Seagrasses are the only flowering plants that live in ocean environments. There are about 60 types of seagrasses that grow completely in the sea. These plants 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 land plants that 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 spreading underground stems called rhizomes, and they often form large underwater areas that look like grasslands. These areas are called "meadows," and some seagrass species look similar to land grasses in the family Poaceae.

Like all plants that make their own food, seagrasses use photosynthesis to grow. They do this in underwater areas where sunlight can reach, usually in shallow, protected coastal waters where they are rooted in sandy or muddy ocean floors. Most seagrass species reproduce underwater, and their entire life cycle happens in the ocean. Earlier beliefs suggested that pollination occurred only through ocean currents, but this is not true for at least one species, Thalassia testudinum. This species uses both living helpers (like crustaceans and worm larvae) and non-living methods (like water movement) to spread pollen. These helpers are attracted to sticky pollen clumps that the plant produces, instead of using nectar like land plants.

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 marine animals, similar to the diversity found in coral reef environments.

Overview

Seagrasses are a group of plants that live in the ocean and evolved from land plants multiple times. Seagrass species can be identified by these traits:

• They grow only in estuaries or marine environments.
• Pollination happens underwater using special pollen.
• Seeds are made underwater and spread by living and non-living factors.
• Seagrass leaves have a thin outer layer, no pores for gas exchange, and are the main part used for making food through sunlight.
• The underground stems help the plants stay rooted.
• Roots can survive in areas with little oxygen and rely on oxygen carried from the leaves and stems. They also help move nutrients.

Seagrasses greatly affect the physical, chemical, and living conditions of coastal waters. While they provide important benefits, such as serving as homes for many marine animals and supporting fish populations, much about their biology is still not fully understood. There are 26 seagrass species in North American coastal waters. Studies show that seagrass habitats are declining globally. Ten species are at higher risk of disappearing (14% of all seagrass species), and three are classified as endangered. Losing seagrass and reducing their variety could harm marine life and the people who rely on the resources and benefits seagrasses provide.

Seagrasses create vital coastal ecosystems. The worldwide decline of these underwater meadows, which provide food and shelter for many marine animals, highlights the need to protect and study these valuable resources.

Evolution

About 140 million years ago, seagrasses developed from early monocots, a group of grass-like flowering plants that successfully adapted to life in the ocean. Monocots are flowering plants (angiosperms) whose seeds usually have one embryonic leaf, called a cotyledon.

Terrestrial plants may have first appeared around 450 million years ago, evolving from a group of green algae. Later, some of these land plants returned to the ocean and became seagrasses. 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 shallow continental shelves of all continents except Antarctica.

Recent studies of the genomes of Zostera marina and Zostera muelleri have provided new insights into how flowering plants adapted to life in the sea. During their return to the ocean, some genes were lost (such as those related to stomata) or became less active (like those involved in making terpenoids), while others were gained, such as those linked to sulfation.

Genome research also shows that seagrasses adapted to the ocean by making major changes to their cell walls. However, scientists do not yet fully understand the structure of these cell walls. Seagrasses likely combine traits from both land plants and marine algae, with new features that help them survive in their environment. This environment includes challenges like high salt levels (non-living stressors) and threats from animals that eat them or bacteria that live on them (living stressors).

Taxonomy

Today, seagrasses are a group of flowering plants that live in the ocean. This group includes about 60 species divided into five families: Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae. These families are part of the order Alismatales, as described by the Angiosperm Phylogeny Group IV System. The genus Ruppia, which grows in brackish water (a mix of salt and fresh water), is not considered a true seagrass by all scientists. Some researchers have placed Ruppia in the Cymodoceaceae family, but the APG IV system and The Plant List Webpage do not agree with this classification.

Cell walls

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

In addition to these sugars, a type of protein called hydroxyproline-rich glycoprotein is important in the cell walls of land plants. These proteins, called arabinogalactan proteins, are found in many seed plants, as well as in ferns, lycophytes, and mosses. These proteins have long sugar chains (usually more than 90% of the molecule) attached to small protein parts (less than 10% of the molecule). Different species and tissues have unique sugar structures, which may affect the physical properties of the cell walls. In 2020, scientists first identified and studied these proteins in seagrass. While the basic structure of these proteins is similar to those in land plants, the sugar parts in seagrass appear to have special features that may help the plants manage water balance.

