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Brown Algae

The large group of multicellular algae, which comprises the class Phaeophyceae, are actually the brown algae, and it even includes many seaweeds which are located in colder waters environments of the northern hemisphere. Brown Algae also belongs to a large group of eukaryotic organisms, and they even have more than 1500 species of them. Brown Algae shows a very diverse nature in their morphology, habitats and distribution, and many other attributes. Brown algae also play a very important role in the food cycle in nature, and they are also considered as one of the important sources of food for humans.

Brown Algae

Brown Algae

Brown Algae are a large group of multicellular algae, which comprises the class Phaeophyceae and it includes many cold-water environment seaweeds that are found in the northern hemisphere. Most of the species belonging to the Brown algae group is found in the marine environment where they play two important roles, which are following:

  • The first role brown algae perform in the marine environment is they act as an important source of food, and thus they maintain the food cycle and food web there.
  • And, the second role which they perform there is that they act as potential habitat in the marine environment.

Example 1: Let's pick Macrocystis (kelp that belongs to order Laminariales), for example, and we will find that they can form prominent underwater kelp forests and may reach up to 200 feet (60 m) in length. And these kelp forests form a high level of biodiversity.

Example 2: One more example is Sargassum; it is a species of brown algae that can create unique floating mats of seaweeds. They can create these unique floating mats of seaweeds in the tropical waters of the Sargasso Sea, and these floating mats will serve as a habitat for many marine species.

Sargassum: Sargassum is a genus (it belongs to the family Sargassaceae) that contains around 150 species of brown algae, and it is also known as Sea holly or gulfweed. They generally occur as pelagic algae (which is a free-floating form of them) in an open sea or attached to rocks along the coasts in the temperate regions. Sometimes massive quantities of Sargassum species in the form of seaweeds wash ashore in the Caribbean. Thus, Sargassum can cause a negative impact on beach tourism, although these rotten algae do not pose a health risk or threat to humans.

Brown Algae: Overview

Brown algae belong to a large group of eukaryotic organisms, i.e., group Heterokontophyta, which is distinguished from other eukaryotic organisms' groups most prominently by having chloroplasts in them, which are surrounded by four membranes. The chloroplasts in this group, which are surrounded by four membranes, also suggest a symbiotic relationship of a basal eukaryotic organism with another eukaryotic organism, and thus it makes this group very different from other eukaryotic groups. Most of the brown algae species contain a pigment named fucoxantin, and this pigment is responsible for their distinctive greenish-brown colour, which is why they are called Brown Algae. Brown algae are very different among the heterokont group in terms of developing into their multicellular forms with the help of their differentiated tissues, and many genetic studies have shown that their closest relatives are yellow-green algae.

A total of between 1500 and 2000 species of brown algae has been registered so far worldwide, and many of these species have become a subject for extensive research. One of such brown algae species that has become a subject of extensive research is Ascophyllum nodosum, which has been a subject for extensive research because of its commercial importance. The Ascophyllum nodosum species of brown algae also have environmental significance as they play a major role in carbon fixation, and thus it increases their importance. Many species of brown algae, such as species of brown algae which belongs to order Fucales, are commonly found growing alongside the rocky seashores. Also, many species of brown algae that form kelp forests, such as kelps, are used by humans as a source of food.

Brown Algae: Morphology

The size of the brown algae species is very diverse and can exist in wide ranges, and that's why the morphological study of brown algae is even more difficult. The smallest members of the brown algae group grow as feathery and tiny tufts, which forms threadlike cells, and their size is not more than a few centimetres (or a few inches) in length. Some species of brown algae have many stages in their life cycle where these species consist of only a few cells, which makes the entire alga of microscopic size. Many other species of the brown algae group grow up to larger sizes.

Leathery kelps (that have been described in the introduction part) and the rockweeds are often considered the most conspicuous brown algae species in their habitats. Kelp species of brown algae can have various size ranges, ranging from 2 feet (60 centimetres tall) in sea palm Postelsia to over 150 feet tall (around 50 metres) in the grown giant kelp Macrocystis pyrifera. This giant kelp Macrocystis pyrifera is also the largest grown and tall species in all the algae. If talking about the form, species of brown algae range from leafy free-floating mats (which is formed by species of Sargassum) to small crusts of cushions. The free-floating leafy mats of Sargassum species may consist of 30-centimetre-long flattened branches resembling a fan (as found in Pandia) or delicate felt-like strands of cells (as found in Ectocarpus).

