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5.3.3: Red and Green Algae - Biology

5.3.3: Red and Green Algae - Biology


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Learning Objectives

  • Distinguish between different groups of algae using life cycle, morphological features, and cellular composition.
  • Connect adaptations in the red and green algae to habitat characteristics and ecology.
  • Identify structures and phases in the Polysiphonia and Spirogyra life cycles; know the ploidy of these structures.

Red algae and green algae are included in the supergroup Archaeplastida. It was from a common ancestor of these protists that the land plants evolved, since their closest relatives are found in this group. Molecular evidence supports that all Archaeplastida are descendants of an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium. This primary endosymbiosis resulted in the chloroplasts of all photosynthetic eukaryotes discussed in this book! The red and green algae include unicellular, multicellular, and colonial forms.

Glaucophyta

Glaucophytes are unicellular, phototrophic eukaryotics found in freshwater ecosystems. Glaucophytes are likely an ancestral branch in the Archaeplastida and are often used as evidence of the cyanobacterial origin of chloroplasts. They have chloroplast-like organelles, called cyanelles or muroplasts, that have peptidoglycan between the two membranes. They have the same pigments as cyanobacteria and red algae: chlorophyll a and phycobilins. Much like the cyanobacteria, they appear blue-green due to the interaction of chlorophyll and phycocyanins (glauco- comes from the Greek glaukos meaning blue-green). Non-motile cells have a rigid cell wall composed of cellulose. Motile cells have two whiplash flagella. Sexual reproduction has yet to be documented in this group.

Rhodophyta

Red algae descended from the same endosymbiotic event as the Glaucophyta. The red algae are almost exclusively marine. Some are unicellular but most are multicellular. Approximately 6,000 species have been identified. They have true chloroplasts with two membranes (no remnant peptidoglycan) containing chlorophyll a. Like the cyanobacteria, they use phycobilins as antenna pigments - phycoerythrin (which makes them red) and phycocyanin. Red pigment allows the red algae to photosynthesize at deeper depths than the green or brown algae, harnessing more of the blue light waves that penetrate deeper into the water column. Unlike green algae and plants, red algae store carbohydrates as Floridean starch in the cytosol. Some are used as food in coastal regions of Asia. Agar, the base for culturing bacteria and other microorganisms, is extracted from a red alga.

Selection Pressures and Drivers

An important aspect of understanding the life history traits of the Rhodophyta is understanding the challenges of living in a marine environment.

  1. Access to sunlight: Most colors of light cannot penetrate into deeper water, as they are scattered by water molecules. The wavelengths of light that reach deepest into the ocean are blue and green. Many fish that live in the deep ocean are red. Because red light does not penetrate to the depths where they live, this makes them virtually undetectable by sight. Remember, we see things because of the light that bounces off of them. Red pigments reflect red light, so no red light, no reflected light. Red algae are using a similar strategy--absorb the wavelengths of light that are not red--with a different goal: to use that absorbed light to make food. The phycoerythrin in their chloroplasts reflects red light, giving them a red appearance, and absorbs the blue light that is able to penetrate to deeper areas in the water column.

  2. Fertilization: The ocean is an expansive environment, often with large areas of open space between populations of organisms. In this environment, successful fertilization of an egg by a nonmotile sperm--red algae lack flagella--presents a challenge. Having multicellular haploid and diploid phases provides red algae more opportunities to produce gametes and spores. A diploid stage that clones the zygote, the carposporophyte, provides more opportunities to do meiosis from each fertilization event.

  3. Salinity: Marine environments are relatively high in salinity. A possible adaptation for this is to have sulfated polysaccharides in the cell wall, such as the galactans present in Rhodophyta. This is a strategy present in (potentially all) marine algae and is inferred to be an adaptation for salinity-tolerance. See this open-access article for further information: https://doi.org/10.1371/journal.pone.0018862

Morphology

Red algae have a diverse range of morphologies. Unicellular forms may live solitarily or as colonies but, unlike other members of the Archaeplastida, lack flagella. Flagella are absent from the Rhodophyta, lost at some point in their evolutionary history. Multicellular forms can be filamentous, leafy, sheet-like, coralloid, or even crust-like (some examples in Figure (PageIndex{4}) and Figure (PageIndex{5})). The strange coralline red algae have calcerous deposits in the cell walls that make the thallus hard, like a coral. These can take a variety of forms and are able to live at depths other algae cannot (over 500 feet deep for some!).

Cells of multicellular species are connected via incomplete cytokinesis, resulting in pit connections (Figure (PageIndex{6})).

Polysiphonia Life Cycle

Red algae have a haplodiplontic (alternation of generations) life cycle that has an extra diploid stage: the carposporophyte. Polysiphonia is the model organism for the Rhodophyta life cycle. The gametophytes of Polysiphonia are isomorphic (iso- meaning same, morph- meaning form), meaning they have the same basic morphology.

Male Gametophyte

The male gametophyte has elongated structures that emerge from the tips of the thallus branches. These are spermatangia, where spermatia are produced by mitosis.

Female Gametophyte and Carposporophyte

The female gametophyte produces an egg that is contained within a structure called the carpogonium. This structure has a long, thin projection called a trichogyne (trich- meaning hair, -gyne meaning female). During fertilization, a spermatium fuses with the trichogyne and the nucleus of the spermatium travels down the tube to the egg. When the nucleus of the spermatium fuses with the egg, a zygote is produced. This zygote is retained and nourished by the female gametophyte as it grows.

