Brown algae (sg.:alga) are a large group ofmulticellularalgae comprising theclassPhaeophyceae. They include many seaweeds located in colder waters of theNorthern Hemisphere. Brownalgae are the majorseaweeds of the temperate and polar regions. Many brown algae, such as members of the orderFucales, commonly grow along rocky seashores. Most brown algae live in marine environments, where they play an important role both as food and as a potentialhabitat. For instance,Macrocystis, akelp of the orderLaminariales, may reach 60 m (200 ft) in length and forms prominent underwaterkelp forests that contain a high level of biodiversity.[4] Another example isSargassum, which creates unique floating mats of seaweed in the tropical waters of theSargasso Sea that serve as the habitats for many species. Some members of the class, such as kelps, are used by humans as food.
Between 1,500 and 2,000species of brown algae are known worldwide.[5] Some species, such asAscophyllum nodosum, have become subjects of extensive research in their own right due to their commercial importance. They also have environmental significance throughcarbon fixation.[4]
Brown algae belong to theStramenopiles, aclade ofeukaryotic organisms that are distinguished fromgreen plants by havingchloroplasts surrounded by four membranes, suggesting that they were acquired secondarily from asymbiotic relationship between a basal eukaryote and a red or green alga. Most brown algae contain the pigmentfucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among Stramenopiles in developing into multicellular forms withdifferentiatedtissues, but they reproduce by means offlagellatedspores andgametes that closely resemble cells of single-celled Stramenopiles. Genetic studies show their closest relatives to be theyellow-green algae.
Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlikecells no more than a few centimeters (a few inches) long.[6] Some species have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of brown algae grow to much larger sizes. Therockweeds and leatherykelps are often the most conspicuous algae in their habitats.[7] Kelps can range in size from the 60-centimeter-tall (2 ft) sea palmPostelsia to the giant kelpMacrocystis pyrifera, which grows to over 50 m (150 ft) long[8][9] and is the largest of all the algae. In form, the brown algae range from small crusts or cushions[10] to leafy free-floating mats formed by species ofSargassum. They may consist of delicate felt-like strands of cells, as inEctocarpus, or of 30-centimeter-long (1 ft) flattened branches resembling a fan, as inPadina.
Regardless of size or form, two visible features set the Phaeophyceae apart from all other algae. First, members of the group possess a characteristic color that ranges from anolive green to various shades ofbrown. The particular shade depends upon the amount offucoxanthin present in the alga.[11] Second, all brown algae aremulticellular. There are no known species that exist as single cells or as colonies of cells,[11] and the brown algae are the only major group ofseaweeds that does not include such forms. However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms.[citation needed] They can change color depending on salinity, ranging from reddish to brown.
Two specimens ofLaminaria hyperborea, each showing the rootlikeholdfast at lower left, a dividedblade at upper right, and a stemlikestipe connecting the blade to the holdfast.
Whatever their form, the body of all brown algae is termed athallus, indicating that it lacks the complexxylem andphloem ofvascular plants. This does not mean that brown algae completely lack specialized structures. But, because some botanists define "true" stems, leaves, and roots by the presence of these tissues, their absence in the brown algae means that the stem-like and leaf-like structures found in some groups of brown algae must be described using different terminology.[12] Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts.
Aholdfast is a rootlike structure present at the base of the algae. Like a root system in plants, a holdfast serves to anchor the alga in place on thesubstrate where it grows, and thus prevents the alga from being carried away by the current. Unlike a root system, the holdfast generally does not serve as the primary organ for water uptake, nor does it take in nutrients from the substrate. The overall physical appearance of the holdfast differs among various brown algae and among various substrates. It may be heavily branched, or it may be cup-like in appearance. A single alga typically has just one holdfast, although some species have more than one stipe growing from their holdfast.
Astipe is a stalk or stemlike structure present in an alga. It may grow as a short structure near the base of the alga (as inLaminaria), or it may develop into a large, complex structure running throughout the algal body (as inSargassum orMacrocystis). In the most structurally differentiated brown algae (such asFucus), the tissues within the stipe are divided into three distinct layers or regions. These regions include a central pith, a surrounding cortex, and an outer epidermis, each of which has an analog in the stem of a vascular plant. In some brown algae, the pith region includes a core of elongated cells that resemble thephloem of vascular plants both in structure and function. In others (such asNereocystis), the center of the stipe is hollow and filled with gas that serves to keep that part of the alga buoyant. The stipe may be relatively flexible and elastic in species likeMacrocystis pyrifera that grow in strong currents, or may be more rigid in species likePostelsia palmaeformis that are exposed to the atmosphere at low tide.
