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76-551: See classification Brown algae ( sg. : alga ) are a large group of multicellular algae comprising the class Phaeophyceae . They include many seaweeds located in colder waters of the Northern Hemisphere . Brown algae are the major seaweeds of the temperate and polar regions. Many brown algae, such as members of the order Fucales , commonly grow along rocky seashores. Most brown algae live in marine environments, where they play an important role both as food and as

152-513: A symbiotic relationship between a basal eukaryote and a red or green alga. Most brown algae contain the pigment fucoxanthin , 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 with differentiated tissues , but they reproduce by means of flagellated spores and gametes that closely resemble cells of single-celled Stramenopiles. Genetic studies show their closest relatives to be

228-426: A diffuse, unlocalized production of new cells that can occur anywhere on the thallus. 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. These filaments may be haplostichous or polystichous, multiaxial or monoaxial forming or not a pseudoparenchyma . Besides fronds, there are

304-560: A fan, as in Padina . 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 an olive green to various shades of brown . The particular shade depends upon the amount of fucoxanthin present in the alga. Second, all brown algae are multicellular . There are no known species that exist as single cells or as colonies of cells, and

380-471: A large, complex structure running throughout the algal body (as in Sargassum or Macrocystis ). In the most structurally differentiated brown algae (such as Fucus ), 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,

456-457: A motile single-celled propagule ; this single cell asexually reproduces by undergoing 2–5 rounds of mitosis as a small clump of non-motile cells, then all cells become single-celled propagules and the clump dissolves. With a few generations under Paramecium predation, the "clump" becomes a persistent structure: only some cells become propagules. Some populations go further and evolved multi-celled propagules: instead of peeling off single cells from

532-504: A possible brown alga. However, modern research favors reinterpretation of this fossil as a terrestrial fungus or fungal-like organism. Likewise, the fossil Protosalvinia was once considered a possible brown alga, but is now thought to be an early land plant . A number of Paleozoic 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

608-464: A potential habitat . For instance, Macrocystis , a kelp of the order Laminariales , may reach 60 m (200 ft) in length and forms prominent underwater kelp forests that contain a high level of biodiversity. Another example is Sargassum , which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. Some members of

684-418: A separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox ), as a great part of species have the capacity for somatic embryogenesis (e.g., land plants, most algae, many invertebrates). One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into

760-410: A single cell organism to one of many cells. Genes borrowed from viruses and mobile genetic elements (MGEs) have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cells and sperm. Such fused cells are also involved in metazoan membranes such as those that prevent chemicals from crossing

836-427: A single large pneumatocyst between the top of the stipe and the base of the blades. In contrast, the giant kelp Macrocystis pyrifera bears many blades along its stipe, with a pneumatocyst at the base of each blade where it attaches to the main stipe. Species of Sargassum also bear many blades and pneumatocysts, but both kinds of structures are attached separately to the stipe by short stalks. In species of Fucus ,

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912-414: A single species. Although such symbiosis is theorized to have occurred (e.g., mitochondria and chloroplasts in animal and plant cells— endosymbiosis ), it has happened only extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming

988-407: A slug-like mass called a grex , which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte . A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism

1064-486: A stipe. It is the most abundant of algal fossils found in a collection made from Carboniferous strata in Illinois . Each hollow blade bears up to eight pneumatocysts 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 order Laminariales (kelps). Several fossils of Drydenia and a single specimen of Hungerfordia from

1140-476: A thallus is called thalloid , thallodal , thalliform , thalline , or thallose . Even though thalli do not have organized and distinct parts ( leaves , roots , and stems ) as do the vascular plants , they may have analogous structures that resemble their vascular "equivalents". The analogous structures have similar function or macroscopic structure, but different microscopic structure; for example, no thallus has vascular tissue . In exceptional cases such as

1216-418: Is a rootlike structure present at the base of the alga. Like a root system in plants, a holdfast serves to anchor the alga in place on the substrate 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

1292-399: 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

1368-413: Is also considered probable in some green algae (e.g., Chlorella vulgaris and some Ulvophyceae ). In other groups, generally parasites, a reduction of multicellularity occurred, in the number or types of cells (e.g., the myxozoans , multicellular organisms, earlier thought to be unicellular, are probably extremely reduced cnidarians ). Multicellular organisms, especially long-living animals, face

