Algae: Chlorophyceae, Phaeophyceae, Rhodophyceae, Practice Problems and FAQs

Ponds can increase the aesthetic beauty of places. Hence garden ponds are very common in parks, resorts and even at homes. It makes the area very beautiful if we add fishes and other ornamental plants to the pond. We always like to spend more time near water bodies like ponds, lakes etc., because it can make us relax. But the maintenance of these water bodies are also very important, right?. If proper maintenance is not done, then the beauty of the pond will deteriorate.

Can you tell me what are the common factors that affect the ponds or lakes because of the less maintenance? One visible thing you might have noticed is the formation of green layers. Once it is formed, there will be bad odour from the pond, slowly the water will change its colour and the fishes will die inside it.

Fig: Algae in pond

So what do you think about those green layers on the ponds and lakes that have formed? Yes, they are a group of plants called algae. But they are very small and do not look like a plant at all. Right? But you can see their green colour, which is due to the presence of chloroplast. Using this they perform photosynthesis. Now the concept of algae as plants is convincing.

Can algae be only green in colour? The answer is no. They can be of other colours like red, brown, olive green etc. What is giving them those beautiful colours? The difference in the colour is due to the presence of different pigments present in them. Such varieties of algae are observed in oceans and also in fresh waters. Now it is obvious that they can be in different habitats. They also provide many economically important products like agar, carrageen, etc. Some algae are also used as food, fodder and medicines. They can even be the indicators of pollution. A small plant can do these many things in the environment!! So we should learn more about this group of plants called algae. Now let’s take a deep dive into the details of algae in this article.

Table of contents:

• Classification of plants
• Algae
• General characteristics of algae
• Classification of algae
• Differences between red, brown and green algae
• Economic importance of algae
• Practice Problems
• FAQs

Classification of plants

According to the traditional system of classification of plants by Eichler in 1883, Kingdom Plantae was classified into two sub kingdoms. They are Cryptogamae and Phanerogamae. He classified plants like this mainly on the basis of the reproductive structures called flowers. Those plants with hidden flowers and seeds are included under the subkingdom Cryptogamae and those that bear flowers and seeds were classified in the subkingdom phanerogamae.

Divisions of Cryptogamae

Cryptogamae is composed of three divisions and they are the Thallophyta, Bryophyta and Pteridophyta. All these divisions are composed of lower plants. The Thallophyta possess plants with undifferentiated bodies, called thallus. The major difference between these divisions are their sex organs. Thallophytes doesnot have jacketed sex organs, but bryophytes have jacketed sex organs whereas pteridophytes possess vascular tissues in the plant body.

Fig: Classification of the Plant Kingdom

Sub divisions of Thallophyta

Thallophytes had two sub divisions, they are as follows:

• Algae
• Fungi

Algae

They are the thallophytes which are composed of chlorophyll pigment and show an autotrophic mode of nutrition. They are able to prepare their own food.

Fig: Algae

Fungi

They are achlorophyllous (lack chlorophyll) thallophytes and show heterotrophic mode of nutrition. That means they depend on other organisms for food.

Fig: Fungi

Now we will discuss more about the sub division algae.

Algae

The chlorophyll containing, photosynthetic, non-vascular, thalloid and aquatic plants are called algae. Carolus Linnaeus termed the plants with such characteristics as ‘algae’. Robert Whittaker placed them in Kingdom Plantae. They have no true roots and true shoots with leaves. They lack flowers, fruits and seeds. Even after that, they are included under the plant kingdom by Eichler. Approximately 1 million species of algae were discovered! They are the first plant group that emerged from the unicellular aquatic organisms like cyanobacteria or blue-green algae. Let's see how algae evolved.

Fig: Algae

Evolution of algae

Primordial soup, or prebiotic soup, was the hypothetical set of conditions present on the Earth around 4 to 4.2 billions of years ago. We know that life first appeared in water. The primitive surface of the Earth was exposed to high levels of UV radiation. Large doses of these UV rays would kill any cell by damaging its genetic material. Moreover, the surface temperature was also very high. So life could not sustain itself on land, but water was deep enough and cool to shield organisms from UV radiation. Thus, life could have originated only in water.

