Algae

🌊 Algae: General Introduction and Importance

Algae are a diverse group of photosynthetic organisms that play a crucial role in aquatic ecosystems. They are characterized by their simple structure, often lacking the complex tissues found in higher plants.

Class Activities for Students 🧑‍🎓

Students can engage in various activities to understand algae better:

Microscopic Examination: Observe live algal slides under a microscope to study their cells, chloroplasts, and reproductive organs. Students can draw and label the observed structures.
Research and Reporting: Investigate and report on various products derived from algae, such as food supplements (spirulina, chlorella), industrial products, and pharmaceuticals. They can present their findings through displays or presentations.
Creative Expression: Students can use different art forms or digital media to express their appreciation for the beauty and significance of algae in nature.
Collection and Analysis: Collect algal samples from various aquatic habitats (freshwater, marine, terrestrial) and compare their diversity and abundance. This activity helps students understand the factors influencing algal distribution.

Characteristics of Kingdom Plantae 🌿

While algae are diverse, some general characteristics apply:

They are photosynthetic, multicellular, eukaryotes.
Their bodies are less regular in shape.
They have photoautotrophic nutrition.
They grow throughout their life.
They are either fixed or free-floating.
They do not show locomotion.
The cell is surrounded by a cell wall and contains a central vacuole and plastids. Centrioles are absent.
Food reserve is in the form of starch with fat.
They show slow response towards external stimuli.
Reproduction occurs through both sexual and asexual methods.

Important Features of Algae ✨

Here are some key characteristics of algae:

They are photosynthetic, non-vascular thalloid aquatic plants.
Their body is filamentous or thalloid.
Vascular and mechanical tissues are absent.
They are usually covered with mucilage which protects them from desiccation and decay.
Most reproduce asexually by means of spores. The spores are motile or non-motile.
Sex organs (gametangia) are unicellular and non-jacketed.
Their cell wall is made of cellulose and pectin.
Storage food material is starch.
Sexual reproduction takes place by the fusion of gametes. It may be isogamous or anisogamous or oogamous.
Embryo stage is absent in their life cycle because zygote divides by meiosis (zygotic meiosis).
Carotene and chlorophyll are common in all algae.

Classification of Photosynthetic Pigments, Storage Material, and Complexity

Based on these characteristics, photosynthetic pigments, storage material, and complexity of their structure, algae are classified into red, brown, and green algae.

1. Red Algae (Rhodophyceae) ❤

Includes about 3000 species.
Most are marine but some (e.g., Batrachospermum) are freshwater.
Motile or flagellate stage is absent.
Plant body varies from unicellular, filamentous, parenchymatous to ribbon-like.
Photosynthetic pigments include chlorophyll-a, carotenoids, chlorophyll-d and phycobilins. Phycobilins are of two types: phycoerythrin (red coloured) and phycocyanin and allophycocyanin (blue coloured).
Reserve food is floridean starch, similar to glycogen.
Vegetative reproduction occurs through monospores, tetraspores, carpospores and gemmae.
Sexual reproduction is oogamous. Male sex organ is called antheridium and produces spermatia (male gametes). Female sex organ is carpogonium and possesses an elongate organ called trichogyne.
They show alternation of generation. Examples include Porphyridium, Spermothamnion, Porphyra, Batrachospermum.

2. Brown Algae (Phaeophyceae) 🤎

Includes approximately 2000 species.
Most of them are marine.
Body consists of a branched filamentous structure in lower forms and parenchymatous structure in higher forms.
The plant body is differentiated into hold fast, stipe and lamina.
Photosynthetic pigments include chlorophyll-a, carotenoids (carotenes and xanthophylls) and chlorophyll-c.
Brown colour is due to the presence of large amounts of xanthophylls called fucoxanthin.
Food reserve is laminarin (carbohydrate) and mannitol.
Conduction tubes are present in large brown algae.
Sexual reproduction takes place by motile and non-motile spores.
Sexual reproduction is isogamous, anisogamous or oogamous. Alternation of generations is found in most brown algae. Examples include Alaria, Fucus, Laminaria, Sargassum, Nereocystis, Macrocystis.

3. Green Algae (Chlorophyceae) 💚

This group comprises about 7000 species.
Majority are freshwater; some grow on moist soil, walls, and rocks in ocean.
Filamentous or heterotrichous or parenchymatous thallus.
Cell wall contains cellulose.
Photosynthetic pigments are chlorophyll-a, chlorophyll-b, carotenes and xanthophylls.
Chloroplast generally contains starch as storage of food which is stored on protein body called pyrenoid.
Reproduction takes place by vegetative, asexually and sexually.

2.4.1 Spirogyra 💧

Spirogyra is a common genus of freshwater green algae.

Systematic Position 📊

Kingdom	Plantae
Division	Thallophyta
Class	Chlorophyceae
Order	Conjugales
Family	Zygnemaceae
Genus	Spirogyra

Occurrence 🏞

Spirogyra grows as free-floating, extensive dirty mass and is often called "pond scum." It is frequently found in freshwater stagnant reservoirs, tanks, ditches, rivers, and slow-running streams and rivers. It looks like silky green filaments and is also called "water silk." The body is covered by mucilage, giving it a slimy feel. The plant grows throughout the year but prefers to grow during cold seasons. This genus contains about 300 species all over the world.

Morphology 🔬

The plant body is a slender, long unbranched filament. It can reach several meters long and 1/22 mm in diameter. Each filament is made up of thousands of rectangular cells arranged end to end. The young filaments are attached to the substratum by a modified basal cell called hapetron or holdfast. The adult plants are free-floating. All cells of a filament are capable of division for growth.

