Posidonia oceanica

Posidonia oceanica
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Order:	Alismatales
Family:	Posidoniaceae
Genus:	Posidonia
Species:	P. oceanica
Binomial name
Posidonia oceanica (L.) Delile
Posidonia oceanica range

Posidonia oceanica, commonly known as Neptune grass or Mediterranean tapeweed, is a

The Posidonia has a very high carbon absorption capacity, being able to soak up 15 times more carbon dioxide every year than a similar sized piece of the Amazon rainforest.^[5]

Morphology

Posidonia oceanica has roots (which mainly serve to anchor the plant to the substrate), rhizome and tapeform leaves.

The rhizomes, up to 1 cm thick, grow both horizontally (plagiotropic rhizomes), and vertically (orthotropic rhizomes). The former, thanks to the presence at the bottom of lignited roots up to 15 cm long, anchor the plant to the substrate. The latter, which increase height, have the function of combatting sanding due to continuous sedimentation. The two types of growth give rise to the so-called "kill", a terrace formation that consists of a network of strata of rhizomes, roots and trapped sediments. In this way, posidonias colonise an environment that algae could hardly occupy due to the lack of roots.

The leaves arise from orthotropic rhizomes, are cyntiform and bright green in colour that turns brown over time. They can reach a length of approximately 1.5 metres high. On average they have a width of 1 cm and have 13 to 17 parallel ribs. The apexes are rounded and are often lost by the action of waves and currents.

They are organised in bushes of 6 or 7 leaves, with the oldest on the outside and the youngest on the inside. The leaves are divided into three categories:

• Adult leaves, which have a lamina with photosynthetic function and a base separated from the foliar edge by a concave structure called "ligula";
• Intermediate leaves, which have no basis;
• Young leaves, which normally have a length of less than 50 mm. In autumn the plant loses the outermost adult leaves, which become brown and are photosynthetically inactive. During the winter, new leaves are produced.

Reproduction

Posidonia oceanica reproduces both sexually and asexually (by stolons).

Sexual reproduction occurs through the production of flowers and fruits. The flowers are hermaphrodite and are grouped in a herringbone-shaped inflorescence, green in colour and contained between floral bracts. The peduncle binds to the rhizome in the centre of the bunch. The gynaecium is formed by a unilocular ovary that continues with a style and ends in the stigma. The androecium consists of three stamens with short anthers. Flowering depends on environmental factors (light and temperature) and endogenous factors (age and size of the plant) and takes place in September and October in the meadows closest to the sea surface, while in the deepest ones it is postponed for two months. Pollen inside the anthers is spherical in shape, but becomes filamentous as soon as it is released into the water. There are no recognition mechanisms between pollen and stigma that prevent self-fertilisation. Pollination is hydrophilic and can lead to fruit formation, although some of them do not reach maturation, which occurs after six months. Once ripe, the fruits separate and float on the surface.

The fruit, slightly fleshy and called in some places "sea olive", is similar to a drupe and has a porous pericarp and rich in an oily substance that allows flotation. When it rots, it releases a seed (coated by a thin membrane, but without a true and proper tegument), which falls to the bottom and, if it finds the right conditions of depth, stability and type of sediment, germinates and gives rise to a new plant. In order for it to be consolidated, it is necessary that in finds a moisturised substrate. Humidification consists of the degradation of plant remains, so the plant can be implanted in "soils" previously colonised by other plants, such as macroalgae or other phanerogams. Thus, a true ecological succession is generated in which posidonia represents the last stage. Germination begins with the release of a small white root of radical pole and a leaf of the apical pole. With sexual reproduction, the plant colonises new areas, diffuses meadows in other areas and guarantees genetic variability.

Asexual reproduction by stolons, which allows the expansion of meadows, is carried out through plagiargiotropic rhizomes, which grow about 7 cm a year and colonise new spaces. The high accumulation of sediments and the reduction of the space available for horizontal growth stimulates the vertical growth of the rhizomes, thus forming the bushes.

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Growth

Posidonia growth rate is amongst the lowest of marine angiosperm with its horizontal rhizomes growing from 1 to 6 centimetres per year, whilst the vertical rhizomes grow from 0.1 to 4 centimetres per year. To grow a new leaf it takes approximately 50.68 days, exceptionally lower than other types of seagrass like Zostera noltii, which takes an approximate 13.71 days.

Taxonomy

The genus Posidonia is named after

Description

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Posidonia oceanica is a flowering plant which grows in dense meadows or along sandy channels in the waters of the Mediterranean. It is found at depths of 1–35 metres (3.3–114.8 ft),^[8] depending on water clarity. Subsurface rhizomes and roots stabilize the plant, while erect rhizomes and leaves reduce silt accumulation.

The leaves are ribbon-like, appearing in tufts of 6 or 7, and up to 1.5 metres (4.9 ft) long.^{[citation needed]} Average leaf width is around 10 millimetres (0.39 in).^{[citation needed]} The leaves are bright green, perhaps turning brown with age, and have 13 to 17 parallel veins. The leaf terminus is rounded, at times absent, usually because of damage or wear. Leaves are arranged in groups, with older leaves on the outside, longer and differing in form from the younger leaves they surround.

The rhizomatous stems have a dual growth habit—one grows down 150 centimetres (59 in) or so beneath the substrate, and the other grows above it. All the stems are approximately 10 millimetres (0.39 in) thick and upright in habit. This arrangement of the rhizomes eventually forms a mat; the surface contains the active parts of the plant, whereas the center is a dense network of roots and decomposing stems.^{[citation needed]}

P. oceania's common name is 'Neptune grass'.^[9]

In 2006 a huge

Dead rhizomes with olive-mill waste are used for compost.^[14]

Distribution and habitat

This species is found only in the Mediterranean Sea, where it is in decline, occupying an area of about 3% of the basin. This corresponds to a surface area of about 38,000 square kilometres (15,000 sq mi). Posidonia grows best in clean waters, and its presence is a marker for lack of pollution.^[15] The presence of Posidonia can be detected by the masses of decomposing leaves on beaches. Such plant material has been used for composting, but Italian laws prohibit the use of marine algae and plants for this purpose.^[16]

The

Preventing the erosion of the Mediterranean Coast

Posidonia oceanica has similar characteristics to terrestrial plants, such as roots, rhizomatous stem and cintiform leaves up to one metre long arranged in clumps of 6 to 7. It flowers in autumn and produces floating fruits in spring, commonly known as sea olives.

It forms underwater meadows of considerable ecological importance. It constitutes the climax community of the Mediterranean Sea and plays an important role in protecting the coastline from erosion. It is home to many animal and plant organisms that find food and protection in the meadows. It is considered a good.

Functions

• They fix CO₂ and produce oxygen.
• They protect the sandy coastline against erosion.
• They are a reserve of biodiversity due to the large number of animal and plant species that find within the habitat a place for feeding, protection and nursery for juveniles.
• They produce a large part of the sediment that forms part of the sand on our beaches.

See also

• List of long-living organisms
• Largest organisms

References