Seagrasses are the only embryophytes (commonly referred to as "land plants") which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the clade of monocotyledons). Seagrasses evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago.
The name seagrass stems from the many species with long and narrow leaves, which grow by rhizome extension and often spread across large "meadows" resembling grassland; many species superficially resemble terrestrial grasses of the family Poaceae.
Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their life cycle underwater. While it was previously believed this pollination was carried out without pollinators and purely by sea current drift, this has been shown to be false for at least one species, Thalassia testudinum, which carries out a mixed biotic-abiotic strategy. Crustaceans (such as crabs, Majidae zoae, Thalassinidea zoea) and syllid polychaete worm larvae have both been found with pollen grains, the plant producing nutritious mucigenous clumps of pollen to attract and stick to them instead of nectar as terrestrial flowers do.
Seagrasses form dense underwater seagrass meadows which are among the most productive ecosystems in the world. They function as important carbon sinks and provide habitats and food for a diversity of marine life comparable to that of coral reefs.
Overview Seagrasses are a paraphyletic group of marine angiosperms which evolved in parallel three to four times from land plants back to the sea. The following characteristics can be used to define a seagrass species:
It lives in an estuarine or in the marine environment, and nowhere else. The pollination takes place underwater with specialized pollen. The seeds which are dispersed by both biotic and abiotic agents are produced underwater. Material was copied from this source, which is available under a Creative Commons Attribution 3.0 International License. The seagrass species have specialized leaves with a reduced cuticle, an epidermis which lacks stomata and is the main photosynthetic tissue. The rhizome or underground stem is important in anchoring. The roots can live in an anoxic environment and depend on oxygen transport from the leaves and rhizomes but are also important in the nutrient transfer processes.Larkum A. W. D., R. J. Orth, and C. M. Duarte (2006) Seagrass: Biology, Ecology and Conservation, Springer, The Netherlands.
Seagrasses profoundly influence the physical, chemical, and biological environments of coastal waters. Though seagrasses provide invaluable ecosystem services by acting as breeding and nursery ground for a variety of organisms and promote commercial fisheries, many aspects of their physiology are not well investigated. There are 26 species of seagrasses in North American coastal waters. Several studies have indicated that seagrass habitat is declining worldwide. Ten seagrass species are at elevated risk of extinction (14% of all seagrass species) with three species qualifying as endangered. Seagrass loss and degradation of seagrass biodiversity will have serious repercussions for marine biodiversity and the human population that depends upon the resources and ecosystem services that seagrasses provide.
Seagrasses form important coastal ecosystems.Hemminga, M. A., and Duarte, C. M. eds (2000). "Seagrasses in the human environment," in Seagrass Ecology (Cambridge: Cambridge University Press), 248–291. The worldwide endangering of these sea meadows, which provide food and habitat for many marine species, prompts the need for protection and understanding of these valuable resources.
Evolution
Around 140 million years ago, seagrasses evolved from early monocots which succeeded in conquering the marine environment. Monocots are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf or cotyledon., in
Terrestrial plants evolved perhaps as early as 450 million years ago from a group of green algae. Seagrasses then evolved from terrestrial plants which migrated back into the ocean. Between about 70 million and 100 million years ago, three independent seagrass lineages (Hydrocharitaceae, Cymodoceaceae complex, and Zosteraceae) evolved from a single lineage of the monocotyledonous flowering plants.Les, D.H., Cleland, M.A. and Waycott, M. (1997) "Phylogenetic studies in Alismatidae, II: evolution of marine angiosperms (seagrasses) and hydrophily". Systematic Botany 22(3): 443–463.
Other plants that colonised the sea, such as salt marsh plants, mangroves, and marine algae, have more diverse evolutionary lineages. In spite of their low species diversity, seagrasses have succeeded in colonising the continental shelves of all continents except Antarctica.
Recent sequencing of the genomes of Zostera marina and Zostera muelleri has given a better understanding of angiosperm adaptation to the sea. During the evolutionary step back to the ocean, different genes have been lost (e.g., stomatal genes) or have been reduced (e.g., genes involved in the synthesis of terpenoids) and others have been regained, such as in genes involved in sulfation.
Genome information has shown further that adaptation to the marine habitat was accomplished by radical changes in cell wall composition. However the cell walls of seagrasses are not well understood. In addition to the ancestral traits of land plants one would expect habitat-driven adaptation process to the new environment characterized by multiple abiotic (high amounts of salt) and biotic (different seagrass grazers and bacterial colonization) stressors. The cell walls of seagrasses seem intricate combinations of features known from both angiosperm land plants and marine macroalgae with new structural elements.
