young and adult fish, algae, mollusks, worms, and tiny organisms. Few species were originally thought to eat seagrass leaves because they are not very nutritious. However, recent studies show that many animals, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins, and crabs, eat seagrass. Some fish that visit seagrass areas raise their young in nearby mangroves or coral reefs.

Seagrass helps trap sediment and slow water movement, which allows suspended particles to settle. This process benefits coral by reducing sediment, improving photosynthesis for both coral and seagrass.

Seagrass provides important ecosystem services and is considered an ecosystem engineer. This means seagrass changes its environment in physical and chemical ways. Many seagrass species have underground roots and rhizomes that stabilize sediment and reduce erosion. These roots also help oxygenate the sediment, creating a better home for organisms living in it. Seagrass improves water quality by trapping heavy metals, pollutants, and extra nutrients. The long blades of seagrass slow water movement, reducing wave energy and protecting coasts from erosion and storms. Seagrass also produces oxygen, which helps the water stay healthy. These meadows store more than 10% of the ocean’s total carbon. Per hectare, seagrass holds twice as much carbon dioxide as rainforests and removes about 27.4 million tons of CO₂ each year.

Seagrass provides food for many marine animals. Sea turtles, manatees, parrotfish, surgeonfish, sea urchins, and pinfish eat seagrass. Many smaller animals eat the algae and tiny creatures that live on or around seagrass blades. Seagrass also creates habitat in areas that would otherwise have no vegetation. The three-dimensional structure of seagrass supports many species for shelter and feeding. About 17 species of coral reef fish spend their entire juvenile stage on seagrass flats. These areas also act as nurseries for valuable fish species, such as the gag grouper (Mycteroperca microlepis), red drum, and common snook. Some fish use seagrass meadows during different stages of their life cycle. A recent study found that two popular fish, the common snook and spotted sea trout, rely on seagrass meadows for feeding during reproduction. Because reproduction requires a lot of energy, these fish need seagrass areas nearby. Many important invertebrates, like bay scallops (Argopecten irradians), horseshoe crabs, and shrimp, also live in seagrass habitats. Animals such as West Indian manatees, green sea turtles, and sharks often visit seagrass areas. The variety of life in seagrass habitats supports tourism and helps coastal economies in places like the Gulf of Mexico and the Caribbean.

In 2022, scientists discovered the world’s largest known seagrass ecosystem near the Bahamas. This discovery was made possible by using cameras attached to tiger sharks between 2016 and 2022, which allowed scientists to see the ocean floor from a new perspective.

• Thalassia testudinum seagrass bed
• White-spotted puffers, often found in seagrass areas
• Underwater footage showing a seagrass meadow with bull huss and conger eel

Seagrass microbiome

The idea of the holobiont describes how a host organism, such as a plant or animal, interacts with its associated microbes and viruses, treating them as a single biological unit. This concept has been studied in many model systems, though some scientists disagree with defining diverse host-microbe relationships as a single unit. Over time, the holobiont and hologenome ideas have changed, and it is clear that microbes play a key role in the biology and ecology of their host. They help provide nutrients like vitamins and energy, support the host’s defenses, and influence the host’s evolution.

Most research on host-microbe interactions has focused on animals, such as corals, sponges, and humans. However, there is also a large amount of research on plant holobionts. Microbial communities in plants affect their growth, survival, and fitness. These communities are shaped by factors like nutrient availability and the plant’s defense systems. Microbes live in different plant habitats, including the rhizoplane (the surface of roots), the rhizosphere (the area around roots), the endosphere (inside plant tissues), and the phyllosphere (all above-ground surfaces). These differences occur because biotic and abiotic factors interact with plants in distinct ways, such as in anoxic sediments versus a moving water column.

Most studies on seagrass microbiomes use Zostera marina as the main species because it is widely distributed and provides important ecosystem services. In seagrasses, microbial populations change vertically, with Alpha and Gamma proteobacteria dominating the upper parts of leaves, while Delta and Gamma proteobacteria are more common in the rhizosphere. Although these groups share some phyla, the microbial communities above and below the seagrass surface are very different. Microbes in these regions have unique roles: they help break down waste, defend against pathogens, and control epiphytes on mature leaves. In reproductive tissues and seeds, certain microbes support plant growth. In the rhizosphere, microbes are involved in nitrogen fixation, sulfur oxidation, and managing compounds, with fungi and archaea also present.

The microbial community in the rhizosphere of Posidonia oceanica is as complex as those in terrestrial soils, containing thousands of species per gram of soil. However, the chemistry in P. oceanica rhizosphere is dominated by sugars like sucrose and phenolic compounds.