The globose structures you see growing from the female gametophyte thallus are called cystocarps. A cystocarp is composed of both female gametophyte tissue (n) and carposporophyte tissue (2n). The outer layer of the cystocarp, the pericarp (peri- meaning around) is derived from the female gametophyte and is haploid. The interior of the cystocarp consists of the carposporophyte, which is diploid, and produces structures called carposporangia, inside of which it produces carpospores by mitosis. All of these--carposporophyte, carposporangia, and carpospores--are diploid.

Tetrasporophyte

The diploid carpospores are released into the ocean waters, where they will be carried on currents to another location. If a carpospore lands in an appropriate environment, it will grow by mitosis into a tetrasporophyte (2n). The tetrasporophyte produces tetrasporangia (2n) within the branches of the thallus. Each tetrasporangium produces four unique, haploid tetraspores by meiosis. Tetraspores (n) are released and will grow by mitosis into either male or female gametophytes, completing the life cycle.

Full Life Cycle Diagram

Summary of Characteristics for Red Algae

  • Morphology: Unicellular to multicellular, no flagellated stages. Cells of multicellular species are connected via incomplete cytokinesis, resulting in pit connections.

  • Cell wall composition: Cellulose and galactans

  • Chloroplasts: 2 membranes, pigments are chlorophyll a and phycobilins (primarily phycoerythrin, providing their red color)

  • Storage carbohydrate: Floridean starch

  • Life cycle: Alternation of generations with an extra diploid stage, the carposporophyte

  • Ecology: Primarily marine (97% of species)

Green Algae

The most abundant group of algae is the green algae. The nature of the evolutionary relationships between the green algae are still up for debate. As of 2019, genetic data supports splitting the green algae into two major lineages: chlorophytes and streptophytes. The streptophytes include several lineages of green algae (such as the charophytes) and all land plants. Streptophytes and chlorophytes represent a monophyletic group called Viridiplantae (literally “green plants”). The green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. They have chlorophyll a and b, have lost phycobilins but gained carotenoids, and store carbohydrates as starch inside plastids. Although some of the multicellular forms are large, they never develop more than a few types of differentiated cells and their fertilized eggs do not develop into an embryo.

Green algae are an important source of food for many aquatic animals. When lakes and ponds are "fertilized" with phosphates and nitrates (e.g., from sewage and the runoff from fertilized fields and lawns), green algae often form extensive algal "blooms". Members of this group can be found in freshwater and marine habitats, and many have adapted to life on land, either inside of lichens or free-living (see Figure (PageIndex{12})).

Selection Pressures and Drivers

  1. Sun Damage. Green algae represent a diverse group of organisms with diverse life history traits, many of which are shared with land plants. The development of carotenoids-- yellow, orange, and red pigments that act in both light harvesting and sun protection--offers this group increased access to sunlight while simultaneously protecting against UV damage. UV rays do not penetrate very far into the water column, so organisms moving into shallower waters or terrestrial environments would need to deal with this new challenge. Many terrestrial species of green algae appear orange, rather than green, due to the production of large amounts of carotenoids.

Morphology

These algae exhibit great diversity of form and function. Similar to red algae, green algae can be unicellular or multicellular. Many unicellular species form colonies and some green algae exist as large, multinucleate, single cells. Green algae primarily inhabit freshwater and damp soil, and are a common component of plankton. Chlamydomonas is a simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior flagella that guide it toward light sensed by its eyespot (Figure (PageIndex{13})). More complex species exhibit haploid gametes and spores that resemble Chlamydomonas.

The alga Volvox is one of a colonial organism, which behaves in some ways like a collection of individual cells, but in other ways like the specialized cells of a multicellular organism (Figure (PageIndex{14})). Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion. Individual Volvox cells move in a coordinated fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell specialization in this organism.

Volvox can reproduce both asexually and sexually. In asexual reproduction, the gonidia develop into new organisms that break out of the parent (which then dies). In sexual reproduction, the presence of an inducing chemical causes the following:

  • The gonidia of the males to develop into clusters of sperm.
  • The gonidia of the females to develop into new spheres each of whose own gonidia develops into a pair of eggs.
  • The sperm break out of the male parent and swim to the female where they fertilize her eggs.
  • The zygotes form a resting stage that enables Volvox to survive harsh conditions (Figure (PageIndex{15})).

The genome of Volvox carteri consists of 14,560 protein-encoding genes - only 4 more genes than in the single-celled Chlamydomonas reinhardtii! Most of its genes are also found in Chlamydomonas. The few that are not encode the proteins needed to form the massive extracellular matrix of Volvox.

Species in the genus Caulerpa exhibit flattened fern-like foliage and can reach lengths of 3 meters (Figure (PageIndex{16})). Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.

True multicellular organisms, such as the sea lettuce, Ulva, are also represented among the green algae (Figure (PageIndex{17}) and Figure (PageIndex{18})).

Spirogyra Life Cycle

Though green algae display a diversity of life cycles, many have a haplontic life cycle. A model organism for the green algae is Spirogyra (Figure (PageIndex{19})). Spirogyra is a unicellular green algae that grows in long, filamentous colonies, making it appear to be a multicellular organism. Even though it is technically unicellular, its colonial nature allows us to classify its life cycle as haplontic. In the haploid vegetative cells of the colony, the chloroplasts are arranged in spirals, containing darkened regions called pyrenoids where carbon fixation happens. Each haploid cell in the filament is an individual, which makes sexual reproduction between colonies an interesting process.