GIF: Primordial soup

Origin of prokaryotic chemoautotroph and phototroph

The first cell to evolve was a prokaryotic chemoautotroph in the primitive Earth. They could utilise the chemicals in the water and convert it to energy needed for their survival. From chemoautotrophs, the next group of organisms evolved, which can convert the sunlight to energy. Those organisms are called photoautotrophs. They were prokaryotic, unicellular, photosynthetic and classified under the Kingdom Monera. They are the cyanobacteria. These were later engulfed by the prokaryotic cell and lead to the formation of ancestral eukaryotic cells. This is explained through the endosymbiotic theory.

Fig: Chemoautotrophs

Endosymbiotic theory

Endosymbiotic theory or symbiogenesis is the evolutionary theory which explains the origin of eukaryotic cells from the prokaryotic organisms. This theory states that the cell organelles like mitochondria, chloroplasts and possibly other organelles are descended from the free living prokaryotes.

Chemoautotrophs can be aerobic or anaerobic. The aerobic chemoautotroph got engulfed by the prokaryotic cells, which has a nucleus formed by covering of infoldings of cell membrane around the nucleoid. The engulfed aerobic chemoautotroph was not digested. This turns into the cell organelle called mitochondria, which can generate energy. As a result the first eukaryotic cell was formed. Then the cyanobacterium is engulfed by the cell and turns into a photosynthetic organelle called chloroplast. Now the cell formed is composed of nucleus, mitochondria and chloroplast. This cell is the eukaryotic plant ancestor. From this cell, first algae came, from which all other land plants evolved slowly.

Fig: Evolution of first plant cell

General characteristics of algae

Algae are a highly heterogeneous group of thallophytes which exhibit high diversity in various aspects of their habitat, organisation and specialisations. Now we will discuss some of the general characteristics of algae.

Habitat

Algae are predominantly aquatic plants which can be found in both marine and freshwater habitats. Some algae occurs in moist surfaces of stones, soil, wood and even snow. The aquatic algae are called hydrophytes. Terrestrial algae are called saprophytes. There are some algae that reside with fungi to form lichens.

Lichens

The mutualistic or symbiotic relationship between algae or cyanobacteria with fungi forms another organism called lichen. Algae is living among filaments of fungi species. The function of algae is to prepare food, in return fungi provide protection to algae. The algal partner of the lichen is called phycobiont and the fungal partner is called mycobiont.

Fig: Lichen

Body of algae

The size and the shape of the algal plant body is very diverse.

Size of algae

The thallus of algae can range from microscopic, colonial, aggregates of cells, fine filaments to flattened sheets of cells. The smallest algae is the unicellular form called Chlamydomonas and the largest algae is Macrocystis or kelp, which can grow up to 60 metres in length.

Fig: Chlamydomonas GIF: Kelp

Morphology of algae

The small and simple forms of algae can be free living. But the large forms of algae have leaf-like blades called lamina with photosynthetic tissues. They also have stem-like stalks called stipe and a hold fast for attaching to the substratum.

Fig: Morphology of algae

Thallus organisation

Based on the features of their plant body algae can be classified into two. They are as follows:

• Unicellular algae
• Multicellular algae

Unicellular algae

The algae with single celled bodies are included under the category of unicellular algae. They are the simplest type of algae. Some unicellular algae can be motile, like Chlamydomonas and some are non motile like Chlorella. The motile algae use flagella for their movement.

Fig: Motile and non-motile unicellular algae

Multicellular algae

The algae with complex and multicellular thallus are called multicellular algae. They can be colonial, palmelloid, dendroid, filamentous, siphonous and parenchymatous. The filaments and sheets are attached to the substratum by means of hold fast which is the anchoring structure.

Colonial algae

In colonial algae the independent organisms or algal cells are loosely aggregated into colonies. Examples include Volvox.

Fig: Volvox

Palmelloid algae

In palmelloid algae the thallus is multicellular and the cells are surrounded by a mucilaginous matrix. This mucilage covering around the plant body, protects them from the epiphytic growth and decaying effect of the water. This mucilage can also prevent desiccation happens during the low tides. Examples include Tetraspora.