1444-558: Is also typically considered to involve cellular differentiation . The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae , such as Eudorina and Volvox ,

1520-448: Is debatable: The vast majority of living organisms are single celled, and even in terms of biomass, single celled organisms are far more successful than animals, although not plants. Rather than seeing traits such as longer lifespans and greater size as an advantage, many biologists see these only as examples of diversity, with associated tradeoffs. During the evolutionary transition from unicellular organisms to multicellular organisms,

1596-441: Is flattened. It may be a single or a divided structure, and may be spread over a substantial portion of the alga. In rockweeds , for example, the lamina is a broad wing of tissue that runs continuously along both sides of a branched midrib . 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. In some brown algae, there

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1672-670: Is inexact, as living multicellular organisms such as animals and plants are more than 500 million years removed from their single-cell ancestors. Such a passage of time allows both divergent and convergent evolution time to mimic similarities and accumulate differences between groups of modern and extinct ancestral species. Modern phylogenetics uses sophisticated techniques such as alloenzymes , satellite DNA and other molecular markers to describe traits that are shared between distantly related lineages. The evolution of multicellularity could have occurred in several different ways, some of which are described below: This theory suggests that

1748-533: Is observable in Drosophila ). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates . Because the first multicellular organisms were simple, soft organisms lacking bone, shell, or other hard body parts, they are not well preserved in

1824-425: Is presumed land-evolved - multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds ) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis , though multicellularity

1900-584: Is termed a thallus , indicating that it lacks the complex xylem and phloem of vascular 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. Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts. A holdfast

1976-502: Is the vegetative tissue of some organisms in diverse groups such as algae , fungi , some liverworts , lichens , and the Myxogastria . A thallus usually names the entire body of a multicellular non-moving organism in which there is no organization of the tissues into organs . Many of these organisms were previously known as the thallophytes , a polyphyletic group of distantly related organisms. An organism or structure resembling

2052-506: The Cambrian explosion shortly after the Marinoan. The predation hypothesis suggests that to avoid being eaten by predators, simple single-celled organisms evolved multicellularity to make it harder to be consumed as prey. Herron et al.  performed laboratory evolution experiments on the single-celled green alga, Chlamydomonas reinhardtii , using paramecium as a predator. They found that in

2128-528: The Cryogenian period and consisted of two global glaciation events known as the Sturtian and Marinoan glaciations. Xiao et al . suggest that between the period of time known as the " Boring Billion " and the snowball Earth, simple life could have had time to innovate and evolve, which could later lead to the evolution of multicellularity. The snowball Earth hypothesis in regards to multicellularity proposes that

2204-577: The Ediacaran is not necessary for complex life and therefore is unlikely to have been the driving factor for the origin of multicellularity. A snowball Earth is a geological event where the entire surface of the Earth is covered in snow and ice. The term can either refer to individual events (of which there were at least two) or to the larger geologic period during which all the known total glaciations occurred. The most recent snowball Earth took place during

2280-458: The Lemnoideae , where the structure of a vascular plant is in fact thallus-like, it is referred to as having a thalloid structure, or sometimes as a thalloid . Although a thallus is largely undifferentiated in terms of its anatomy, there can be visible differences and functional differences. A kelp , for example, may have its thallus divided into three regions. The parts of a kelp thallus include

2356-636: The Xenophyophorea that can reach 20 cm. Multicellularity has evolved independently at least 25 times in eukaryotes , and also in some prokaryotes , like cyanobacteria , myxobacteria , actinomycetes , Magnetoglobus multicellularis or Methanosarcina . However, complex multicellular organisms evolved only in six eukaryotic groups: animals , symbiomycotan fungi , brown algae , red algae , green algae , and land plants . It evolved repeatedly for Chloroplastida (green algae and land plants), once for animals, once for brown algae, three times in