Another important part of plant cell walls is a group of strong, cross-linked polymers called lignin, which help make the walls rigid. Seagrass cell walls also contain lignin, but usually in smaller amounts compared to land plants. This suggests that seagrass cell walls combine features found in both land plants and marine algae, along with new structures. Dried seagrass leaves could be used for making paper or as insulation, so understanding their cell wall composition may have practical applications.

Sexual recruitment

Seagrass populations are currently at risk due to human-caused problems. The ability of seagrasses to survive changes in their environment depends partly on genetic differences, which are created through reproduction using seeds. By producing new individuals, seagrasses increase their genetic diversity, helping them spread to new areas and adapt to changes in their surroundings.

Seagrasses use different strategies to colonize new areas. Some species create seed banks with small seeds that have tough outer coverings and can stay in a sleeping state for several months. These seagrasses often live for a short time and can quickly recover from disturbances by growing close to the original seagrass meadows. Examples include species like Halophila, Halodule, Cymodocea, Zostera, and Heterozostera. In contrast, other seagrasses produce dispersal propagules, which are floating fruits containing large, non-dormant seeds. This strategy is common in long-lived seagrass species such as Posidonia, Enhalus, and Thalassia. The seeds of these long-lived species can travel farther than those of short-lived species, allowing them to grow in areas with less light by developing from seedlings near the parent meadows.

The seagrass Posidonia oceanica is one of the oldest and largest seagrass species on Earth. A single individual can form meadows covering nearly 15 kilometers and may live for hundreds to thousands of years. Posidonia oceanica meadows help shape and protect Mediterranean coastlines, making this seagrass a key habitat for conservation efforts. Recent observations suggest that Posidonia oceanica is flowering and producing new plants more often than in the past. This species has special traits that help it survive during reproduction. Its seeds contain large amounts of nutrients, which support the growth of shoots and roots even during the first year of seedling development. In the early stages of germination, when leaves are not yet developed, Posidonia oceanica seeds use sunlight to make food, increasing their chances of successfully growing into plants. Seedlings also have flexible root systems that form sticky root hairs, helping them attach to rocky surfaces. However, many details about the sexual reproduction of Posidonia oceanica remain unclear, such as when its seeds use sunlight to make food or how seeds stay attached to surfaces until their roots fully develop.

Intertidal and subtidal

Seagrasses in the intertidal and subtidal zones experience changing environmental conditions because of tides. Subtidal seagrasses are often in areas with less light due to many natural and human-made factors that increase the amount of particles in water, which block sunlight. Scientists can use artificial intelligence to estimate light conditions in subtidal areas accurately, allowing faster solutions than older methods. Seagrasses in the intertidal zone are regularly exposed to air, leading to very hot and cold temperatures, intense sunlight, and drying out. These extreme conditions can cause seagrass to die during low tide. Drying out 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 stress from being exposed to air. They also adjust their ability to use sunlight, such as reducing how efficiently they make food during bright sunlight and increasing protection from too much light.

In contrast, subtidal seagrasses adapt to less light because water and particles in the water reduce how much light reaches them. Seagrasses in deeper subtidal areas typically have longer and wider leaves than those in shallower areas or the intertidal zone. This helps them take in more sunlight, which supports more growth. To use less light better, subtidal seagrasses increase their chlorophyll levels and change the balance of two types of chlorophyll to absorb more light efficiently. Because seagrasses in the intertidal and subtidal zones face very different light conditions, they have unique ways to adjust their photosynthesis and protect themselves from too much sunlight.

Seagrasses take in large amounts of inorganic carbon to produce food. Seagrasses and other underwater plants use both carbon dioxide and bicarbonate (a form of carbon) for photosynthesis. Even when exposed to air during low tide, intertidal seagrasses can still use carbon dioxide from the air to make food. This means the sources of carbon used for photosynthesis may differ between intertidal and subtidal seagrasses. Since the carbon in plant tissues changes based on the type of inorganic carbon used, seagrasses in the intertidal and subtidal zones may have different patterns of carbon in their tissues.

Seagrass meadows

Seagrass beds, also called meadows, can be made up of one type of plant or a mix of several species. In temperate regions, 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 up to thirteen species found in the Philippines.