Fig: Tetraspora

Dendroid algae

In dendroid algae the multicellular thallus is branched and looks like a microscopic tree. Examples include Echallocystis.

Filamentous algae

The filamentous algae has a thallus may be attached or free floating and may be branched or unbranched. Examples include Polysiphonia, Ectocarpus etc.

Fig: Ectocarpus

Siphonous algae

In the siphonous algae the thallus will be aseptate, multinucleate and elongated without differentiating into cells. Examples include Vaucheria.

Tissues in the plant body

There are three types of cells found in algae. They are prokaryotic, eukaryotic and mesokaryotic. Mesokaryotic cells are those cells which come intermediate between the prokaryotic and eukaryotic cells.

The tissues of the multicellular algae are not complicated. They are made up of simple cells that have cellulose cell walls. Since the algae are usually submerged in the water, they do not have vascular tissues in their body. Conduction of water is not even needed in the giant forms like kelps.

Buoyancy keeps the algal body erect in the water. So mechanical tissues are absent in the water. They can move along with the sea tides, because of their flexible bodies.

GIF: Movement of algae (Kelps) in water

Pigments of algae

Algae exhibit three major colours, because of the presence of different types of photosynthetic pigments in them. The pigments are seen in plastids. In some cases plastids may contain pyrenoids and starch also. The major group of algal pigments are as follows:

• Chlorophylls
• Carotenoids
• Phycobilins

Chlorophylls

Chlorophyll a is present in all algal groups. Chlorophyll b is present in an algal group called Chlorophyceae, chlorophyll c is present in Phaeophyceae and Bacillariophyceae, and chlorophyll d is present in Rhodophyceae.

Carotenoids

Carotenoids are fat soluble, yellow, orange, brown or red accessory photosynthetic pigments. Three major groups of carotenoids are carotenes, xanthophylls and carotenoid acids. Fucoxanthin is a xanthophyll found in phaeophyceae.

Phycobilins

The water soluble and strongly fluorescent red or blue accessory photosynthetic pigments are called phycobilins. These are found in algal groups like Cyanophyceae and Rhodophyceae. The three main kinds of phycobilins are phycoerythrin, phycocyanins and allophycocyanins.

Nutrition

Algae belong to the phytoplankton group. They generally have autotrophic nutrition. But certain algae can also follow a heterotrophic and mixotrophic nutrition.

Photoautotrophic nutrition

Those algae that have chloroplasts in their cells are called photoautotrophs. They perform photosynthesis with the help of chlorophylls using light energy, water and carbon dioxide. In this way they synthesise their own food from inorganic substances. Examples include Volvox.

Fig: Volvox

Heterotrophic nutrition

In the heterotrophic mode of nutrition, they utilise exogenous organic nitrogen or carbon sources. Through this kind of nutrition they can also use nutrients in soil or water to synthesise essential oils, fats and proteins. Examples include Scenedesmus

Mixotrophic nutrition

In mixotrophic nutrition, the algae utilise both inorganic and exogenous organic sources to fulfil the nutritional needs. Examples include Chlamydomonas.

Fig: Chlamydomonas

Reserve food

Various types of polysaccharides, lipids and proteins are stored by algae. Different groups of algae store different types of food materials.

Fig: Nutrition in algae

Reproduction

The reproduction in algae can be of three types. They are as follows:

• Vegetative
• Asexual
• Sexual

Vegetative reproduction

Usually the vegetative reproduction occurs through the cell division and fragmentation of the thallus.

Cell division or binary fission

When the mother cell divides into two daughter cells, new plants are formed from them. This can be exclusively seen in Chlamydomonas and Pleurococcus.

Fragmentation

The algal body breaks into several parts or fragments and each fragment will develop into a new individual. This is called fragmentation and is usually seen in filamentous forms of algae like Ulothrix, Spirogyra etc.

Fig: Fragmentation in Spirogyra

Asexual reproduction

Only one parent is involved in the asexual reproduction. The protoplast of an algal cell divides into several protoplasts and then they will escape from the mother cell to develop into new plants. These are called asexual spores.

