Phaeophyta, Brown Algae
Sargassum C. Agardh 1820
Phaeophyta, Order: Fucales; Family: Sargassaceae.
The origin of the genus’ scientific name, Sargassum, is vernacular, a local name for the plant that penetrated into scientific terminogoly, meaning ‘sea alga’.
Sargassum plants are externally differentiated quite clearly. The alga resembles a higher plant in shape. Stalk and leaf- like organs can easily be discerned, as can air vesicles. The base of the alga is wide, in a cone-like fashion. The ‘stalks’ and ‘leaves’ are rigid and covered with protrusions. A central vein can be seen running the length of the ‘leaf’. The vesicles are globular and contain gas, and their function is to keep the plant afloat.
Through a microscope, the ‘stalk’ is seen to be constructed of three layers: the external layers contain small cells that are rich in small chloroplasts, and the middle layer is comprised of larger elongated cells that resemble vascular plants.
The alga reaches a height of 40 cm and sometimes more. The ‘leaves’ are usually smaller than 5 cm. The width of the thallus varies.
The general colour of the Sargassum is brown, with shades ranging from very light yellowish-brown to deep chocolate brown.
The Sargassum is unique in its’ ‘leaf’ and ‘stalk’ structure, which resembles that of higher plants. The flotation vesicles are round and located upon pedicles, rather than sunk into the ‘stalk’. The sporangia are connected to separate ‘stalks’. The species is quite easy to identify.
The Sargassum is quite common in the subtidal zone, and individuals have been found at depths of 70 m and more.
Biology and reproduction
The family’s thalli are characterized by triangular apical cells. The alga is dioecious. Its reproductive organs are located within special branches - cylindrical or sharpened, that branch repeatedly, usually dichotomously. The reproductive branches are short and measure about half the length of the thallus. Most individuals are fertile and can be found throughout the year, but at the end of summer and during autumn, there is a significant decline in population, and many individuals are torn from their substrate and flung towards the beach. The reason for this may be sensitivity to low temperatures that slow their growth rate.
Seasonality and distribution
The vulgare is the most common Sargassum species along the coast. It is mainly distributed in the subtidal zone, near the low-tide line. Sargassum species are especially vulnerable to atmospheric influences, and species exposed to the air are rare, this in spite of the thallus, which is covered
by an oily layer that impedes the loss of water when the plant is exposed to the air.
The various species of Sargassum represent the majority of the globe’s algae biomass. Notable is the Sargassum Sea in the Atlantic Ocean, west of Africa between latitudes 20o and 35o, named for the huge Sargassum natans that float upon the water, covering extensive tracts of sea.
The Sargassum genus covers about 150 to 200 reported species. These are usually distinguished by the shapes of their respective ‘stalks’, ‘leaves’, flotation vesicles and reproductive organs. The identification is primarily based on the location of the plants’ reproductive organs. The shapes of the algae vary in response to environmental conditions, and to define a species properly, the sporangium must be observed and cross-sections taken of the thallus. Individuals belonging to the same species may differ in size and relationship between ‘leaf’ width and length. The varied thallus shape and the differences between individuals is probably one reason for the fact that one species may have a variety of names.
In the Mediterranean, five species are known: Sargassum vulgare C. Agardh = Sargassum salicifolium Naccari Sargassum acinarium (Linneaus) C. Agardh = Sargassum linifolium (Turner) C. Agardh Sargassum trichocarpum J. Agardh Sargassum flavifolium Kutzing Sargassum hornschuchii C. Agardh and one species, Sargassum muticum (Fensholt) Yendo, is a ‘cultural refugee’, a Japanese species that was originally cultivated in artificial pools and escaped to the sea.
The most frequently reported species in the Mediterranean is the Sargassum vulgare C. Agardh, but it is not certain whether the species is all that common. Along Israel’s shore, at least three species can be found, Sargassum vulgare, Sargassum linifolium (both reported by Edlestain also in deep water), and Sargassum trichocarpum, which was found in Rosh Hanikra (Northern Israel), and identified in Italy. Sargassum hornschuchii C. Agardh has been reported in Egypt and Turkey.
Research at the Israeli Oceanographic and Limnological Research Institute near Haifa by Friedlander and Ben Amotz revealed vegetarian hormonal-like materials )Cytokinine) in the thallus of Sargassum vulgare. These materials can assist plant germination.