When two colonies of Spirogyra meet that are of a complementary mating type (+/-), sexual reproduction occurs. The two colonies align, each cell across from a complementary cell on the other filament. A conjugation tube extends from each cell in one colony (Figure (PageIndex{20})), inducing formation of a tube on the cells in the other colony. The conjugation tubes from each colony fuse together.

The contents of one cell will move through the conjugation tube and fuse with the contents of the complementary cell, resulting in a diploid zygote (Figure (PageIndex{21})). The zygote appears as a large, egg-like structure contained within the complementary cell. It has a thick wall that provides resistance to desiccation and cold, allowing colonies of Spirogyra to overwinter, when needed. The other colony is now a filament of empty cells that will be broken down by some decomposer. When conditions are right, the zygote undergoes meiosis to produce another vegetative colony of haploid cells.


Figure (PageIndex{21}): Cells in various stages of conjugation. Of the cells that have formed conjugation tubes and connected, the one farthest to the left has just recently finished the transfer and fusion of its cytoplasm, but the zygote hasn't fully formed yet. In the cell on the far right, there is a fully formed zygote. It is dark in color and has thick walls. The chloroplasts are not individually distinguishable within it. Photo by Maria Morrow, CC-BY-NC.

Full Life Cycle Diagram

Summary of Characteristics for Green Algae

  • Morphology: Unicellular to multicellular; two whiplash flagella on motile cells
  • Cell wall composition: Cellulose
  • Chloroplasts: 2 membranes, pigments are chlorophyll a, chlorophyll b, and carotenoids
  • Storage carbohydrate: Starch
  • Life cycle: Varies, but primarily haplontic. Some marine species have alternation of generations.
  • Ecology: Freshwater, marine, and terrestrial species

Summary

Glaucophytes, red algae, and green algae are part of the Archaeplastida. Glaucophytes are thought to be one of the earliest lineages to diverge due to the presence of remnant peptidoglycan between the membranes of its chloroplast-like cyanelles. Unsurprisingly, glaucophytes and red algae share the same pigments as Cyanobacteria.

Red algae (phylum Rhodophyta) are united by several synapomorphies (shared derived characteristics). They lack flagella, have pit connections between cells, and store carbohydrates as Floridean starch. The sulfated galactans in their cell walls allows them increased fitness in marine environments, while the pigment phycoerythrin allows them to photosynthesize deeper in the water column. They have an alternation of generations life cycle with an extra diploid phase, the carposporophyte, that clones the zygote. These characteristics can be connected to the environmental stressors presented by the marine habitats most red algae are found in.

Green algae represent several distinct lineages. Like plants, they store carbohydrates as starch within their plastids and have the pigments chlorophyll a and b, as well as carotenoids. Organisms in this group have haplontic (e.g. Spirogyra) or haplodiplontic (e.g. Ulva) life cycles. Many green algae are unicellular, forming complex colonies. Green algae can be found in marine, freshwater, and terrestrial environments (including within lichens!).


Similarities and differences between plants and algae

We all know plants such as pines, oaks or beeches. We all know seaweed such as sea lettuce, wakame or others used in Japanese cuisine. But do we really know what plants and algae are? Well, as we might suppose the answer is no. All the organisms mentioned above evolutionarily are related and belong to the same group, that of the plants. However, another group of organisms less known and less evolutionarily related are also called algae, the cyanobacteria, belonging to the kingdom of bacteria. If you want to know what the similarities and differences between algae and plants are, read on because we will reveal the answer.


Characteristics of Algae (With Diagram)

Algae exhibit a very wide range of morphological diversity. The simplest forms are unicellular, microscopic, motile or non-motile eukaryotic cells. They may be spherical (Protococcus, Chlorella), or pyriform (Chlamydomonas). When motile (Volvox, Chlamydomonas) the cells are generally provided with a pair of eukaryotic flagella. Diatoms show a characteristic type of non-flagellar locomotion.

Motile or non-motile algae may form a colony, known as a coenobium. There are also many multicellular algae. These may form uniseriate or multiseriate filaments which may be branched or un-branched. The branched filaments may have prostrate and erect branches (heterotrichous habit).

The multiseriate filaments may form a cylindrical thallus or sometimes a flat thalloid structure. The siphonaceous algae have coenocytic body (multinucleate, without septa) which may be simple or complex and elaborate. The brown algae which are exclusively marine and always multicellular, often have large complex thalli. Diatoms are unicellular algae, but they have a cell which is unique. It consists of two overlapping halves or valves, like those of a petridish.

Some lower forms of algae have a doubtful systematic position. Many of them, like the chrysomonads are amoeboid. Euglenoids, have a flexible cell-covering. They are without a rigid cell wall and resemble protozoa in many ways. The dinoflagellates are also peculiar in having a typically flattened cell with an equatorial constriction, known as a girdle. However, all such atypical organisms are photosynthetic which justifies their inclusion in algae.

Morphological features of some representative types of algae are shown in Fig. 5.31:

Cellular Characteristics of Algae:

Algae—being eukaryotic organisms—have a cellular organization like that of other photosynthetic eukaryotes. Algal cells have a double-membrane bound nucleus, mitochondria, vacuoles, chloroplastids, Golgi bodies, endoplasmic reticulum and 80S ribosomes. The prokaryotic blue-green algae are now considered as bacteria (cyanobacteria), though like other algae they carry out oxygenic photosynthesis. They have been treated elsewhere along with bacteria.