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2432-429: The fungi ( chytrids , ascomycetes , and basidiomycetes ) and perhaps several times for slime molds and red algae. The first evidence of multicellular organization, which is when unicellular organisms coordinate behaviors and may be an evolutionary precursor to true multicellularity, is from cyanobacteria -like organisms that lived 3.0–3.5 billion years ago. To reproduce, true multicellular organisms must solve

2508-561: The placenta and the brain body separation. Two viral components have been identified. The first is syncytin , which came from a virus. The second identified in 2002 is called EFF-1 , which helps form the skin of Caenorhabditis elegans , part of a whole family of FF proteins. Felix Rey, of the Pasteur Institute in Paris, has constructed the 3D structure of the EFF-1 protein and shown it does

2584-437: The yellow-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 threadlike cells no more than a few centimeters (a few inches) long. 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. The rockweeds and leathery kelps are often

2660-517: The Cryogenian period in Earth's history could have been the catalyst for the evolution of complex multicellular life. Brocks suggests that the time between the Sturtian Glacian and the more recent Marinoan Glacian allowed for planktonic algae to dominate the seas making way for rapid diversity of life for both plant and animal lineages. Complex life quickly emerged and diversified in what is known as

2736-988: The Phaeophyceae come from Miocene diatomite deposits of the Monterey Formation in California . Several soft-bodied brown macroalgae, such as Julescraneia , have been found. Based on the work of Silberfeld, Rousseau & de Reviers 2014. Choristocarpaceae Discosporangiaceae Ishigeaceae Petrodermataceae Onslowiales Dictyotales Syringodermatales Lithodermataceae Phaeostrophiaceae Stypocaulaceae Cladostephaceae Sphacelariaceae Bachelotiaceae Desmarestiales Sporochnales Ascoseirales Ralfsiales Cutleriaceae Tilopteridaceae Phyllariaceae Nemodermatales Sargassaceae Durvillaeaceae Himanthaliaceae Multicellular Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are

2812-545: The Upper Ordovician , but the taxonomic affinity of these impression fossils is far from certain. Claims that earlier Ediacaran fossils are brown algae have since been dismissed. While many carbonaceous fossils have been described from the Precambrian , they are typically preserved as flattened outlines or fragments measuring only millimeters long. Because these fossils lack features diagnostic for identification at even

2888-525: The Upper Devonian of New York have also been compared to both brown and red algae. Fossils of Drydenia consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species of Laminaria , Porphyra , or Gigartina . The single known specimen of Hungerfordia branches dichotomously into lobes and resembles genera like Chondrus and Fucus or Dictyota . The earliest known fossils that can be assigned reliably to

2964-538: The alga that bear the reproductive structures. Gas-filled floats called pneumatocysts provide buoyancy in many kelps and members of the Fucales . These bladder-like structures occur in or near the lamina , so that it is held nearer the water surface and thus receives more light for photosynthesis. Pneumatocysts are most often spherical or ellipsoidal , but can vary in shape among different species. Species such as Nereocystis luetkeana and Pelagophycus porra bear

3040-475: The appearance of metazoans are deregulated in cancer cells, including genes that control cell differentiation , adhesion and cell-to-cell communication . There is a discussion about the possibility of existence of cancer in other multicellular organisms or even in protozoa . For example, plant galls have been characterized as tumors , but some authors argue that plants do not develop cancer. In some multicellular groups, which are called Weismannists ,

3116-430: The atmosphere at low tide. Many algae have a flattened portion that may resemble a leaf, and this is termed a blade , lamina , or frond . The name blade is most often applied to a single undivided structure, while frond may be applied to all or most of an algal body that is flattened, but this distinction is not universally applied. The name lamina refers to that portion of a structurally differentiated alga that

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3192-419: The brown algae are the only major group of seaweeds 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. They can change color depending on salinity, ranging from reddish to brown. Whatever their form, the body of all brown algae

3268-464: 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. DNA sequence comparison also suggests that the brown algae evolved from the filamentous Phaeothamniophyceae , Xanthophyceae , or the Chrysophyceae between 150 and 200 million years ago. In many ways,

3344-434: 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. Thus, all heterokonts are believed to descend from a single heterotrophic ancestor that became photosynthetic when it acquired plastids through endosymbiosis of another unicellular eukaryote. The closest relatives of