Many algae have a flattened portion that may resemble a leaf, and this is termed ablade,lamina, orfrond. The nameblade is most often applied to a single undivided structure, whilefrond may be applied to all or most of an algal body that is flattened, but this distinction is not universally applied. The namelamina refers to that portion of a structurally differentiated alga that is flattened. It may be a single or a divided structure, and may be spread over a substantial portion of the alga. Inrockweeds, for example, the lamina is a broad wing of tissue that runs continuously along both sides of a branchedmidrib. The midrib and lamina together constitute almost all of a rockweed, so that the lamina is spread throughout the alga rather than existing as a localized portion of it.
Species likeFucus vesiculosus produce numerous gas-filledpneumatocysts (air bladders) to increase buoyancy.
In some brown algae, there is a single lamina or blade, while in others there may be many separate blades. Even in those species that initially produce a single blade, the structure may tear with rough currents or as part of maturation to form additional blades. These blades may be attached directly to the stipe, to a holdfast with no stipe present, or there may be an air bladder between the stipe and blade. The surface of the lamina or blade may be smooth or wrinkled; its tissues may be thin and flexible or thick and leathery. In species likeEgregia menziesii, this characteristic may change depending upon the turbulence of the waters in which it grows.[6] In other species, the surface of the blade is coated with slime to discourage the attachment ofepiphytes or to deterherbivores. Blades are also often the parts of the alga that bear the reproductive structures.
Gas-filled floats calledpneumatocysts providebuoyancy in manykelps and members of theFucales. These bladder-like structures occur in or near thelamina, so that it is held nearer the water surface and thus receives more light for photosynthesis. Pneumatocysts are most often spherical orellipsoidal, but can vary in shape among different species. Species such asNereocystis luetkeana andPelagophycus porra bear a single large pneumatocyst between the top of the stipe and the base of the blades. In contrast, the giant kelpMacrocystis pyrifera bears many blades along its stipe, with a pneumatocyst at the base of each blade where it attaches to the main stipe. Species ofSargassum also bear many blades and pneumatocysts, but both kinds of structures are attached separately to the stipe by short stalks. In species ofFucus, the pneumatocysts develop within the lamina itself, either as discrete spherical bladders or as elongated gas-filled regions that take the outline of the lamina in which they develop.
Growth inDictyota dichotoma occurs at each frond tip, where new cells are produced.
The brown algae include the largest and fastest growing of seaweeds.[6] Fronds ofMacrocystis may grow as much as 50 cm (20 in) per day, and the stipes can grow 6 cm (2.4 in) in a single day.[13]
Growth in most brown algae occurs at the tips of structures as a result of divisions in a singleapical cell or in a row of such cells. They are single cellular organisms.[7] As this apical cell divides, the new cells that it produces develop into all the tissues of the alga. Branchings and other lateral structures appear when the apical cell divides to produce two new apical cells. However, a few groups (such asEctocarpus) grow by a diffuse, unlocalized production of new cells that can occur anywhere on the thallus.[11]
The simplest brown algae are filamentous—that is, their cells are elongate and have septa cutting across their width. They branch by getting wider at their tip, and then dividing the widening.[14]
These filaments may be haplostichous or polystichous, multiaxial or monoaxial forming or not apseudoparenchyma.[15][16] Besides fronds, there are the large in sizeparenchymatic kelps with three-dimensional development and growth and different tissues (meristoderm,cortex andmedulla) which could be consider the trees of the sea.[17][18] There are also theFucales andDictyotales smaller than kelps but still parenchymatic with the same kind of distinct tissues.