Fig: Asexual reproduction

Types of asexul spores

There are two types of asexual spores, mitospores and meiospores.

Mitospores

These spores are formed through mitosis. Mitospores have the same set of chromosomes as that of parent cells.

Meiospores

These are formed through meiosis. But meiospores will show genetic variations which helps in the species adaptation.

Common asexual spores

Zoospores, aplanospores, hypnospores etc., are some of the common examples of asexual spores.

Zoospores

Thin walled and flagellated, motile spores are called zoospores. They are formed during the favourable seasons. Examples include Chlamydomonas.

Fig: Zoospores

Aplanospores

Thin walled, uninucleate and non flagellated, non motile spores are called aplanospores. They are formed under unfavourable conditions. Examples include Spirogyra.

Fig: Aplanospore

Hypnospores

The thick walled and non flagellated spores are called hypnospores. Examples include Ulothrix and Vaucheria.

Sexual reproduction

The sex organs or gametangia are not protected with the covering of sterile cells or we can say they are non-jacketed. Sexual reproduction involves the fusion of gametes.

Fig: Sexual reproduction

Types of sexual reproduction

There are three major types of sexual reproduction. They are as follows:

• Isogamy
• Anisogamy
• Oogamy

Isogamy

The fusion of morphologically and physiologically similar types of gametes is called isogamy. The gametes can be motile or flagellated and non motile or non-flagellated, but similar in size. Such gametes are called isogametes. Examples include Spirogyra, Chlamydomonas.

Fig: Isogamy

Anisogamy

Fusion of two gametes that are dissimilar in size is called anisogamy. The gametes can be motile or non motile here. This is also known as heterogamy. Examples include Ectocarpus and Eudorina.

Fig: Anisogamy

Oogamy

Fusion of large, non-motile female gamete with small, motile male gamete is called oogamy. Examples include Volvox and Fucus.

Fig: Oogamy

Life cycle

Sexually reproducing algae have two phases in their life cycle generally. These are the haplontic phase and the diplontic phase. There will be alternation of generations between the haplontic and diplontic phases. The embryo stage is absent in the life cycle of algae. Most algal groups have a haplontic life cycle. But, in algae three types of life cycles can be observed generally. These are as follows:

• Haplontic life cycle
• Diplontic life cycle
• Haplo-diplontic life cycle

Haplontic life cycle

In haplontic life cycle gametophyte or the gamete bearing phase is haploid (n) and is the dominant stage. Here the gametes fused to form zygote and the zygote undergoes meiosis to form spores. Now the spores develop into the gametophyte. Sporophyte stage is absent here. Examples include Volvox and Spirogyra.

Fig: Schematic representation of the haplontic life cycle

Diplontic life cycle

In the diplontic life cycle the sporophyte or the spore bearing phase is diploid (2n) and dominant. Here the spores produced by meiosis undergoes gametogenesis to produce gametes. Then the fusion of gametes produces zygote which develops into the sporophyte. Examples include Fucus.

Fig: Schematic representation of the diplontic life cycle

Haplo-diplontic life cycle

The haplontic and diplontic phase are equally prominent in this type of life cycle. Here the sporophyte undergoes meiosis and produces spores. Spores develop into gametophytes. Gametophytes then produce gametes by mitosis. Fusion of gametes results in the formation of zygote which then develops into sporophyte. Examples include Ectocarpus, Polysiphonia and kelps.

Fig: Schematic representation of the haplo-diplontic life cycle

Fig: Haplo-diplontic life cycle in algae

Classification of algae

The algae are classified into three major classes on the basis of pigmentation, chemical nature of assimilatory products, type of flagellation etc. The classes of algae are as follows:

• Chlorophyceae
• Phaeophyceae
• Rhodophyceae

Chlorophyceae

Chlorophyceae are the most recent algae in evolutionary history and it is the ancestor of all green plants. The members of Chlorophyceae are commonly called green algae and include around 7000 living species.