3420-439: The brown algae the ability to develop complex structurally multicellular organisms like the kelps. Genetic and ultrastructural evidence place the Phaeophyceae among the heterokonts (Stramenopiles), a large assemblage of organisms that includes both photosynthetic members with plastids (such as the diatoms ) as well as non-photosynthetic groups (such as the slime nets and water molds ). Although some heterokont relatives of

3496-425: The challenge of cancer , which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process. Cancer in animals ( metazoans ) has often been described as a loss of multicellularity and an atavistic reversion towards a unicellular-like state. Many genes responsible for the establishment of multicellularity that originated around

3572-625: The class, such as kelps, are used by humans as food. Between 1,500 and 2,000 species of brown algae are known worldwide. Some species, such as Ascophyllum nodosum , have become subjects of extensive research in their own right due to their commercial importance. They also have environmental significance through carbon fixation . Brown algae belong to the Stramenopiles , a clade of eukaryotic organisms that are distinguished from green plants by having chloroplasts surrounded by four membranes, suggesting that they were acquired secondarily from

3648-704: The clump, the clump now reproduces by peeling off smaller clumps. Multicellularity allows an organism to exceed the size limits normally imposed by diffusion : single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism. Whether all of these can be seen as advantages however

3724-449: The composite lichen , although dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more. This theory states that a single unicellular organism, with multiple nuclei , could have developed internal membrane partitions around each of its nuclei. Many protists such as the ciliates or slime molds can have several nuclei, lending support to this hypothesis . However,

3800-407: The evolution of the brown algae parallels that of the green algae and red algae , as all three groups possess complex multicellular species with an alternation of generations . Analysis of 5S rRNA sequences reveals much smaller evolutionary distances among genera of the brown algae than among genera of red or green algae, which suggests that the brown algae have diversified much more recently than

3876-459: The expression of genes associated with reproduction and survival likely changed. In the unicellular state, genes associated with reproduction and survival are expressed in a way that enhances the fitness of individual cells, but after the transition to multicellularity, the pattern of expression of these genes must have substantially changed so that individual cells become more specialized in their function relative to reproduction and survival. As

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3952-446: The first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other that they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism. Each respective organism would become a separate lineage of differentiated cells within

4028-715: The fossil record. One exception may be the demosponge , which may have left a chemical signature in ancient rocks. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon ( Gabonionta ). The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits. Until recently, phylogenetic reconstruction has been through anatomical (particularly embryological ) similarities. This

4104-425: The highest level, they are assigned to fossil form taxa according to their shape and other gross morphological features. A number of Devonian fossils termed fucoids , from their resemblance in outline to species in the genus Fucus , have proven to be inorganic rather than true fossils. The Devonian megafossil Prototaxites , which consists of masses of filaments grouped into trunk-like axes, has been considered

4180-486: 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. A stipe is a stalk or stemlike structure present in an alga. It may grow as a short structure near the base of the alga (as in Laminaria ), or it may develop into

4256-408: The lamina or blade may be smooth or wrinkled; its tissues may be thin and flexible or thick and leathery. In species like Egregia menziesii , this characteristic may change depending upon the turbulence of the waters in which it grows. In other species, the surface of the blade is coated with slime to discourage the attachment of epiphytes or to deter herbivores . Blades are also often the parts of

4332-430: The large in size parenchyma tic kelps with three-dimensional development and growth and different tissues ( meristoderm , cortex and medulla ) which could be consider the trees of the sea. There are also the Fucales and Dictyotales smaller than kelps but still parenchymatic with the same kind of distinct tissues. The cell wall consists of two layers; the inner layer bears the strength, and consists of cellulose ;

4408-453: The latter of which consists of up to 500–50,000 cells (depending on the species), only a fraction of which reproduce. For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". Some authors suggest that

4484-549: The major groups of multicellular algae to be reliably distinguished. Among the brown algae, only species of the genus Padina deposit significant quantities of minerals in or around their cell walls. Other algal groups, such as the red algae and green algae , have a number of calcareous 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. Fossils comparable in morphology to brown algae are known from strata as old as