Regardless of its form or size, there are two unique characteristics, or visible features of Phaeophyceae, which set it apart from all the other algae species. These two visible features of Phaeophyceae are following:

  • The first feature is that all the Phaeophyceae or brown algae species are multicellular, and there is not a single species from the brown algae group, which is found to be existing as colonies of cells or in single-cell form.
  • The second visible feature of Phaeophyceae is that members from this group possess a characteristic colour that can range from various shades of brown colour to olive-green shade. The particular shade for a member of this group totally depends on the amount of fucoxanthin (a type of pigment) present.

It is also notable that the brown algae group is the only major algae group of seaweeds which do not include any colonial or single-cell forms. But this is not due to their evolutionary history because it seems to result from the classification of species. This can be seen when all the groups of algae that have single-cell forms of colonial forms are hypothesized as the closest relatives of brown algae. Brown algae group members can also change their colour, which ranges from reddish to brown, depending on the salinity of the habitat.

Brown Algae: Growth

Brown algae also possess some unique characteristics in terms of their growth and development, as many seaweeds that belongs to the brown algae group are the largest and fastest-growing seaweeds. For example, look at the Macrocystis member of this group. The stipes of Macrocystis can grow up to 6 cm (which is equal to 2.5 inches) in a single day, and their fronds may grow as much as 50 cm (which is equal to 20 inches) per day. Growth in most members of the brown algae group occurs at the tip of their structures, and it occurs due to divisions in the single apical cell or in a row of such apical cells. These apical cells are single-cell organisms, and when they divide, the new cells produced by them develop into all the tissues of the alga of the brown algae group. When the apical cells divide to produce two new apical cells, branching and other lateral structures appear on the alga of this group. However, many groups of brown algae, such as Ectocarpus, grow by a diffuse and unlocalized production of new cells, and these new cell productions can appear anywhere on the thallus.

Brown Algae: Visible structures

No matter what the form of brown algae species are, the body of all the members of their group is termed as thallus. The term thallus for brown algae's body indicate that it lacks phloem of vascular plants and complex xylem in them but saying this doesn't mean that body of brown algae completely lack all the specialized structures. Because some botanists and researchers define leaves, roots, and "true" stems by the presence of tissues that are mentioned above (absent tissues in the brown algae). Therefore, the presence of leaf-like and stem-like structures, which is found in some brown algae species, have to be described using different terminology. Moreover, not all the member species of brown algae have complex structures, but some of them that typically possess one or more characteristic parts.

Following are the descriptions of complex visible structures present in some members of brown algae:

i) Holdfast:

A holdfast in the alga is a root-like structure that is present at the base of the alga. The holdfast in the algae serves to anchor them in the place on the substrate where they grow, which is very much similar to the root system present in plants, and thus it also prevents them from being carried away from the current of the water. But unlike the root system that is present in plants, the holdfast (present in the brown algae) usually does not act as the primary organ in them for up taking water, and it is not even used for taking in the nutrients in the brown algae from the substrate. These two important factors actually make the holdfast very different from the traditional root system present in the plants. The overall appearance of this holdfast organ generally differs among various species of brown algae, and there is no common pattern observed among the different species and even different substrates. The appearance of holdfast can be seen as a cup-like structure, or it may be heavily branched. A single alga from the brown algae species' groups generally have only one holdfast in it, but it is often seen that some species of brown algae have more than one stipe growing on their holdfast.

ii) Stipe:

The next unique part of a brown algae species' body is its stipe which is a stem or stalk-like structure, and it is usually found growing near the base of them (Which is very common in species like Laminaria). It is sometimes can be found growing in the algal body, which later can develop into a complex and large structure throughout their body, which is commonly seen in species of brown algae such as Macrocystis or Sargassum). The tissues present in the stipe part are divided into three distinct regions or layers, and it is commonly seen in most structurally differentiated brown algae species such as Fucus etc. Following are the three distinct regions of the tissues that are present in the stipe part of brown algae:

  • An outer epidermis,
  • A surrounding cortex
  • A central pith

Each of all the three layers that are mentioned above has an analogue in the stem of a vascular plant. In some species of brown algae, the centre of the stipe part is hollow and filled with gas which serves as a buoyancy mechanism to keep that part of alga buoyant. Whereas, in other species of brown algae, the pith region of the stipe includes a core of elongated cells that resembles the phloem part both in function and structure (present in the vascular plants). The stipe of brown algae can be more rigid in species like Postelsia palmaeformis, which usually grow and are exposed to the atmosphere at low tide or maybe elastic and relatively flexible in species like Macrocystis pyrifera (Brown Algae species that grows in strong current regions).

iii) Frond or Blade or Lamina:

Many species of the brown algae group have a flattened part that may resemble a leaf, and this part is termed as a frond, blade or lamina. The term frond can be applied to most or all of an algal body that is flattened, whereas the name blade is most often applied to the algal body that is present in a single undivided structure. But it is even found that the distinction between these two terms is not universally applied. Now, talking about the name lamina, the term 'lamina' refers to the portion of the algal body which is flattened. The lamina may be spread over a substantial portion of the algal body and may be present in a single or in a divided structure. For example, rockweeds species of brown algae can be observed in which the lamina is a broad wing of tissue that runs continuously along both sides of a midrib (in branched form). The lamina or midrib present in rockweeds together constitutes almost all of the rockweeds. In rockweeds, the lamina is spread throughout the algal body rather than existing as a localized portion of their body, and thus these lamina and midrib constitute almost all of the rockweed's body.