GIF: Green algae

Salient features of Chlorophyceae

The salient features of class Chlorophyceae are as follows:

• These are freshwater or marine algae with a unicellular or multicellular body.
• Some green algae are terrestrial and they can grow anywhere.
• Cells are eukaryotic and contain cellulosic cell walls inside and pectose in the outer layer.
• They occur in various shapes and sizes.
• The chief pigments include chlorophyll a and b that are located in definite chloroplast. It possess 𝛼, 𝛽 and 𝛾 carotenes too.
• Thallus is usually green due the dominance of chlorophyll a and b.
• Reserve food materials include starch and very rarely oils. Starch is stored in pyrenoids.
• Vegetative reproduction occurs through fragmentation or by the formation of different types of spores.
• Asexual reproduction occurs through both mitospores and meiospores. The common asexual spores are zoospores, aplanospores, hypnospores, akinetes and autospores.
• Palmella stage is found in the asexual reproduction, during the desiccation.
• Sexual reproduction is isogamous, anisogamous or oogamous.
• Male gametes are flagellated and hence motile.
• Flagella are isokont or identical and acronematic (rod-like or whiplash flagella).
• The motile cell of primitive forms may contain eye-spot or stigma in the chloroplast.
• Life cycle is mostly haplontic. Diplontic and haplo-diplonticntic life cycles are also present.

Chloroplasts of Chlorophyceae

The plastids of green algae are called chloroplasts, because they contain chlorophylls a and b and less amount of carotenoids. The form and number of chloroplasts are different in different groups of green algae. Single chloroplast is seen in Volvox, but there are several in Caulerpa. The chloroplasts of green algae can be of the following shapes:

• Cup-shaped chloroplast is seen in Chlamydomonas.
• Girdle shaped chloroplast is seen in Pleurococcus.
• Reticulate chloroplast is seen in Oedogonium.
• Collar shaped chloroplast is seen in Ulothrix.
• Band like or ribbon shaped chloroplast is seen in Debarya.
• Spiral chloroplast is seen in Spirogyra.
• Stellate shaped chloroplast seen in Zygnema.

Fig: Diversity in chloroplast

Pyrenoid

The presence of starch grains and pyrenoids within chloroplasts is a salient feature of green algae. Pyrenoids are granular, proteinaceous bodies seen in association with starch grains. Pyrenoids are believed to be involved in starch synthesis.

Fig: Pyrenoid in Chlamydomonas

Examples of Chlorophyceae

Some common examples of the green algae are Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara, and Chlorella.

Fig: Common examples of green algae

Phaeophyceae

It includes the brown algae which is different from other algae because of their characteristic olive green, golden yellow or deep brown colour. There are about 2000 species of brown algae and are mostly marine. This colouration is mainly due to the presence of the xanthophyll called fucoxanthin. They can be observed in different sizes and forms.

Fig: Brown algae

Salient features of Phaeophyceae

The salient features of class phaeophyceae are as follows:

• These are marine algae with a unicellular or multicellular body.
• They possess well organised and highly differentiated thallus.
• Absence of unicellular, colonial and unbranched filamentous forms.
• They are mostly branched, filamentous or flat ribbon shaped.
• They possess eukaryotic cells.
• They have a special type of excretory granule present in vesicles.
• A cellulosic wall usually covers vegetative cells.
• They also possess a gelatinous coating of algin outside the cell wall.
• Thallus size ranges from microscopic epiphytes to huge forms like kelps. Kelps or Macrocystis ranges from 60m or more in length.

GIF: Macrocystis

• Thallus has a holdfast to attach to the substratum.
• They possess a stalk called stipe and frond (leaf-like photosynthetic organ).

Fig: Plant body brown algae

• The plant body is composed of air vesicles that provide buoyancy.
• Major photosynthetic pigments include chlorophyll a and b, beta carotene and xanthophylls like fucoxanthin and violaxanthin. Minor pigments include diatoxanthin and diadinoxanthin.
• In addition to plastids, protoplast contains centrally located vacuole and nucleus.
• Major reserve foods include laminarin and mannitol.
• In some cases, sucrose and glycerol may also be present.
• Trumpet hyphae is a food conducting tube in large brown algae. They carry food from the blade to the holdfast.
• Major cell wall constituents are cellulose, fucinic acid and alginic acid.
• The outer coat of the cell wall is made up of algin-fucoidan.