Taxonomy Today, seagrasses are a polyphyletic group of marine angiosperms with around 60 species in five families (Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae), which belong to the order Alismatales according to the Angiosperm Phylogeny Group IV System. The genus Ruppia, which occurs in brackish water, is not regarded as a "real" seagrass by all authors and has been shifted to the Cymodoceaceae by some authors. The APG IV system and The Plant List WebpageThe Plant List (2020). Ruppia. Available online at: http://www.theplantlist.org/1.1/browse/A/Ruppiaceae/Ruppia/ (accessed September 22, 2020). do not share this family assignment. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
Family Image Genera Description Zosteraceae The family Zosteraceae, also known as the seagrass family, includes two genera containing 14 marine species. It is found in temperate and subtropical coastal waters, with the highest diversity located around Korea and Japan. Phyllospadix Zostera Hydrocharitaceae The family Hydrocharitaceae, also known as tape-grasses, include Canadian waterweed and frogbit. The family includes both fresh and marine aquatics, although of the sixteen genera currently recognised, only three are marine. They are found throughout the world in a wide variety of habitats, but are primarily tropical. Enhalus Halophila Thalassia Posidoniaceae The family Posidoniaceae contains a single genus with two to nine marine species found in the seas of the Mediterranean and around the south coast of Australia. Posidonia Cymodoceaceae The family Cymodoceaceae, also known as manatee-grass, includes only marine species. Some taxonomists do not recognize this family. Amphibolis Cymodocea Halodule Syringodium Thalassodendron Total species: 61 species
Cell walls
Seagrass cell walls contain the same polysaccharides found in angiosperm land plants, such as cellulose. However, the cell walls of some seagrasses are characterised by sulfated polysaccharides, which is a common attribute of macroalgae from the groups of red, brown and also green algae. It was proposed in 2005 that the ability to synthesise sulfated polysaccharides was regained by marine angiosperms. Another unique feature of cell walls of seagrasses is the occurrence of unusual pectic polysaccharides called apiogalacturonans.
In addition to polysaccharides, glycoproteins of the hydroxyproline-rich glycoprotein family, are important components of cell walls of land plants. The highly glycosylated arabinogalactan proteins are of interest because of their involvement in both wall architecture and cellular regulatory processes. Arabinogalactan proteins are ubiquitous in seed land plants and have also been found in ferns, lycophytes and mosses. They are structurally characterised by large polysaccharide moieties composed of arabinogalactans (normally over 90% of the molecule) which are covalently linked via hydroxyproline to relatively small protein/peptide backbones (normally less than 10% of the molecule). Distinct glycan modifications have been identified in different species and tissues and it has been suggested these influence physical properties and function. In 2020, AGPs were isolated and structurally characterised for the first time from a seagrass. Although the common backbone structure of land plant arabinogalactan proteins is conserved, the glycan structures exhibit unique features suggesting a role of seagrass arabinogalactan proteins in osmoregulation.
Further components of secondary walls of plants are cross-linked phenolic polymers called lignin, which are responsible for mechanical strengthening of the wall. In seagrasses, this polymer has also been detected, but often in lower amounts compared to angiosperm land plants. Thus, the cell walls of seagrasses seem to contain combinations of features known from both angiosperm land plants and marine macroalgae together with new structural elements. Dried seagrass leaves might be useful for papermaking or as insulating materials, so knowledge of cell wall composition has some technological relevance.
Sexual recruitment
Seagrass populations are currently threatened by a variety of anthropogenic stressors. The ability of seagrasses to cope with environmental perturbations depends, to some extent, on genetic variability, which is obtained through sexual recruitment. By forming new individuals, seagrasses increase their genetic diversity and thus their ability to colonise new areas and to adapt to environmental changes. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
Seagrasses have contrasting colonisation strategies. Some seagrasses form seed banks of small seeds with hard pericarps that can remain in the dormancy stage for several months. These seagrasses are generally short-lived and can recover quickly from disturbances by not germinating far away from parent meadows (e.g., Halophila sp., Halodule sp., Cymodocea sp., Zostera sp. and Heterozostera sp.). In contrast, other seagrasses form dispersal propagules. This strategy is typical of long-lived seagrasses that can form buoyant fruits with inner large non-dormant seeds, such as the genera Posidonia sp., Enhalus sp. and Thalassia sp.Kuo J, Den Hartog C. (2006) "Seagrass morphology, anatomy, and ultrastructure". In: Larkum AWD, Orth RJ, Duarte CM (Eds), Seagrasses: Biology, Ecology and Conservation, Springer, pages 51–87. Accordingly, the seeds of long-lived seagrasses have a large dispersal capacity compared to the seeds of the short-lived type, which permits the evolution of species beyond unfavourable light conditions by the seedling development of parent meadows.