Thecell wall consists of two layers; the inner layer bears the strength, and consists ofcellulose; the outer wall layer is mainlyalgin, and is gummy when wet but becomes hard and brittle when it dries out.[15] Specifically, the brown algal cell wall consists of several components with alginates andsulphatedfucan being its main ingredients, up to 40% each of them.[19] Cellulose, a major component from most plant cell walls, is present in a very small percentage, up to 8%. Cellulose and alginate biosynthesis pathways seem to have been acquired from other organisms through endosymbiotic and horizontal gene transfer respectively, while the sulphated polysaccharides are of ancestral origin.[20] Specifically, the cellulose synthases seem to come from the red alga endosymbiont of the photosynthetic stramenopiles ancestor, and the ancestor of brown algae acquired the key enzymes for alginates biosynthesis from anactinobacterium. The presence and fine control of alginate structure in combination with the cellulose which existed before it, gave potentially the brown algae the ability to develop complex structurally multicellular organisms like the kelps.[21]
Genetic and ultrastructural evidence place the Phaeophyceae among theheterokonts (Stramenopiles),[22] a large assemblage of organisms that includes bothphotosynthetic members withplastids (such as thediatoms) as well as non-photosynthetic groups (such as theslime nets andwater molds). Although some heterokont relatives of the brown algae lack plastids in their cells, scientists believe this is a result of evolutionary loss of that organelle in those groups rather than independent acquisition by the several photosynthetic members.[23] Thus, all heterokonts are believed to descend from a singleheterotrophic ancestor that became photosynthetic when it acquired plastids throughendosymbiosis of another unicellular eukaryote.[24]
The closest relatives of the brown algae include unicellular and filamentous species, but no unicellular species of brown algae are known. However, most scientists assume that the Phaeophyceae evolved from unicellular ancestors.[25]DNA sequence comparison also suggests that the brown algae evolved from the filamentousPhaeothamniophyceae,[26]Xanthophyceae,[27] or theChrysophyceae[28] between 150[1] and 200 million years ago.[2] In many ways, the evolution of the brown algae parallels that of thegreen algae andred algae,[7] as all three groups possess complex multicellular species with analternation of generations. Analysis of 5SrRNA sequences reveals much smaller evolutionary distances among genera of the brown algae than among genera of red or green algae,[2][29] which suggests that the brown algae have diversified much more recently than the other two groups.
The occurrence of Phaeophyceae asfossils is rare due to their generally soft-bodied nature,[30] and scientists continue to debate the identification of some finds.[31] Part of the problem with identification lies in theconvergent evolution of morphologies between many brown and red algae.[32] Most fossils of soft-tissue algae preserve only a flattened outline, without the microscopic features that permit the major groups of multicellular algae to be reliably distinguished. Among the brown algae, only species of the genusPadina deposit significant quantities of minerals in or around their cell walls.[33] Other algal groups, such as thered algae andgreen algae, have a number ofcalcareous members. Because of this, they are more likely to leave evidence in the fossil record than the soft bodies of most brown algae and more often can be precisely classified.[34]
Fossils comparable in morphology to brown algae are known from strata as old as the UpperOrdovician,[35] but thetaxonomic affinity of these impression fossils is far from certain.[36] Claims that earlierEdiacaran fossils are brown algae[37] have since been dismissed.[26] While manycarbonaceous fossils have been described from thePrecambrian, they are typically preserved as flattened outlines or fragments measuring only millimeters long.[38] Because these fossils lack features diagnostic for identification at even the highest level, they are assigned to fossilform taxa according to their shape and other gross morphological features.[39] A number ofDevonian fossils termedfucoids, from their resemblance in outline to species in the genusFucus, have proven to be inorganic rather than true fossils.[30] The Devonian megafossilPrototaxites, which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga.[11] However, modern research favors reinterpretation of this fossil as a terrestrialfungus or fungal-like organism.[40] Likewise, the fossilProtosalvinia was once considered a possible brown alga, but is now thought to be an earlyland plant.[41]
A number ofPaleozoic fossils have been tentatively classified with the brown algae, although most have also been compared to known red algae species.Phascolophyllaphycus possesses numerous elongate, inflated blades attached to a stipe. It is the most abundant of algal fossils found in a collection made fromCarboniferous strata inIllinois.[42] Each hollow blade bears up to eightpneumatocysts at its base, and the stipes appear to have been hollow and inflated as well. This combination of characteristics is similar to certain modern genera in the orderLaminariales (kelps). Several fossils ofDrydenia and a single specimen ofHungerfordia from the Upper Devonian ofNew York have also been compared to both brown and red algae.[32] Fossils ofDrydenia consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species ofLaminaria,Porphyra, orGigartina. The single known specimen ofHungerfordia branches dichotomously into lobes and resembles genera likeChondrus andFucus[32] orDictyota.[43]
The life cycle of a representative species Laminaria. Most Brown Algae follow this form of sexual reproduction.A closeup of a Fucus's conceptacle, showing the gametes coming together to form a fertilized zygote.
