Sargassum is a genus of brown (class Phaeophyceae) macroalgae (seaweed) in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they generally inhabit shallow water and coral reefs, and the genus is widely known for its planktonic (free-floating) species. While most species within the class Phaeophyceae are predominantly cold water organisms that benefit from nutrients upwelling, genus Sargassum appears to be an exception to this general rule. Any number of the normally benthic species may take on a planktonic, often pelagic existence after being removed from reefs during rough weather; however, two species (S. natans and S. fluitans) have become holopelagic—reproducing vegetatively and never attaching to the seafloor during their lifecycle. The Atlantic Ocean's Sargasso Sea was named after the algae, as it hosts a large amount of sargassum.
Sargassum was named by the Portuguese sailors who found it in the Sargasso Sea after a species of rock rose (Helianthemum) that grew in their water wells at home and that was called sargaço in Portuguese (Portuguese pronunciation: [sɐɾˈɣasu]).
The Florida Keys and its smaller islands are well known for their high levels of Sargassum covering their shores. Gulfweed was observed by Columbus. Although it was formerly thought to cover the entirety of the Sargasso Sea, making navigation impossible, it has since been found to occur only in drifts.
Sargassum is also cultivated and cleaned for use as an herbal remedy. Many Chinese herbalists prescribe powdered Sargassum, either the species S. pallidum, or more rarely, hijiki, S. fusiforme, in doses of 0.5 gram, to be dissolved in warm water and drunk as a tea. It is called "hai zao" in Traditional Chinese Medicine, where it is used to resolve "heat phlegm."
Species of this genus of algae may grow to a length of several metres. They are generally brown or dark green in color and consist of a holdfast, a stipe, and a frond. Oogonia and antheridia occur in conceptacles embedded in receptacles on special branches. Some species have berrylike gas-filled bladders which help keep the fronds afloat to promote photosynthesis. Many have a rough sticky texture, which together with a robust but flexible body, helps it to withstand strong water currents.
The thick masses of Sargassum provide an environment for a distinctive and specialised group of marine animals and plants, many of which are not found elsewhere.
Sargassum is commonly found in the beach drift near Sargassum beds where they are also known as gulfweed, a term also used to include all seaweed species washed up on shore.
Sargassum species are found throughout tropical areas of the world and are often the most obvious macrophyte in near-shore areas where Sargassum beds often occur near coral reefs. The plants grow subtidally and attach to coral, rocks or shells in moderately exposed or sheltered rocky or pebble areas. These tropical populations often undergo seasonal cycles of growth and decay in concert with seasonal changes in sea temperature. In some cases (e.g., the Sargasso Sea) there are floating populations of Sargassum.
Sargassum muticum is a large brown seaweed of the class Phaeophyceae. It grows attached to rocks by a perennial holdfast up to 5 cm in diameter. From this holdfast the main axis grows to a maximum of 5 cm high. The leaf-like laminae and primary lateral branches grow from this stipe. In warm waters it can grow to 12 m long, however in British waters it gives rise to a single main axis with secondary and tertiary branches which are shed annually. Numerous small 2–6 mm stalked air vesicles provide buoyancy. The reproductive receptacles are also stalked and develop in the axils of leafy laminae. It is self-fertile.
Sargassum adrift in Gulf Stream
The Gulf has the second largest concentration of sargassum of any body of water in the world. A fair amount of it washes out through the Straits of Florida in the Gulf Stream and ends up in the Sargasso Sea in the Atlantic Ocean off the East Coast of the United States.
We rounded Hatteras in fair weather, and saw the line between the brilliant blue Gulf Stream full of gulf weed and the muddy grayish shore water as clearly defined as that between the sidewalk and the roadway in a street.
Sargassum crisis in the Caribbean Sea
In the summer of 2015, large quantities of different species of Sargassum accumulated along the shores of many of the countries bathed by the Caribbean Sea. Some of the affected islands and regions include the Caribbean coast of Mexico, the Dominican Republic, Barbados and Tobago.
The algae washes ashore, piles up on beaches, and decays, often causing a foul odor, releasing fumes of sulphur hydroxide that rusts metals, that can turn taps black in shore houses, damages mod cons, and causes respiratory problems, particulary for asthmatics. 52 patients were recorded by a single doctor in Guadeloupe with sargassum related symptoms. Insurance problems arise for tourist operators and homeowners, where the household and business losses do not fall into previous insurance categories. Wildlife also suffers, for example baby sea turtles that die on their way to the open water. The affected countries and territories are discussing causes of the outbreak, potential solutions, and the negative affects on tourism. Currently, it is cleaned by spade and barrow onshore, and it can be collected by raking boats offshore. Barrages of shallow nets floated by long buoys can be used to ward off algea drifts, depending on wave height and current.  
Researchers say that the Sargassum outbreak started in 2011, but it has become worse over the years. There are several factors that could explain the proliferation of Sargassum in the area in recent years. These include the rise of sea temperature and the change of sea currents due to climate change. Also, nutriments from agricultural fertilizers and wastewater from the cities, that end up in the sea, could also make the algae bloom.
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