Fig: Cell of brown algae

• Generally they possess white fucosan vesicles in cells.
• Pyrenoids are either absent or are solitary and stalked.
• Vegetative reproduction takes place through fragmentation.
• Motile reproductive bodies, such as pear shaped zoospores and gametes possess two laterally inserted and heterokont flagella.
• In heterokont flagellation, there will be two flagella. One is whiplash type and the other is tinsel type.
• Male gametes are flagellate.
• Sporangia are unilocular or single chambered or can be multilocular.
• Sexual reproduction occurs through isogamy, anisogamy or oogamy.
• Gametes are fused in water or within oogonium.
• Life cycle involves alternation of generations.
• They mainly possess haplontic or diplontic life cycles.
• In some forms, similar structures can be seen in both the generations and it is called as isomorphic alternation of generation.

Examples of Phaeophyceae

The common examples of phaeophyceae include Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Fig: Common examples of brown algae

Rhodophyceae

It includes the red algae which are characterised by the distinctive red colouration. Red algae are the most beautifully colured members of sea weeds. 5000 species of red algae have been discovered. Out of these 50 species are freshwater forms and the rest are marine. There is an exception to this, which is the unicellular red algae Porphyrum. It is a terrestrial algae growing in moist soil. In general marine red algae prefer deeper waters where they receive only blue-violet light.

Fig: Red algae

Red algae are the most ancient algae and the oldest plant fossil is 1.6 billion years old. They were predominantly marine.

Fig: Fossil of the ancient red algae

Salient features of Rhodophyceae

• Thallus is unicellular and microscopic in some algae like Porphyrum. But in great majority forms it is multicellular and filamentous, parenchymatous or pseudoparenchymatous.
• Thallus is red-colured due to the presence of excessive amounts of the pigments phycoerythrin and phycocyanin.
• The main foods include floridian starch, floridosides and mannoglycerate.
• The major constituents of the cell wall are cellulose, pectic compounds and polysulphate esters.
• Total absence of a flagellated and motile stage in the life cycle.
• In most multicellular forms, vegetative cells are interconnected by pit connections, often called plasmodesmata.
• They usually reproduce through vegetative reproduction.
• Asexual reproduction occurs through non-motile spores.
• Sexual reproduction is highly specialsied and is an advanced type of oogamy.
• Spermatangia are the male sex organs, and carpogonia are the female organs.
• Male gametes are non-flagellated and non-motile spermatia.
• Post-fertilisation development is elaborate, highly specialised and different from that of other algae.
• Most members exhibit haplontic or diplontic life cycles.
• They show alternation of generations.

Fig: Ultrastructure of red algae

Examples of Rhodophyceae

The common examples of class Rhodophyceae are Polysiphonia, Porphyra, Gracilaria and Gelidium.

Fig: Examples of red algae

Differences between green, brown and red algae

The major differences between green, brown and red algae are as follows:

Economic importance of algae

Algae have always been directly or indirectly linked with the life of mankind and some animals as sources of food, fodder and manure. They are used in agriculture, horticulture, fisheries etc. At the same time some algae may be harmful to man and animals.

Positive impacts

Some of the economic importance of algae are as follows:

• Marine algae are commonly used as food, since they contain greater amounts of minerals and vitamins.
• The important algae consumed by human beings are as follows:

Chlorophyceae like Monostroma, Caulerpa and Ulva.

Phaeophyceae like Laminaria, Alaria and Ecklonia.

Rhodophyceae like Porphyra, Gracilaria, Gelidium and Eucheuma.

• Seaweeds can be used to make vermifuge, anaesthetics and ointments due to their vitamin and mineral contents.
• Laminaria prevents goitre and blood pressure.

Fig: Laminaria

• Sargassum (Hijiki) can deal with inflammation and fever.