Motile unicellular or coenobial algae, as well as motile asexual and sexual spores, all have eukaryotic flagella with two central and nine pairs of fibrils surrounded by a membrane. The flagella are anchored to the basal bodies situated in the protoplast.

In some forms, the flagella are of two types — whiplash type having a stiff basal part and a flexible upper part, and tinsel type with fine hairy outgrowths. The flagella, often in a pair may be attached at the anterior end of the cell, or laterally. In some, like yellow-green algae (e.g. Botrydium, Vancheria), the pair of flagella are of unequal length in zoospores.

The algal chloroplasts vary greatly in size, shape and number. They may contain one or more pyrenoids, or none. The pyrenoids are colourless proteinaceous bodies which are involved in synthesis of starch during photosynthesis. Some photosynthetic flagellates, like Euglena, as well as zoospores of some algae e.g. Stigeoclonium have a red-coloured carotenoid-containing eye-spot which serves as a photo-receptive organ guiding locomotion. Some photo autotrophic flagellates have also contractile vacuoles.

Algal cell, in general, is bound by a cell-wall. The flagellates and the amoeboid forms lack a rigid cell wall. The composition of the cell wall is variable in different taxonomic groups. Generally, it is made of complex polymeric carbohydrates. In the green algae, the cell wall is mainly composed of cellulose. In many marine green algae, mannans are also present along with cellulose. Galactans are present in the cell wall of red algae.

Pectic substances are often associated with the polymeric carbohydrates. The walls of many algae are often reinforced with a variety of other materials, such as silica, calcium salts, alginic acid etc. The flagellates lack a cell wall and their cells are covered by a flexible modified membrane, which is generally known as a pellicle.

All types of algae contain photosynthetic pigments. These include different chlorophylls, carotenoids and phycobiliproteins. The pigments are located in chloroplasts. The proportion of different pigments imparts the characteristic colour of different groups of algae. Chlorophyll a is present in all groups of algae.

Chlorophyll b is present in mainly in the green algae and in traces in the englenoid flagellates. Chlorophyll is present in small amounts in the brown algae, yellow-green algae, golden-brown algae and the diatoms. Chlorophyll d is present specifically in the red algae. Chlorophyll e is found in some yellow-green algae.

Among the carotenoids, β-carotene is present universally in all algal groups, a-carotene is present in some green algae, brown algae, red algae and in diatoms, y-carotenes have been detected in some green algae and englenoids. Xanthophyll’s which are oxygenated carotenes also occur extensively in algae. Lutein is found in green algae and in small amount in red algae.

Fucoxanthin is the main xanthophyll’s of brown algae and gives the characteristic colour of these algae. It is also present in the golden-brown algae and diatoms. Zeaxanthin is present in red algae. Several other xanthophyll’s, like alloxanthin, dinoxanthin, heteroxanthin etc. occur in specific groups.

The phycobiiiproteins, characteristically present in cyanobacteria, also occur in red algae and brown crypto-monads. Phycobiiiproteins are of two types — phycocyanin which is a blue pigment, and phycoerythrin, a red pigment. Both types are found in algae, as also in cyanobacteria.

A non-plastidial pigment found in some algae is haematochrome. This pigment is present in some green algae, like Trentepohlia giving a red colour to these terrestrial algae. Sphaerella, a unicellular alga related to Chlamydomonas growing in the Arctic and Alpine regions, is rich in this pigment.

The alga grows so densely that the snow appears red and it causes the phenomenon called ‘red snow’. The chloroplasts of algae, specially those of green algae, are of various forms depending on the genus. Generally, in green algae there is 3 single chloroplast. In other groups, there are numerous small chloroplasts.

Some characteristic forms of chloroplasts in green algae are shown in Fig. 5.32:

Another cellular feature that varies in different algal groups is the nature of the storage carbohydrates. In green algae, as well as in crypto-monads and dinoflagellates, the storage product is starch. A starch-like polysaccharide, called floridean starch is the reserve substance in red algae.

In the brown algae, a dextrin-like carbohydrate, known as laminarin is the reserve material and a related polymer, chrysolaminarin occurs in the yellow-green and golden-brown algae, as also in diatom. The englenoids store paramylum. Besides the polysaccharides, most algae have oil-drops in the cells. Brown algae have also soluble mannitol.

Some useful characteristics of taxonomic importance of different algal divisions are summarized in Table 5.2:


Red Algae: Rhodophyta

D E N N I S A X E R Photograph / Getty Images

There are more than 6,000 species of red algae. Red algae gain their often brilliant colors thanks to the pigment phycoerythrin. The ability to absorb blue light allows red algae to live at greater depths than either brown or green algae.

Coralline algae, a subgroup of red algae, is important in the formation of coral reefs. Several types of red algae are used in food additives, and some are regular parts of Asian cuisine. Examples of red algae include Irish moss, coralline (Corallinales), and dulse (Palmaria palmata).


Similarities of Cyanobacteria with Red Algae and Bacteria

(i) Flagellated or motile cells are absent in both cyanobacteria and red algae.

(ii) The blue (phycocyanin) and red (phycoerythrin) pigments occurring in cyanobactena are chemically similar to those occurring in red algae and are located on phycobilisomes in both groups.

(iii) Both the groups have a common pattern of fatty acid formation which differs from other plants in that the lipid content does not increase as the thallus grows.