Seagrass beds are rich in life and support hundreds of species from different groups, such as juvenile and adult fish, algae, mollusks, worms, and nematodes. Few species were originally thought to eat seagrass leaves because they have low nutritional value. However, new research shows that many animals, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins, and crabs, rely on seagrass as a food source. Some fish that visit seagrass areas raise their young in nearby mangroves or coral reefs.

Seagrass plants trap sediment and slow water movement, allowing suspended particles to settle. This process helps protect coral by reducing sediment, which improves photosynthesis for both coral and seagrass.

Seagrass beds provide many benefits to the environment. They are called ecosystem engineers because they change their surroundings in physical and chemical ways. Their roots and rhizomes (underground plant parts) stabilize sediment, reduce coastal erosion, and add oxygen to the soil, creating a better home for organisms living in the sediment. Seagrass also improves water quality by trapping heavy metals, pollutants, and excess nutrients. The long blades of seagrass slow water movement, reducing wave energy and protecting coasts from erosion and storm surges. Because seagrass is underwater, it produces a lot of oxygen, which helps oxygenate the water. Seagrass meadows store more than 10% of the ocean’s total carbon. Per hectare, they hold twice as much carbon dioxide as rainforests and can absorb about 27.4 million tons of CO₂ each year.

Seagrass meadows are a food source for many marine herbivores, such as sea turtles, manatees, parrotfish, surgeonfish, sea urchins, and pinfish. Many smaller animals eat the algae and invertebrates that live on or among seagrass blades. These meadows also provide habitat in areas that would otherwise lack vegetation. Their three-dimensional structure in the water supports many species for shelter and feeding. Scientists estimate that 17 species of coral reef fish spend their entire juvenile stage on seagrass flats. These habitats also serve as nurseries for commercially and recreationally important fish, such as gag grouper (Mycteroperca microlepis), red drum, and common snook. Some fish use seagrass meadows during different life stages. A recent study found that two popular fish, common snook and spotted sea trout, rely on seagrass meadows for foraging during reproduction. Because reproduction requires a lot of energy, these fish need seagrass meadows nearby to complete the process. Many commercially important invertebrates, like bay scallops (Argopecten irradians), horseshoe crabs, and shrimp, also live in seagrass habitats. Charismatic animals, such as West Indian manatees, green sea turtles, and sharks, are often seen in these areas. The variety of marine life in seagrass habitats makes them popular tourist attractions and important sources of income for coastal communities in 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 of seagrass meadow with bull huss and conger eel

Seagrass microbiome

The idea of a holobiont, which describes a host and its associated microbes and viruses as a single biological unit, has been studied in many systems. However, some scientists criticize this concept because it groups different types of host-microbe relationships into one unit. The holobiont and hologenome ideas have changed since they were first introduced. It is clear that microbes play important roles in the biology and ecology of their host by providing nutrients, helping with defense, and influencing the host’s evolution.

Most research on host-microbe interactions has focused on animals like corals, sponges, and humans. However, there is also a large amount of research on plant holobionts. Microbial communities in plants affect plant growth, survival, and fitness. These communities are shaped by factors like nutrient availability and plant defense systems. Plant-associated microbes live in different areas, such as the rhizoplane (root surface), rhizosphere (area around roots), endosphere (inside plant tissue), and phyllosphere (above-ground surfaces). These differences occur because biotic and abiotic factors interact with plants in unique ways, leading to varied microbial communities in different environments.

Most studies on seagrass microbiomes use Zostera marina because it is widely distributed and provides important ecosystem services. Microbial populations in seagrasses 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 some microbial groups overlap, the communities above and below the ground differ significantly. Microbes in different regions have distinct roles, such as breaking down waste, defending against pathogens, and controlling epiphytes on mature leaves. Other microbes help plants grow by producing nutrients in reproductive tissues and seeds. 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 in terrestrial soils, containing thousands of species per gram of soil. However, the rhizosphere of P. oceanica has high levels of sugars like sucrose and phenolics.

Scientists are currently interested in the idea of a "core seagrass microbiome," which refers to a consistent group of microbes found in a host. However, defining and measuring this concept is challenging because research methods, study scales, and goals vary. Studies have tried to identify core microbiomes across different populations, species, and regions, but results are mixed. One major difficulty is the wide range of taxonomic detail used in seagrass research, making comparisons difficult.