Fig: Sargassum

• Hypnea (red algae) is used to cure stomach troubles.
• Seaweed meal increases butter fat content of milk of feeding cattle.
• Stock feed and commercial feed are regularly processed for many cattle and sheep from the species of Laminaria, Ascophyllum and Fucus.
• Chara is used as a green manure. It is used to overcome the deficiency of calcium in the field.

Fig: Chara

• Excessive growth of Cladophora in water indicates pollution due to accumulation of heavy metals.
• Nori are the sheets that are used to cover sushi. It is made from red algae and is a rich source of calcium, magnesium and many important antioxidant compounds.

Fig: Nori

• Microalgae can be used for the treatment of waste water by virtue of their capacity to assimilate nutrients. The process is called bioremediation. Examples include Chlorella, Porphyra, Chlamydomonas.

Fig: Porphyra

• Various commercial products are obtained from algae. Examples include agar, Carrageenin and algin.
• Agar is a gelatinous mixture used as a base for different culture media. It is also used as the thickening agent in the soup and sauces. It is obtained from Gelidium, Gracilaria, Hypnea etc.

Fig: Agar from algae

• Alginates obtained from Laminaria, Fucus etc., are used in the preparation of insecticides. It is also used as a gelling and stabilising agent in the pharmaceutical industry.
• Carrageenin is used as a thickening agent in the food industry. It is obtained from Irish moss or Chondrus.

Fig: Chondrus

• Dried and burnt ash of brown algae is a good fertiliser.
• The main source of algal biofertilizer is Azolla.
• Casting of body parts can be done from the loose paste of alginate.
• Algae are the primary producers of aquatic environments.
• Chlorella is used in space research. It has been found very suitable for keeping the air in space vehicles pure.

Fig: Chlorella

Negative impacts

Some of the negative impacts of algae are as follows

• Algal blooms will reduce the efficiency of fish and other aquatic forms to find food.

Fig: Algal bloom

• Algal blooms will cause entire populations of aquatic forms to leave an area or die.
• Algal blooms can cause thick, green muck which will have bad impacts on clear water, businesses, property values and recreation.
• It may create toxins that are detrimental to fish and other animals.
• Algal toxins consumed by shellfish and small fish may move up the food chain and will have a bad impact on larger animals such as turtles, sea lions, birds and dolphins.
• Algal blooms will consume more oxygen and as they die and decompose more, will result in hypoxia.

Practice Problems

1. Which of the following statements is/ are correct about the endosymbiotic theory?

1. Explains the origin of eukaryotic cells from the prokaryotic organisms.
2. The aerobic chemoautotroph gets engulfed by the prokaryotic cells and mitochondria is formed.
3. Cyanobacterium is engulfed by the cell and turns into a photosynthetic organelle called chloroplast.
4. Algae is formed from the first eukaryotic ancestral cell.

1. A, B, C, D
2. A, B, C
3. A and D
4. B and C

Solution: Endosymbiotic theory or symbiogenesis is the evolutionary theory which explains the origin of eukaryotic cells from the prokaryotic organisms. This theory states that the cell organelles like mitochondria and chloroplasts and possibly other organelles are descended from the free living prokaryotes. Chemoautotrophs can be aerobic or anaerobic. The aerobic chemoautotroph got engulfed by the prokaryotic cells, which has a nucleus formed by covering of infoldings of cell membrane around the nucleoid. The engulfed cell was not digested. This turns into the cell organelle called mitochondria, which can generate energy. As a result the first eukaryotic cell was formed. Then the cyanobacterium is engulfed by the cell and turns into a photosynthetic organelle called chloroplast. Now the cell formed is composed of nucleus, mitochondria and chloroplast. This cell is the eukaryotic plant ancestor. From this cell, first came the algae, from which all other land plants evolved. Hence the correct option is a.

Fig: Evolution of first plant cell

2. Assertion: The symbiotic relationship between algae and fungi is called lichen.

Reason: Algae provide food to fungi and fungi give protection to algae.

Which of the following statements is correct regarding the assertion and reason given above?

1. Both the assertion and the reason are correct and reason is the correct explanation of the assertion.
2. Both the assertion and the reason are correct and the reason is not the correct explanation of the assertion.
3. The assertion is correct, but the reason is false.
4. Both the assertion and the reason are false.