(iv) In both case, photosynthetic thylakoids occur singly and widely separated.

(v) In both case, the principle constituents of mucilage are sulfated galactoses, uronic acid, glucose, and xylose.

(vi) Pit connections are present in certain cyanobacteria and similar structures are also found in red algae

(vii) In both cases, the major components in the cellulose are xylans, and trehalose and galactose occur in free form in both.

Similarities of Cyanobacteria with Bacteria:

(i) Both, bacteria and cyanobacteria are prokaryotes (i.e., they have nucleus without nuclear membrane, lack membrane-bound plastids, possess 70S ribosomes, lack histone proteins, lack cell organelles, peptidoglycan present in cell wall, etc.).

(ii) The mucilaginous sheath surrounding cyanobacterial cells and the capsule present in many bacteria possess a similar structure as both are made up of extremely fine fibrils.

(iii) Both are sensitive to antibiotics.

(iv) Both lack true sexual reproduction in which plasmogamy, karyogamy, and meiosis take place in a regular sequence at specified points during the life. Bacteria fulfill their requirements of sexuality by means of three processes (conjugation, transformation, transduction) of genetic recombination and these mechanisms have been reported in certain cyanobacteria (e.g., Anacystis nidulans).

(v) Many metabolic processes (e.g., sulfur and nitrogen metabolism) are similar in both groups.

(vi) Some bacteria also fix atmospheric nitrogen (e.g., Rhizobium, Azotobacter) which is an important function of most of the cyanobacteria.

(vii) Both possess certain structural similarities. For example, Thiothrix and some other bacteria are structurally similar to ‘hormogonia’ found in cyanobacteria Beggiatoa, a sulfur bacterium, resembles with Oscillatoria in shape and movement.


The effects of water on light absorption

Red wavelengths are absorbed in the first few metres of water. Blue wavelengths are more readily absorbed if the water contains average or abundant amounts of organic material. Thus, green wavelengths are often the most common light in deep water.

Chlorophylls absorb red and blue wavelengths much more strongly than they absorb green wavelengths, which is why chlorophyll-bearing plants appear green. The carotenoids and phycobiliproteins, on the other hand, strongly absorb green wavelengths. Algae with large amounts of carotenoid appear yellow to brown, those with large amounts of phycocyanin appear blue, and those with large amounts of phycoerythrin appear red.

At one time it was believed that algae with specialized green-absorbing accessory pigments outcompeted green algae in deeper water. Some green algae, however, grow as well as other algae in deep water, and the deepest attached algae include green algae. The explanation of this paradox is that the cell structure of the deepwater green algae is designed to capture virtually all light, green or otherwise. Thus, while green-absorbing pigments are advantageous in deeper waters, evolutionary changes in cell structure can evidently compensate for the absence of these pigments.


Difference Between Red Brown and Green Algae

Definition

Red algae refer to a large group of algae that includes many seaweeds that are mainly red in color while brown algae refer to a large group of algae that are typically olive brown or greenish in color, including many seaweeds. Green algae, on the other hand, refer to photosynthetic algae which contain chlorophyll and store starch in discrete chloroplasts. Thus, this is the basic difference between red brown and green algae.

Classification

Red algae are classified under Rhodophyta, and brown algae are classified under Phaeophyta while green algae are classified under Chlorophyta. Hence, this is one important difference between red brown and green algae.

Type of Photosynthetic Pigments

Furthermore, the type of photosynthetic pigments is the main difference between red brown and green algae. Red algae contain chlorophyll a, chlorophyll d, and phycobilins, while brown algae contain chlorophyll a, chlorophyll c, fucoxanthin, and xanthophylls while green algae contain chlorophyll a, chlorophyll b, and xanthophylls.

Habitat

Another difference between red brown and green algae is that the red algae mainly live in marine habitats, and brown algae exclusively live in marine habitats while green algae mainly live in freshwater.

Unicellular or Multicellular

Moreover, their cellular structure is also a major difference between red brown and green algae. Red algae are mainly multicellular brown algae are exclusively multicellular unicellular species are more prominent in green algae.

Nature of Thylakoids

Besides, the thylakoids of red algae are unstacked while three thylakoids are stacked in brown algae. Green algae, on the other hand, contain thylakoid stacks of 2-20.

Motility

Both red and brown algae are sessile while green algae are motile and contain flagella.

Motility of Sperms

Motility of sperms is also a difference between red brown and green algae. Red algae do not produce motile stages during their life cycle, but brown algae produce motile sperms while green algae produce motile sperms with multiple flagella.

Food Reservation

While red algae reserve food in the form of floridean starch, brown algae reserve food in the form of laminarin green algae reserve food in the form of starch.

Cell wall

The cell wall of red algae is composed of cellulose and sulfated phycocolloids. Moreover, the cell wall of brown algae is composed of cellulose and non-sulfated phycocolloids while the cell wall of green algae is composed of cellulose. Hence, this is another difference between red brown and green algae.

Examples

Some examples of red algae are Irish moss, coralline algae, dulse (Palmaria palmata), etc. Some examples of brown algae are kelp, rockweed (Fucus), Sargassum, etc. while some examples of green algae are sea lettuce (Ulva sp.), which is commonly found in tidal pools, and Codium sp., etc.