There is growing interest in the idea of a "core seagrass microbiome," which refers to a consistent set of microbes found in a specific host. However, defining and measuring this concept is challenging because studies vary in scale, goals, and resolution. Research has explored whether a core microbiome exists across different populations, species, and regions. Findings are mixed, with some studies supporting the idea and others opposing it. A major challenge is the wide range of taxonomic detail used in seagrass studies, which makes comparisons difficult.

Research on seagrass microbiomes has expanded as methods and technology improved. Early studies aimed to confirm the existence of a seagrass holobiont by identifying microbes consistently present on plants but absent in surrounding water or sediment. High-throughput sequencing, a cost-effective way to analyze DNA and RNA, allowed more detailed microbiome studies in aquatic environments.

As research advanced, scientists began examining seagrass communities based on microhabitats (like rhizosphere, endosphere, and phyllosphere), life stages, and geographic regions. Conservation efforts now consider microbial communities in rearing methods because they affect plant health and reproduction. Current research includes studying how microbes interact, how they colonize seagrass, how communities change over time, and how environmental factors influence these changes.

Future research seeks to understand the functions of microbes associated with seagrasses but faces challenges. These communities are often polymicrobial, making it hard to isolate and assign specific roles to individual microbes. Metagenomics, a technique used to study microbial communities, sometimes misses low-abundance microbes, and some pathways are not fully understood. Identifying the causes of seagrass diseases is a growing focus as seagrass populations decline globally.

Threats and conservation

Seagrasses cover only 0.1 to 0.2% of the ocean's surface, but they support important ecosystems. Like many ocean areas, seagrass habitats are declining quickly. Since the late 1800s, over 20% of seagrass areas worldwide have been lost, with seagrass beds decreasing by 1.5% each year. Of the 72 known seagrass species, about 15 (or one-quarter) are listed as Threatened or Near Threatened on the IUCN Red List of Threatened Species. Threats include natural causes, such as storms and disease, and human-related causes, like habitat destruction, pollution, and climate change.

Human activity is the most common threat to seagrass. About 67 seagrass species (93% of all species) are affected by human actions near coasts. Activities like building on coastal land, motorboating, and fishing methods such as trawling can damage seagrass beds or make water cloudy, which harms seagrass. Seagrasses need a lot of light to grow, so changes in water clarity that block light can cause seagrass to die.

Seagrasses are also harmed by climate change. More frequent weather events, rising sea levels, and higher temperatures from global warming can lead to widespread seagrass loss. Another threat is the arrival of non-native species. At least 28 non-native species have become established in seagrass areas worldwide. Of these, 64% are known to harm the ecosystem.

Coastal eutrophication is another major cause of seagrass loss. As human populations grow near coasts, nutrients from sewage and development increase in coastal waters. High nutrient levels can cause problems for seagrass. While some nutrients may help seagrass grow, too many nutrients can lead to rapid growth of algae and phytoplankton. These algae form thick layers on the water’s surface, blocking light from reaching seagrass. Algal blooms also reduce oxygen levels in the water, which harms seagrass. Seagrass relies on oxygen from photosynthesis or water to supply oxygen to its roots. When oxygen levels drop, seagrass growth and survival are harmed. Low oxygen can reduce photosynthesis, increase respiration, and slow growth. Over time, this can lead to seagrass dying, which worsens oxygen levels as organic matter decays.

Studies in the Mediterranean Sea show that seagrass populations depend on factors like temperature, salinity, depth, and water clarity, as well as climate change and human activity. Many areas in the Mediterranean have seen seagrass decline. For example, the southern coast of Latium has lost 27.7% of seagrass since the 1960s, while the Northern Mediterranean basin has lost 18% to 38%. In France, seagrass areas have decreased by 23% in 50 years. In Spain, human activities like illegal trawling and aquaculture farming caused significant seagrass loss. Areas with high human impact had the greatest reductions. Overall, about 29% of seagrass populations worldwide have disappeared since 1879. If warming continues, seagrass like Posidonia oceanica in the Mediterranean may face functional extinction by 2050. These trends are likely part of a larger global pattern.

Protecting seagrass is essential for their survival. Challenges include raising awareness about seagrass and its importance to humans, tracking seagrass populations globally, identifying harmful activities locally, balancing human needs with environmental protection, and funding scientific research. In some regions, like India and China, little effort is made to conserve seagrass. However, seagrass conservation and restoration could support 16 of the 17 UN Sustainable Development Goals.

In China, scientists suggested including seagrass in conservation plans, banning land reclamation near seagrass beds, reducing aquaculture, improving sediment quality, creating seagrass reserves, and educating fishermen and policymakers. Similar ideas were shared in India, where scientists emphasized public engagement and combining traditional knowledge with conservation policies.

World Seagrass Day is celebrated on March 1 each year to highlight the importance of seagrass in marine ecosystems.