Most brown algae, with the exception of theFucales, performsexual reproduction throughsporic meiosis.[47] Between generations, the algae go through separatesporophyte (diploid) andgametophyte (haploid) phases. The sporophyte stage is often the more visible of the two, though some species of brown algae have similar diploid and haploid phases. Free floating forms of brown algae often do not undergosexual reproduction until they attach themselves to substrate. The haploid generation consists of male and femalegametophytes.[48] The fertilization of egg cells varies between species of brown algae, and may beisogamous,oogamous, oranisogamous. Fertilization may take place in the water with eggs and motile sperm, or within theoogonium itself.
Certain species of brown algae can also performasexual reproduction through the production of motile diploidzoospores. These zoospores form in plurilocularsporangium, and can mature into the sporophyte phase immediately.
In a representative speciesLaminaria, there is a conspicuousdiploid generation and smaller haploid generations. Meiosis takes place within severalunilocularsporangium along the algae's blade, each one forming either haploid male or femalezoospores. The spores are then released from the sporangia and grow to form male and female gametophytes. The female gametophyte produces an egg in the oogonium, and the male gametophyte releases motile sperm that fertilize the egg. The fertilized zygote then grows into the mature diploid sporophyte.
In the orderFucales, sexual reproduction isoogamous, and the mature diploid is the only form for each generation. Gametes are formed in specializedconceptacles that occur scattered on both surfaces of the receptacle, the outer portion of the blades of the parent plant. Egg cells and motile sperm are released from separate sacs within the conceptacles of the parent algae, combining in the water to complete fertilization. The fertilized zygote settles onto a surface and then differentiates into a leafythallus and a finger-like holdfast. Light regulates differentiation of the zygote into blade and holdfast.(by samay the great )
Brown algae have adapted to a wide variety of marine ecological niches including the tidal splash zone, rock pools, the whole intertidal zone and relatively deep near shore waters. They are an important constituent of some brackish water ecosystems, and have colonized freshwater on a minimum of six known occasions.[49] A large number of Phaeophyceae are intertidal or upper littoral,[26] and they are predominantly cool and cold water organisms that benefit from nutrients in up welling cold water currents and inflows from land;Sargassum being a prominent exception to this generalisation.
Brown algae growing in brackish waters are almost solely asexual.[26]
Brown algae have aδ13C value in the range of −30.0‰ to −10.5‰, in contrast with red algae and greens. This reflects their different metabolic pathways.[51]
They havecellulose walls withalginic acid and also contain the polysaccharidefucoidan in the amorphous sections of their cell walls. A few species (ofPadina) calcify witharagonite needles.[26]
In addition to alginates, fucoidan and cellulose, the carbohydrate composition of brown algae consists ofmannitol,laminarin andglucan.[52]
The photosynthetic system of brown algae is made of aP700 complex containingchlorophyll a. Their plastids also containchlorophyll c and carotenoids (the most widespread of those beingfucoxanthin).[53]
Brown algae produce a specific type of tannin calledphlorotannins in higher amounts than red algae do.
Brown algae include a number ofedible seaweeds. All brown algae containalginic acid (alginate) in their cell walls, which is extracted commercially and used as an industrial thickening agent in food and for other uses.[54] One of these products is used inlithium-ion batteries.[55] Alginic acid is used as a stable component of a batteryanode. Thispolysaccharide is a major component of brown algae, and is not found in land plants.
Alginic acid can also be used inaquaculture. For example, alginic acid enhances the immune system of rainbow trout. Younger fish are more likely to survive when given a diet with alginic acid.[56]
Brown algae includingkelp beds also fix a significant portion of the earth's carbon dioxide yearly through photosynthesis.[57] Additionally, they can store a great amount of carbon dioxide which can help us in the fight against climate change.[58]Sargachromanol G, an extract ofSargassum siliquastrum, has been shown to have anti-inflammatory effects.[59]
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