Conclusion

Red algae are the red color algae mainly live in marine habitats. They contain chlorophyll a, chlorophyll d, and phycobilins. They store food in the form of floridean starch. On the other hand, brown algae are the brown color algae exclusively found in marine habitats. They contain chlorophyll a, chlorophyll c, fucoxanthin, and xanthophylls as photosynthetic pigments. They reserve food in the form of laminarin. Both red and brown algae are mainly multicellular. In comparison, green algae are the green color algae that mainly live in freshwater. They contain chlorophyll a, chlorophyll b, and xanthophylls. Starch is the main form of food stored by green algae. Therefore, the main difference between red brown and green algae is the type of photosynthetic pigments present, habitat, cellular organization, and the form of food storage.

References:

1. Kennedy, Jennifer. “What Are the 3 Types of Sea Weed (Marine Algae)?” ThoughtCo, ThoughtCo, 13 Sept. 2017, Available Here.

Image Courtesy:

1. “green-moss-nature-outdoor-texture-2798160” By oranfireblade (Pixabay License) via pixabay
2. “Red algae” By Ed Bierman – [1] (CC BY 2.0) via Commons Wikimedia
3. “seaweed-baltic-sea-coast-beach-sea-1614647” By KRiemer (Pixabay License) via pixabay

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things


Types of Algae: Green, Brown and Red Algae (With Diagram)

F.E. Fritsch (1935, 1945) in his book “Structure & reproduction of algae” gave a very comprehensive account of algae.

He divided algae into 11 classes (suffix-phyceae), mainly on the basis of pigmentation, thallus-structure, reserve food, flagellation & modes of reproduction.

i. Chlorophyceae (Green algae)

ii. Xanthophyceae (Yellow- green algae)

iii. Chrysophyceae (Golden Brown algae)

ix. Phaeophyceae (Brown algae)

xi. Cyanophyceae (Myxophyceae) – BGA

R.H. Whittaker grouped only Chlorophyceae, Phaeophyceae & Rhodophyceae under plant kingdom, Cyanophyceae under kingdom-Monera and the rest under kingdom-Protista.

1. Chlorophyceae (Green algae) (Fig. 5.2):

1. About 7000 species are known, mostly freshwater except a few (- 10%) marine forms.

2. The members are multi-cellular but may be unicellular, colonial or coenocytic.

3. Chloroplasts contain photosynthetic pigments (Chl- a, b, carotenes and xanthophylls) similar to those of land plants.

4. Cell wall made up cellulose.

6. Sexual reproduction is isogamous, anisogamous, and oogamous type. Examples: Spirogyra, Ulothrix, Caulerpa,VoIvox, Acetabularia, Chlorella etc.

2. Phaeophyceae (Brown Algae) (Fig. 5.3):

1. It includes about 2000 species, mostly marine.

2. Unicellular forms absent.

3. They appear brown due to large amount of brown coloured xanthophyll pigments called fucoxanthin (C40H56O6).

4. Photosynthetic pigments include Chl-a, c, carotenes &xanthophylls.

5. The plant body is a thallus differentiated into holdfast, stipe and lamina (blade or frond). Photosynthetic lamina is annual while stipe is perennial. Holdfast helps in anchorage. A few species are free-floating e.g. Fucus (rockweed), Sargassum (gulf weed). Sargassum covered thousand of hectors in the Sargasso Sea in the North Atlantic Ocean and it is a menace to shipping as they get attached to the bottom of ships.

6. The larger forms of brown algae are called kelps or sea weeds e.g. Macrocystis (30-60m, the largest sea plant), Nereocystis (20-30m.). The giant kelps contain air vesicles or bladder for buoyancy.

7. Cell wall composed of a mixture of polysaccharides like cellulose, pectose and algin (non- sulphated phycocolloids). Chemically, algin is the calcium salts of alginic acid (a major phycocolloid). Phycocolloids are complex polysaccharides that store in the cell wall of algae, protect them from desiccation and prevent drying or freezing (in winter) when exposed to air in low tide.

8. Food reserve is laminarin and mannitol.

3. Rhodophyceae (Red Algae) (Fig. 5.4):

1. About 5000 species are known, mostly marine except a few fresh water forms (Batrachospermum)

2. They appear red due to phycoerythrin (red pigment, C34H46O8N4) & phycocyanin (the blue pigment, C34H46O8N4). These pigments absorb blue-green region of spectrum i.e. 480-520 nm which can penetrate greater depth of water. Hence, the red algae are the deepest growing algae in the seas where other photosynthetic forms cannot grow.

3. Red algae appear more red in deep water because of excess phycoerythrin than chlorophyll is formed. When growing in shallow water, they appear green due to more chlorophyll. This property of change in pigmentation (colour) is called chromatic adaptation (Gaidukov phenomenon).

4. Nutrition is photoautopophic, except some colourless & parasitic forms like Harveyella which live on other red algae.

5. Reserve food is floridean starch.

6. Cell wall composed of cellulose, pectin & sulphated phycocolloids (agar, carageenin & funori).

7. Vegetative reproduction occurs by fragmentation & regeneration of holdfast. Some reproduce asexually by spores. Sexual reproduction is oogamous type.

8. The thallus of red algae may be unicellular (Porphyridium), filamentous (Batrachospermum, Polysiphonia), pseudofilamentous (Astocystis), parenchymatous (Porphyra), lace-like (Gelidium), ribbon-like (Chondrus) etc.

Green Algae as Ancestors of Land Plants:

Because of morphological, cytological and biochemical similarities, and phylogenetic evidences, it is now believed that green algae are the ancestors of land plants.

