Algae Characteristics and Classification

Algae are non-vascular, primitive plants having no true roots. they produce spores instead of seeds and have mostly both sexual and asexual reproduction. while fungi are eukaryotic heterotrophs that grow hyphae threads to form a mesh network mycelium. they are not plants.

Both agae and Fungi have many charachteristics and are divided into various divisions, classes and orders which we will dicuss later in this articel.

So, Keep reading…!

Characteristics of Algae

Algae show following characters:

  1. Algae (singular alga) are photosynthetic protists. They carry out probably 50% to 60% of all the photosynthesis on the earth.
  2. The sex organs in algae are unicellular the zygote is not protected by parent body. On the other hand, the sex organs of the plants are multicellular and zygote grows to form multicellular embryo. This embryo is protected by parental tissue. –
  3. The algae show a wide range of forms:
  4. Some are unicellular.
  5. The others are filamentous. The filaments are composed of distinct cells or coenocytes. The coenocytes are multinucleate structure and it lacks cross-walls.
  6. Some forms thallus. They are multicellular. They are highly branched and arranged in leaf like extensions, e.g. sea weeds. A body that cannot be differentiated into true roots, stems, and leaves and lacks xylem and phloem is called thallus.
  7. All algae contain photosynthetic pigments, green chlorophyll a, yellow and orange carotenoids. In addition to these pigments, some alga phyla also contain other pigments like xanthophylls and phycoerythrin. They are also important pigment for photosynthesis. The algae are classified on the basis of their pigment composition.
  8. Algae show different types of life cycles. All the algae except Rhodophyta (red algae) develop motile flagellated cells during some stage of their life cycles.
  9. Almost all algae are aquatic. They are present in damp and wet environment during their active growth. They live in ocean, freshwater, ponds, lakes, streams, hot springs, polar ice, moist soil, trees and rocks.

Classification of Algaeclassification of algae (table)

1.      The EuglenoidsEuglenoids

  1. Earlier, Euglenoids were classified with plant kingdom (in algae) and animal kingdom (protozoa). Molecular data shows that Euglenoids are closely related with Zooflagellates. .
  2. They are unicellular.
  3. They have two flagella. One flagellum is long and the other is short.
  4. They are plant like in their pigment. Their pigments are chlorophyll a, chlorophyll b and carotenoids. However, some photosynthetic Euglenoids lose their chlorophyll when they are grown in darkness. They become heterotrophic and ingest the organic matter. Other species of Euglenoids are always colorless and heterotrophic.
  5. Example: Euglena. Euglena has special evolutionary significance. They resemble plants and green algae, as they have similar pigments. They also resemble Zooflagellates (protozoan). It is believed that Euglena is ancestor of both animal and plants.

2.      DinoflagellatesDinoflagellates

  1. They are most unusual protists as they show different structures.
  2. Most dinoflagellates are unicellular.
  3. Their cells are often covered with shells. This shell is composed of interlocking cellulose plates. Some silicates are also impregnated in these plates.
  4. They have two flagella.
  5. Their pigments are Chlorophyll a, Chlorophyll c, Carotenes and Fucoxanthin.
  6. Ecologically, they are one of the important groups of producers (plants) in marine ecosystem.
  7. Dinflagellates show occasional population explosion or bloom. These blooms give orange, red or brown colour to water. So they are also known as red tides.
  8. Example: Gonyaulax, Ceratium.

3.      DiatomsDiatoms

  1. They are usually unicellular.
  2. The do not have any locomotory organs.
  3. Their pigments are Chlorophyll a, Chlorophyll c, Carotenes and Fucoxanthin.
  4. The cell wall of each diatom consists of two shells. These shells overlap and fit together. They look like petri dish. Silica, a glass like material, is also deposited in the shell. This glass like material is arranged in an intricate pattern.
  5. A large number of diatoms are present in aquatic ecosystems. So they are major producer in the aquatic (marine and freshwater) ecosystems. Diatoms are very important in aquatic food chains.
  6. Examples: Diatom, Frequilaria, Pinnularia.

4.      Brown AlgaeBrown Algae

  1. Brown algae include giants of the kingdom protista as they have large size.
  2. All brown algae are multicellular.
  3. Their size is from a few centimeters to 75 meters in length. The largest brown algae are called kelps. The kelps are tough and leathery in appearance. They possess leaf like blades, stem like stipes and root like holdfast.
  4. They are non-motile but their reproductive cells have two flagella.
  5. Their pigments are Chlorophyll a, Chlorophyll c, Carotenes and Fucoxanthin.
  6. Brown algae are common in cooler marine water, especially long coastline in the intertidal zone.
  7. Examples: Focus, Macrocytsis. The largest brown algae are called kelps.


5.      Red AlgaeRed Algae

  1. They are multicellular.
  2. Their body is composed of complex interwoven filaments. These filaments are delicate and feathery. A few red algae form flattened sheets of cells.
  3. They are non-motile. Most of multicellular algae are attached to rocks or other substances by a basal holdfast.
  4. Their pigments are chlorophyll a, Carotenes and Phycoerthrin (red pigment).
  5. Some red algae incorporate calcium carbonate in their cell wall from the oceans. They take part in the building of coral reefs along with coral animals.
  6. Examples: Chondrus, Polysiphonia.