Solution: The chlorophyll containing, photosynthetic, non-vascular, thalloid and aquatic plants are called algae. There are some algae that reside with fungi to form lichens. Hence, the mutualistic or symbiotic relationship between algae or cyanobacteria with fungi forms another organism called lichen. Algae is living among filaments of fungi species in lichen. The function of algae is to prepare food and in return the fungi provide protection to algae. The algal partner of the lichen is called phycobiont and the fungal partner is called mycobiont. Here the assertion and the reason are correct and the reason is the correct explanation of the assertion. Hence the correct option is a.

3. Match the following class of algae (column I) with their examples (column II):

1. A - II, B - III, C - I
2. A - III, B - II, C - I
3. A - I, B - III, C - II
4. A - III, B - I, C - II

Solution: Chlorophyceae are the most recent algae in evolutionary history and it is the ancestor of all green plants. The members of Chlorophyceae are commonly called green algae and include around 7000 living species. Some common examples of green algae are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara. Phaeophyceae includes the brown algae which is different from other algae through their characteristic olive green, golden yellow or deep brown colour. This colouration is mainly due to the presence of the xanthophyll called fucoxanthin. They can be observed in different sizes and forms. The common examples of phaeophyceae are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus. Rhodophyceae includes the red algae which are characterised by the distinctive red colouration. Red algae are the most beautifully colured members of sea weeds. 5000 species of red algae have been found. Out of these, 50 species are freshwater forms and the rest are marine. There is an exception to this, which is the unicellular red algae Porphyrum. It is a terrestrial algae growing in moist soil. In general marine red algae prefer deeper waters where they receive only blue-violet light. The common examples of class Rhodophyceae are Polysiphonia, Porphyra, Gracilaria and Gelidium. Hence the correct option is d.

4. Which is the type of life cycle seen in algae?

1. Haplontic
2. Diplontic
3. Haplodiplontic
4. All of the above

Solution: Sexually reproducing algae have two phases in their life cycle. They are the haploid phase and diploid phase. There will be alternation of generations between the haploid and diploid phases. The embryo stage is absent in the life cycle of algae. Most algal groups have a haplontic life cycle. But, in algae three types of life cycles can be observed. They are the haplontic life cycle, diplontic life cycle and haplo-diplontic life cycle. Haploid phase (n) is the gamete bearing, dominant stage in a haplontic life cycle. Examples include Volvox, Spirogyra. Diploid (2n) phase is spore bearing and dominant in the diplontic life cycle. Examples include Fucus. The haploid (gametophyte) and diploid (sporophyte) phases are equally prominent in the haplo-diplontic type of life cycle. Examples include Ectocarpus, Polysiphonia and kelps. Hence the correct option is d.

FAQs

1. What are the differences between algae and fungi?

Answer: Both algae and fungi occur in aquatic and semi-aquatic habitats and they have a thallus-like body. But there are many differences between the algae and fungi, some of them are as follows:

2. What is nori?

Answer: The dried edible seaweed is called nori. It is made from red algae. Pyropia is called the nori weed. It is rich in calcium, magnesium and antioxidants. Nori has a distinctive and strong flavour. It is used in the making of sushi, as a wrap. The algae undergoes the process of shredding and rack-drying to become the nori. Nori sheets can easily absorb water from the air and degrade.

Fig: Nori

3. What are algal turfs?

Answer: The thick, carpet like beds of seaweed are called algal turfs. It can retain sediments and have competition with corals and kelps which are the foundation species. Algal kelps are usually less than 15 cm tall. It consists of one or more than one species and covers an area of one square metre or more. Monospecific turfs are also found.

4. What is the relationship between sea sponges and algae?

Answer: Algae can live with sea sponges in a symbiotic relationship with each other. The endosymbiotic green algae lives close to the surface of the sponges like breadcrumb sponges. This symbiotic relationship protects algae from predators and algae provide oxygen and sugar to the sponges. Examples of green algae form such a relationship is Zoochlorella.

YOUTUBE LINK: https://www.youtube.com/watch?v=b2szozixtyc

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