Some of the points in support of this view are briefly given below:

1. Both green algae and land plants possess the same kind of photosynthetic pigments, i.e. chlorophyll a, chlorophyll b, and carotenoids.

2. Cell wall contains cellulose and pectose in both.

3. In both the groups reserve food material is starch.

4. The structure of the flagella is similar in motile forms of both the groups.

5. Definite tendency is seen among the members of chlorophyceae to migrate towards land and lead life like land plants.

Land plants have advanced over the members of green algae along the line of folio wing adaptations:

1. Increased structural complexity of the plant body.

2. Highly developed reproductive organs with special adaptation to protect the gametes.

3. Protection of zygote, embryo formation, and great development of post-fertilization stages.


Figure 2. The representative alga, Chara, is a noxious weed in Florida, where it clogs waterways. (credit: South Florida Information Access, U.S. Geological Survey)

Green algae in the order Charales, and the coleochaetes (microscopic green algae that enclose their spores in sporopollenin), are considered the closest living relatives of embryophytes. The Charales can be traced back 420 million years. They live in a range of fresh water habitats and vary in size from a few millimeters to a meter in length. The representative species is Chara (Figure 2), often called muskgrass or skunkweed because of its unpleasant smell. Large cells form the thallus: the main stem of the alga. Branches arising from the nodes are made of smaller cells. Male and female reproductive structures are found on the nodes, and the sperm have flagella. Unlike land plants, Charales do not undergo alternation of generations in their lifecycle. Charales exhibit a number of traits that are significant in their adaptation to land life. They produce the compounds lignin and sporopollenin, and form plasmodesmata that connect the cytoplasm of adjacent cells. The egg, and later, the zygote, form in a protected chamber on the parent plant.

New information from recent, extensive DNA sequence analysis of green algae indicates that the Zygnematales are more closely related to the embryophytes than the Charales. The Zygnematales include the familiar genus Spirogyra. As techniques in DNA analysis improve and new information on comparative genomics arises, the phylogenetic connections between species will change. Clearly, plant biologists have not yet solved the mystery of the origin of land plants.

In Summary: Green Algae: Precursors of Land Plants

Green algae share more traits with land plants than other algae, according to structure and DNA analysis. Charales form sporopollenin and precursors of lignin, phragmoplasts, and have flagellated sperm. They do not exhibit alternation of generations.


Green Algae: Characters and Economic Importance (With Diagram)

Green algae are defined as a group of eukaryotic algae which resemble land plants in having cellulosic cell wall, starch as food reserve and both chlorophyll a as well as chloro­phyll b as photosynthetic pigments.

1. The group contains about 7000 living species.

2. Green algae occur in all types of habitats. Only ten percent of green algae are marine. Majority of the species are fresh water. Several members are sub-aerial. They grow on moist soils, walls, rocks and tree trunks. Strains of Chlorella can bear moderate hot waters. Some forms live in snow or frozen lakes (e.g., Seotiella, Homidium).

3. Some species are epiphytic, endophytic, epizoic or endozoic. Zoo-chlorella is associated with sponges. Characium occurs on crusta­ceans, Cladophora on molluscan shells, while Trichophilus provides green colour to the fur of tree-dwelling sloth (a mammal) found in the rain forests of South America.

The alga gives protective colouration to the sloth. Certain green algae are constituents of lichens. Cephaleuros is parasitic on a number of higher plants. It reduces the yield of tea, coffee, pepper and citrus fruits.

4. Thallus is various— unicellular flagel­late (e.g. Chlamydomonas), unicellular non-flagellate (e.g. Chlorella, Characium, Ac- etabularia or umbrella plant which is several centimeters in length and is differentiated into uni-nucleate holdfast, an elongated stalk and umbrella-like cap.), flagellate colonies (e.g. Volvox), non-flagellate colonies (e.g. Scenedesmus, Hydrodictyon), coenocytic and siphonaceous (e.g. Caulerpa), heterotrichous (with prostrate and vertical branches, e.g. Draparnaldia), and parenchymatous (e.g., Ulva).

5. Cell wall contains cellulose with a few exceptions.

6. Chloroplasts have 2-20 thylakoid lamellae.

7. Photosynthetic pigments are similar to those of higher plants— chlorophyll a, chlo­rophyll b, carotenes and xanthophyll’s. The colour is grass-green due to predominance of chlorophylls.

9. Chloroplasts generally contain 1 to many pyrenoids for storage of starch.

10. In flagellate forms, an eye spot is present in the chloroplasts.

11. Asexual reproduction takes place by both mitospores and meiospores. The common asexual spores are zoospores, aplanospores, hypnospores, akinetes, autospores, etc.

12. Sexual reproduction is effected by isogamy, anisogamy and oogamy. In isogamy both the fusing gametes are morphologically and physiologically similar. They may be flagel­late or non-flagellate. In anisogamy the fusing gametes are structurally similar but differ in size and behaviour.

One of the two gametes is larger and is called macrogamete or female gamete. The other is smaller and is termed microgamete or male gamete. In oogamy there is a large food laden non-flagellate female gamete called egg or oosphere. The male gamete or antherozoid is smaller and motile.

Common Types of Green Algae:

Some of the important types and the importance of green algae are as follows:

It is a micro­scopic (10-30 nm), eukaryotic, unicellular, pyriform, biflagellate green alga of both fresh water and marine habitats, generally rich in am­monia salts. Cell wall consists of glycoprotein.