Structure of Cell 🧬

All cells of a filament are similar. The cell wall is two-layered: an inner layer of cellulose and an outer layer of pectin substances. The pectin substance gelatinizes in the presence of water, giving the characteristic slimy feel to the plant. The nucleus is usually located at the center of the cell, which is suspended by strands of cytoplasm. The cytoplasm is peripheral due to the presence of a large central vacuole. The central vacuole is traversed by several cytoplasmic strands. The cytoplasm consists of spirally arranged ribbon-shaped chloroplasts. The name of the alga is derived from these spiral chloroplasts. Each chloroplast contains numerous small protein bodies called pyrenoids, which store starch. The number of chloroplasts in a cell is variable (1 to 16).

Reproduction 🌱

Spirogyra reproduces by vegetative, asexual and sexual methods.

1. Vegetative Reproduction

Vegetative reproduction takes place by fragmentation. Accidental breaking or injury breaks the filaments into small fragments. Fragmentation also naturally occurs due to the dissolution of middle lamella.

2. Asexual Reproduction

Asexual reproduction is very rare in Spirogyra. It is found in a few species. Asexual reproduction occurs by the following method:

Akinetes: Akinetes are resting spores formed under unfavorable conditions. Akinetes have been observed in Spirogyra farlowii. The spores are formed from individual cells by the formation of an additional wall around them. Akinete is thick-walled and can survive unfavorable conditions. During favorable conditions, they germinate and produce new filaments.
Aplanospores: Aplanospores are thin-walled resting spores which arise singly inside the cells. The protoplast of a cell loses water and contracts. It rounds off and secretes a thin wall around it to become an aplanospore, which can then germinate into a new filament.
Parthenospores (Azygospores): Under failure of normal gametic union, parthenospores are formed. It is reported in S. varians, S. daedalea, S. rhizoides and S. mirabilis. Each parthenospore gives a new filament.

3. Sexual Reproduction

Sexual reproduction is isogamous type, which occurs by conjugation of non-flagellated gametes. Conjugation is the fusion between gametangia. Conjugation takes place by two methods:

a) Scalariform Conjugation 🪜

This type of conjugation takes place between the cells of two different filaments (i.e., scalariform conjugation takes place in heterothallic or dioecious species such as S. tuxcensis, S. cylindrica).

In this process, two filaments come together and lie side by side, parallel to one another throughout their length. Both filaments become intimately glued to one another by mucilage. Two opposite cells of different filaments function as gametangia. Two gametangia develop short lateral outgrowth called protuberance or papillae. The papillae grow in size and come in contact with each other through the anterior end. The common wall between the papillae gets dissolved by an enzyme called cytase. A common tube-like passage is formed, which is called the conjugation tube. The cells of one filament function as male gametangium and those of the other as female gametangium. After the formation of the conjugation tube, their protoplast directly functions as gametes. A male gamete is formed from a male gametangium. The protoplast of a male gamete moves towards the female gamete through the conjugation tube. Protoplasts of two gametes fuse to form a zygote. The zygote secretes a wall around itself and is called a zygospore.

b) Lateral Conjugation ↔

Lateral conjugation takes place in homothallic or monoecious species. Two adjacent cells function as gametangia. It is of two types: indirect and direct.

Indirect Lateral Conjugation: In this conjugation, two adjacent cells of the same filament start to function as male and female gametangia. The longitudinal wall on the other side of septum produces papillae. The papillae grow in size. The common wall between them dissolves and a lateral conjugation tube is formed. The contents of one cell which functions as male gametangium becomes a male gamete and moves towards the female gametangium, which behaves as a female gamete, through the conjugation tube. The protoplasts of both gametes fuse to form a diploid zygospore. This type of conjugation takes place in Spirogyra affinis, S. tennuis, S. simple. Ultimately, the empty and the zygospore containing cells are present alternately in the filament.
Direct Lateral Conjugation: Direct lateral conjugation takes place in S. jogensis. The two cells situated immediately next to the basal cell (hapteron) show direct lateral conjugation. The cell immediately next to the hapteroid cell becomes slightly swollen and functions as a female gametangium, whereas the upper cell functions as a male gametangium. The contents of the male gametangium become somewhat conical, elongated, or rod-shaped and pierces the septum. A pore is formed in the septum through which the male gamete passes and fuses with the female gamete to form a zygospore.

Germination of Zygospore ♻

The zygospore is a thick-walled diploid resting spore. The zygospore is liberated by destruction and decay of the wall of the female cell. The diploid nucleus of the zygospore divides meiotically into 4 haploid nuclei. Out of these, 3 haploid nuclei degenerate, and only 1 survives. The zygospore having a single haploid nucleus germinates by forming a germ tube. The germinating tube divides to form two cells: a lower hyaline rhizoidal cell and an upper green divides which repeatedly divides and forms a new filament of Spirogyra.

Parthenogenesis 🔄

Sometimes the gametes fail to conjugate due to certain physiological changes or changes in the environment. Such gametes develop thick walls and are converted into parthenospores or azygospores. On the return of favorable conditions, they directly give rise to new filaments without undergoing meiosis.

Economic Importance of Algae 💰

Algae are a primitive group of plants. These algae have many valuable uses in food, medicine, laboratory products, and other areas. Some key importances are given below:

Chlorella, Spirogyra, Ulva lactuca and Oedogonium are widely used in different purposes.
Chlorella is rich in protein (after Spirulina) so it is used as source of food.
Chlorella is used in space travel as source of food, oxygen, disposal of CO_2 and sewage treatment.
An antibiotic substance is produced from Chlorella.
Laminaria, a brown algae, is a source of iodine and algin.