Research on seagrass microbiomes has expanded with advances in technology. Early studies aimed to confirm the existence of a seagrass holobiont by identifying microbes unique to seagrass compared to surrounding water and sediment. High-throughput sequencing, a method that analyzes DNA and RNA quickly and affordably, has increased microbiome research in aquatic environments.

Later studies focused on different seagrass microhabitats (rhizosphere, endosphere, phyllosphere), life stages, and geographic areas. Conservation efforts now consider microbial communities in rearing methods because they affect plant health and reproduction. Current research includes studying interactions between microbes, changes in microbial communities, disease and pathogen relationships, and how environmental changes affect microbial populations over time.

Future research aims to understand how seagrass-associated microbes function. However, challenges remain. Seagrass microbiomes are often polymicrobial, making it hard to isolate and assign specific functions. Metagenomics, a technique that studies genetic material from environmental samples, sometimes misses low-abundance microbes, and pathways are not always clearly identified. Identifying the causes of seagrass diseases is a growing focus as seagrass populations decline globally.

Threats and conservation

Although they cover only 0.1 to 0.2% of the ocean, seagrasses create very important ecosystems. Like many other ocean areas, seagrass habitats have been declining rapidly. Since the late 1800s, more than 20% of the world’s seagrass area has been lost, with seagrass beds disappearing at a rate of 1.5% each year. Out of 72 seagrass species worldwide, about one-quarter (15 species) are listed as Threatened or Near Threatened on the IUCN Red List. Threats include natural causes, such as storms and disease, and human-caused issues, such as habitat destruction, pollution, and climate change.

The most common threat to seagrass is human activity. Up to 67 seagrass species (93% of all species) are affected by human actions near coasts. Activities like building on coastal land, using motorboats, and fishing methods such as trawling can physically damage seagrass beds or increase water cloudiness, leading to seagrass dying. Seagrasses need a lot of sunlight, so changes in water clarity that block light can harm them.

Seagrasses are also harmed by changes in global climate. More frequent extreme weather, rising sea levels, and higher temperatures from global warming can cause widespread seagrass loss. Another threat is the introduction of non-native species. At least 28 non-native species have been found in seagrass areas worldwide, and 64% of these invasive species harm the ecosystem.

Coastal eutrophication, or too many nutrients in the water, is another major cause of seagrass loss. Rapid human population growth along coasts has increased nutrients from sewage and development in coastal waters. While some nutrients can help seagrass grow, too many nutrients cause fast growth of algae and tiny plants, which block sunlight and reduce oxygen levels. Algae blooms also create low-oxygen conditions, which harm seagrass. Seagrass needs oxygen for its roots, and low-oxygen water harms its growth and survival. This can lead to seagrass dying, which worsens the situation by further lowering oxygen levels.

Studies in the Mediterranean Sea show that seagrass health depends on factors like temperature, salinity, depth, and water clarity, as well as human activity and climate change. Many areas in the Mediterranean have seen seagrass loss, with reductions of 27.7% along southern Italy, 18%–38% in the northern Mediterranean, 19%–30% in Ligurian coasts, and 23% in France over the past 50 years. In Spain, human activities like illegal trawling and aquaculture farming caused major seagrass loss. Scientists estimate that 29% of seagrass areas worldwide have disappeared since 1879. If warming continues, seagrass species like Posidonia oceanica in the Mediterranean may face functional extinction by 2050. These trends are similar to global patterns.

Protecting seagrass is essential for their survival. Challenges include raising public awareness about seagrass and its importance, tracking seagrass populations globally, identifying local threats, balancing human needs with environmental protection, and increasing scientific research on seagrass. In some regions, like India and China, little effort is made to conserve seagrass. However, protecting and restoring seagrass can support 16 of the 17 UN Sustainable Development Goals.

In China, scientists suggest adding seagrass conservation to national plans, banning land reclamation near seagrass areas, reducing aquaculture farming, creating seagrass reserves, and educating people about seagrass. Similar ideas were shared in India, where scientists emphasized public involvement and integrating traditional knowledge into conservation policies.

World Seagrass Day is held on March 1 to raise awareness about seagrass and its role in marine ecosystems.