Cellulose is absent. There is an apical papilla. Internally, the alga possesses a single nucleus, two contractile vacuoles for osmoregulation and excretion, a cup-shaped chloroplast with a red eye spot or stigma and a pyrenoid for storing starch.

Asexual reproduction occurs through for­mation of zoospores, aplanospores, hypnospores and palmella stage. In palmella stage, a large number of near naked cells devoid of flagella lie inside a mass of mucilage. The stage develops in response to toxic chemicals and unfavorable water conditions.

Zoospores are flagellate spores while aplanospores and hypnospores are non-motile. Aplanospores are thin-walled. Hypnospores are thick-walled.

They often possess reddish pigment haematochrome. Red snow caused by C. nivalis is due to red coloured hypnospores. Sexual reproduction can occur by isogamy, hologamy (fusion of young cells), anisogamy and oogamy. It shows zygotic meiosis and thus life cycle is haplontic.

It is a fresh water green hollow ball like colonial alga of 0.5-2 mm diameter. Colony of Volvox is hollow and has a fixed number of cells (500 to 60,000). It is called coenobium. The cells are interconnected by cytoplasmic strands.

They are biflagellate. Some cells of the posterior region are large. They function as reproductive cells or gonidia. The whole colony or coenobium swims by joint activity of its flagellate cells. The alga rotates during swim­ming. It is, therefore, also called rolling alga. Asexual reproduction occurs by for­mation of daughter colonies. Sexual reproduction is oogamous.

It is an attached, un-branched, green, filamentous alga of fresh aerated waters. Fila­ments are covered by muci­lage. They are attached to a solid substratum by means of a colourless lowermost cell called holdfast. The remaining cells are green. They are cylindrical and quadrate. The cells are uninucleate. They have central vacuoles.

The periph­eral protoplast possesses a single girdle-like or collar shaped chloroplast studded with a few pyrenoids. Every green cell is capable of growth, photosynthesis and reproduction. Vegeta­tive reproduction occurs through fragmentation. Asexual reproduction takes place through zoospores, aplanospores, hypnospores and akinetes.

Sexual reproduction is isogamous. Life cycle haplontic.

It is an un-­branched, mucilage covered green fila­mentous alga that forms free floating masses over the surface of fresh wa­ter ponds. It is called pond scum, water silk or mermaid tresses. All the cells are green, elongated, cylin­drical, capable of growth, division and taking part in reproduction.

A non-­green holdfast occurs in attached spe­cies. A green cell contains 1-16 spi­rally coiled ribbon shaped chloroplasts studded with medianly arranged pyrenoids. There is a single nucleus suspended in the central vacuole by means of cytoplasmic strands.

Spirogyra multiplies vegetatively by fragmentation. Asexual reproduction by spores is rare. Sexual reproduction occurs by conjugation. Conjugation occurs by two methods, scalanform conjugation and lateral conjugation.

In scalariform conjugation opposite cells of two filaments develop conjugation tubes. Gamete of one cell called male gamete is more active, passes through conjugation tube and fuses with gamete or female gamete of the cell of the second filament.

In lateral conjugation, two adjacent cells of the same filament function as male and female cells. Male gamete passes into female cell either through a conjugation tube (indirect lateral conjugation, e.g., S affinis) or through a central pore (direct lateral, e.g., S. jogensis).

Zygote develops into a resting thick walled zygospore. On approach of favourable season next year, zygospore undergoes meiosis but produces only a single filament due to degeneration of three of the four haploid nuclei. Development of zygote is direct. Life cycle is haplontic.

Chara or aquatic horse­tail is a green alga found growing at the bottom of shallow fresh waters like ponds, pools and lakes. Lime incrustation may occur in some spe­cies (hence stonewort). Chara is food for many aquatic animals. It can be used as manure. Mosquito larvae do not occur in Chara waters.The plant is fixed to the substratum by means of highly branched multicellular rhizoids.

The axis of the plant is jointed. The joints represent nodes having whorls of short laterals with an occasional long lateral. Male sex organ is rounded called antheridium (= globule). It lies below the ovate shaped female sex organ called oogonium (= nucule).

Both antheridium and oogonium have multicellular coverings which bring them close to sex organs of bryophytes. Chara can multiply vegetatively through fragmentation, tubers, bulbils, asylum stars and secondary protonemata.

Economic Importance of Green Algae:

A number of green algae are used as food, e.g., Ulva, Caulerpa, Enteromorpha. Chlorella can yield food rich in lipids, proteins, vitamins and minerals.

They can be extracted from Chlorella and Caulerpa.

Cephaleuros virescens causes red rust of tea and reduces yield of tea. It also reduces the yield of coffee, pepper, citrus, etc.

Sewage oxidation ponds contain a number of green algae, e.g., Chlamydomonas, Chlorella, Scenedesmus.

Green Algae as Ancestors of Land Plants:

There are no biochemical, cytological and morphological similarities between land plants and any other group of algae except the green ones or chlorophyta.

The various evidences which favour the chlorophycean origin of land plants are:

(i) Both green algae and land plants possess the same type of chlorophylls, a and b.

(ii) The carotenoid pigments are similar in the two groups.

(iii) Cell wall contains similar cellulose and pectic compounds in the two groups.


Watch the video: #Brownalgae#Greenalgae#Redalgae#ImportanceofAlgae (May 2022).