6.      Green AlgaeGreen Algae

  1. They are unicellular, colonial or multicellular.
  2. The pigments of green algae are chlorophyll a, chlorophyll b and carotenoids. These pigments are present in the chloroplasts.
  3. Most of the green algae have flagella.
  4. The cell walls of green algae have cellulose.
  5. Green algae have many characters common to plant. They have similar pigments, reserve products, and cell walls. Due to these similarities with plants, it is generally believed that plant have evolved from ancestral green algae. The RNA sequencing of plants and green algae also supports this idea. It indicates that the green algae and plants form a monophyletic lineage.
  6. Examples Chlamydomonas (unicellular algae), Desmids, Volvox (colonial algae), Spirogyra (filamentous algae) Ulva (sheet like multicellular body) and Chlorella.


It is a unicellular non-motile unicellular alga. Its habitat is fresh water, pond and ditches. It can be easily cultured and used as experimental organism. Chlorella is used for research on photosynthesis. It is being investigated as alternate source of food.


It has single celled body, but it may grow in the form of groups. The cells are small and spherical. Each cell is covered by a thin cell wall, composed of cellulose. In the cell cytoplasm is present. In the center of cell nucleus is present. Each cell also contains a Cup-Shaped Chloroplast. In the chloroplast Pyrenoid is present or sometimes absent. Pyrenoid is a starch-forming body. In Chlorella flagella are absent i.e. it is Non-motile.


Chlorella has only asexual reproduction. It takes place by aplanospores. At the time of reproduction the cytoplasm of cell divides into 8 or 16 small bodies. Each body is covered by a wall, it is called aplanospore. In this ay 8 or 16 aplanospore are formed in a cell. After maturation of aplanospores the cell wall ruptures and all aplanospore become free. Each aplanospore can develop into a new cell.

Economic Importance of Chlorella:

  1. Chlorella cells have very high food value. They contain about 50% proteins, 20% carbohydrates and 20% fats, amino acids and vitamins, so it can be an alternate source of food.
  2. From chlorella an important antibiotic chlorellin has been isolated. It is useful to control bacterial diseases.
  3. Chlorella is used in important physiological experiments, because it is easily cultivated. It is used in the research on Photosynthesis and respiration.

Importance of Algae

Algae have great economic and environmental importance:

  1. Some algae like kelps are edible and may be used to overcome shortage of food in the world.
  2. Marine algae ¡s also a source of many useful substances like algin, agar, carrageenan and antiseptics
  3. Algae are major products of the aquatic ecosystem. Thus they play a basic role in food chains. They provide food and oxygen to other organisms.

Fungus-like Protists

Resemblances with fungi

Some protists superficially resemble fungi due to following reasons:Fungus-like Protists

  • They are not photosynthetic like fungi.
  • Their bodies are formed of thread like structure called hyphae.

Differences with fungi

However, these protists are not fungi due to following reasons:

  • Many of these protists have centrioles, while fungi lack centrioles.
  • They produce cellulose as major component of cell walls. While the cell wall of fungi is composed of chitin. Due to these differences, these protists are not placed in fungi and are placed in kingdom protista. There are two major groups of fungus- like protists. These are: Slime molds (Myxomycota) and water molds (Oomycotes).

A.     Slime molds (Myxomycota)

There are two forms of slime mold:

Slime molds (Myxomycota)1. Feeding stage: Plasmodium

The feeding stage of a slime mold is called plasmodium. Plasmodium is a multinucleate naked mass of cytoplasm (without cell membrane). It can grow to 30cm to 1ft in diameter. The plasmodium is slimy in appearance. It moves over damp, decaying logs and leaf litter. It often forms network of channels. These channels cover a large area. It ingests bacteria, yeast, spores and decaying organic matter during its creeping movements.

2. Reproductive stage:

Slime mold forms stalked sporangia during unfavorable conditions. Resistant haploid spores are produced within these sporangia by meiosis. When condition becomes favorable, these spores germinate to form biflagellated amoeboid reproductive cells or swarm cells. These swarm cells unite to form diploid zygote. Zygote produces multinticleate plasmodium. Each nucleus of plasmodium is diploid.

Example: The plasmodial slime mold Physarum polycephalum is a model organism. It is used to study many fundamental biological processes like growth, differentiation, cytoplasmic streaming movements and the function of cytoskeleton.

B.     Water mold or OomycotesWater mold or Oomycotes

Oomycotes show close relationship with fungi. They have similar structure like fungi. But they are now taken as more ancient group. They show following characters:

  1. Their cell wall contains cellulose, not chitin.
  2. Their hyphae are aseptate (without cross walls).
  3. There are many pathogenic oomycotes like Phytophthora infestans.

Phytophthora infestans

It has played an infamous role in the history of-man. It attacks on potatoes and caused a disease called late blight of potatoes Phytophthora caused Irish (Ireland) potato famine in 19th century. There were several cool summers in Ireland in 1840.

This mold multiplied unchecked during these suitable conditions. It caused rotting (decay) of the potatoes. Potatoes were the staple of the Irish people. Many people about 250,000 to more than 1 million died due to starvation. People started migrating to other countries like the United States of America (USA) due to this famine.

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