4560-584: The most conspicuous algae in their habitats. Kelps can range in size from the 60-centimeter-tall (2 ft) sea palm Postelsia to the giant kelp Macrocystis pyrifera , which grows to over 50 m (150 ft) long and is the largest of all the algae. In form, the brown algae range from small crusts or cushions to leafy free-floating mats formed by species of Sargassum . They may consist of delicate felt-like strands of cells, as in Ectocarpus , or of 30-centimeter-long (1 ft) flattened branches resembling

4636-534: The multicellular organism emerged, gene expression patterns became compartmentalized between cells that specialized in reproduction ( germline cells) and those that specialized in survival ( somatic cells ). As the transition progressed, cells that specialized tended to lose their own individuality and would no longer be able to both survive and reproduce outside the context of the group. Thallus Thallus ( pl. : thalli ), from Latinized Greek θαλλός ( thallos ), meaning "a green shoot " or " twig ",

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4712-453: The newly created species. This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones . In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as

4788-517: The origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation , rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel 's gastraea theory . About 800 million years ago, a minor genetic change in a single molecule called guanylate kinase protein-interaction domain (GK-PID) may have allowed organisms to go from

4864-425: The other two groups. The occurrence of Phaeophyceae as fossils is rare due to their generally soft-bodied nature, and scientists continue to debate the identification of some finds. Part of the problem with identification lies in the convergent evolution of morphologies between many brown and red algae. Most fossils of soft-tissue algae preserve only a flattened outline, without the microscopic features that permit

4940-412: The other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium. The colonial theory of Haeckel , 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory , which suggests the symbiosis of different species) led to a multicellular organism. At least some - it

5016-541: The outer wall layer is mainly algin , and is gummy when wet but becomes hard and brittle when it dries out. Specifically, the brown algal cell wall consists of several components with alginates and sulphated fucan being its main ingredients, up to 40% each of them. 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

5092-497: The oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life. This hypothesis is based on the correlation between the emergence of multicellular life and the increase of oxygen levels during this time. This would have taken place after the Great Oxidation Event but before the most recent rise in oxygen. Mills concludes that the amount of oxygen present during

5168-474: The pith region includes a core of elongated cells that resemble the phloem of vascular plants both in structure and function. In others (such as Nereocystis ), 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 like Macrocystis pyrifera that grow in strong currents, or may be more rigid in species like Postelsia palmaeformis that are exposed to

5244-437: 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. The brown algae include the largest and fastest growing of seaweeds. Fronds of Macrocystis 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. Growth in most brown algae occurs at

5320-645: The presence of this predator, C. reinhardtii does indeed evolve simple multicellular features. It is impossible to know what happened when single cells evolved into multicellular organisms hundreds of millions of years ago. However, we can identify mutations that can turn single-celled organisms into multicellular ones. This would demonstrate the possibility of such an event. Unicellular species can relatively easily acquire mutations that make them attach to each other—the first step towards multicellularity. Multiple normally unicellular species have been evolved to exhibit such early steps: C. reinhartii normally starts as

5396-773: The problem of regenerating a whole organism from germ cells (i.e., sperm and egg cells), an issue that is studied in evolutionary developmental biology . Animals have evolved a considerable diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi. Loss of multicellularity occurred in some groups. Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g., Microsporidia ) and there have been numerous reversions to unicellularity across fungi (e.g., Saccharomycotina , Cryptococcus , and other yeasts ). It may also have occurred in some red algae (e.g., Porphyridium ), but they may be primitively unicellular. Loss of multicellularity

5472-399: The result of many identical individuals joining together to form a colony . However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". There are also macroscopic organisms that are multinucleate though technically unicellular, such as

5548-425: The simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions. The macro nucleus serves the organism's needs, whereas the micro nucleus is used for sexual reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not

5624-421: The sulphated polysaccharides are of ancestral origin. 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 an actinobacterium . The presence and fine control of alginate structure in combination with the cellulose which existed before it, gave potentially

5700-402: The tips of structures as a result of divisions in a single apical cell or in a row of such cells. They are single cellular organisms. 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 as Ectocarpus ) grow by

5776-417: The work of linking one cell to another, in viral infections. The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity. Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible. This theory suggests that

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