In some other species of brown algae, there is only a single blade or lamina present, while in many other species of brown algae, there can be many separate lamina or blades present. This possibility is even applicable to the species of brown algae, which initially produces a single blade and later the single blade structure tear as part of their maturation or through rough current habitat, which result in the formation of additional blades. The blades can be directly attached to the stipe part of the brown algae, to a holdfast part where no spite is present in the algal body, or an air bladder may be present between the blade and the stipe. The surface of the frond or blade or lamina may be wrinkled or smooth, whereas tissues of this part of brown algae can be thick and leathery or thin and flexible. There are many species of brown algae, such as Egregia menziesii, in which the characteristics mentioned above can change, and this change in characteristics depend upon the turbulence of the water in which these algae grow. Whereas in many other species of them, the surface of the lamina or blade is found coated with slime, and this is due to deter herbivores or to discourage the attachment of epiphytes. This lamina or blade also carry or have the reproductive structures or parts of the brown algae.

iv) Pneumatocysts:

Pneumatocysts are the part of brown algae's body that are gas-filled floats, and they provide buoyancy in members of the Fucales group and in many kelps. In appearance, pneumatocysts are bladder-like structures that usually occurs near or in the lamina of brown algae, and they help the brown algae to hold nearer the water surfaces. Thus, pneumatocysts help brown algae to get more sunlight (for photosynthesis) by holding them nearer the water. Their shape in brown algae is usually ellipsoidal or spherical, but it can also vary with different shapes among different species. Many brown algae species like Nereocystis luetkeana and Pelagophycus porra carry a single large pneumatocyst between the base of the lamina or blade and the top of the stipe. Whereas some other species of brown algae, such as Macrocystis pyrifera, which forms giant kelp, have many blades in it along its stipes, and each lamina bears a pneumatocyst at its base where it attaches to the main stipe of the algal body. There are many species of the Sargassum group that also carry many blades or lamina and pneumatocysts, but these both kinds of structures are attached separately. Both structures (pneumatocysts and blades) are attached separately by short stalks to the stipe part of the Sargassum species. Whereas in many species of Fucus, this pneumatocyst part develops within the blade of algae itself, either as elongated gas-filled regions or as discrete spherical bladders that make the outline of the blades in which these pneumatocysts are developed.

Brown Algae: Tissue Organization

The simplest ones from the brown algae group are filamentous species, the cells of these species have septa cutting across their width and are elongated across the algal body. These cells in the filamentous species of brown algae branch by getting wider at their tip, and then the widening is divided further between them. The filaments of brown algae may be polystichous or haplostichous and monoaxial or multiaxial, and they may form a pseudoparenchym or maybe not. Apart from the fronds of brown algae, there are parenchymatic kelps that are large in size with the three-dimensional growth and development of these cells. There are different tissues, such as the medulla, cortex, and meristoderm etc., in the parenchymatic kelps, and these tissues could be considered the trees of the sea. There are other species of brown algae, such as Dictyotales and Fucales, which are smaller than kelps, but still, they are parenchymatic with the same kind of different tissues present in them.

The cell wall of the brown algae consists of the following two layers:

  1. The inner layer: This layer of cell wall bears the strength, and it consists of cellulose
  2. The outer layer: This layer of the cell wall is usually algin, and it is gummy when wet, but it becomes brittle and hard when it dries out.

The cell wall of brown algal specifically consists of many components, with the sulphated fucan being the main element as it accounts for up to 40% of the cell wall ingredients. Cellulose, which is present in the inner layer of the cell wall of brown algae and is a major component from most plant cell walls, is present marginally in a small amount (which is approximately equal to 8%) in the cell wall. Alginate biosynthesis pathways and cellulose seem to have been acquired from other organisms during their evolutionary history through horizontal gene transfer and endosymbiotic, respectively. In contrast, the sulphated fucan or sulphated polysaccharides have trackback to their ancestral origin. The cellulose synthases specifically seem to have been acquired from the red algae species endosymbiont by forming a close relationship with their photosynthetic stramenopiles ancestors. At the same time, the ancestors of brown algae have acquired their key enzymes (which are used for alginates photosynthesis) from an actinobacterium.

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