[Biology] Algae

Open Mindmap For Revision


  • Algae comprise a diverse group of non-flowering, photosynthetic and oxygen-producing plants that lack typical plant structures i.e. roots, stem, leaves.
  • In terms of size they range from microscopic single cells to gigantic seaweeds.
  • The study of algae is named as phycology


  • Algae are eukaryotic organisms that contain a true nucleus and membrane bound organelles.
  • algae lack typical plant structures such as roots, stems, or leaves
  • However, they contain chlorophyll and possess the ability to carry out photosynthesis.
  • Algae can be unicellular or multicellular.
  • Unicellular algae are found most often in aquatic habitats such as plankton.
  • In addition, algae can occur in moist soil or on the surface of moist rocks and wood.
  • They have diverse interactions with other organisms, including symbiosis and parasitism. The most profound relation is that of lichens.
  • Algae possess a symbiotic relation with fungi in the form of lichens.
  • Some species of algae have flagella with the “9+2” pattern of microtubules.
  • A well-defined, eukaryotic nucleus is present in algal cells and multiple chromosomes are observed in mitotic phase of algal species.
  • Most algae are photoautotrophic in nature and carry out photosynthesis. The chlorophyll (photosynthetic pigment) is present in thylakoids of chloroplast.
  • Some forms, however, are chemoheterotrophic and acquire energy from chemical reactions and nutrients from pre-formed organic matter.
  • Algae can be found nearly everywhere on earth: oceans, rivers, lakes, in the snow of mountaintops, on forest and desert soils, on rocks or even on other algae.


Some salient algal groups are discussed as under


  • Red algae are also known as Rhodophyta.
  • Rhodophyta are red because of the pigment phycoerythrin, which reflects red light and absorbs blue light.
  • They are commonly found in tropical marine locations.
  • Red algae inhabit solid surfaces including tropical reefs or attached to other algae.
  • Their cell walls consist of cellulose and many different forms of carbohydrates.
  • These algae reproduce asexually through monospores that are carried by water currents for germination.
  • Red algae also undergo reproduction by sexual means and undergo alternation of generations.
  • They form a number of different seaweed types.


  • Palmaria
  • Delesseria
  • Chondrus


  • Green algae are also termed as Chlorophyta.
  • They consist of the green pigment chlorophyll within the chloroplast and are able to carry out photosynthesis.
  • Their habitat ranges from the ocean to freshwater and sometimes to land.
  • They can be hostile, with some species fouling beaches.
  • Green algae are food for humans and sea animals.


Blue-green Algae (NOT AN ALGAE)

  • Cyanobacteria or blue-green algae is a phylum of bacteria that acquire food through photosynthesis
  • Cyanobacteria are not algae. They are named as blue-green algae due to their photosynthetic and aquatic nature.
  • They resemble the eukaryotic algae in many ways, including morphological characteristics and ecological niches.
  • Due to such resemblances they were at one time misunderstood as algae, hence the name blue-green algae
  • They are found in almost all habitats of ocean from fresh water, stone of deep purple, sea marsh, to the ground.


  • Nostoc
  • Oscillatoria
  • Spirulina


  • Photo autotrophy is the principal mean of nutrition in algae.
  • They are termed as “self-feeders” as they use light energy and a photosynthetic apparatus to produce their own food from carbon dioxide and water.
  • However, algae also groups comprising of heterotrophic species, which acquire their organic food molecules by consuming other organisms.
  • Numerous algae are mixotrophs as they use different modes of nutrition (such as autotrophy and heterotrophy), depending on the availability of resources.
  • According to their nutritional strategies, algae are into classified four groups:
    1. Obligate heterotrophic algae
    2. Obligate phototrophic algae
    3. Facultative mixotrophic algae
    4. Obligate mixotrophic algae


They are mostly heterotrophic, but are capable of sustaining themselves by phototrophy


  • Gymnodium gracilentum
  • Dinophyta


Their primary mode of nutrition is phototrophy, but they can increase growth by phagotrophy or osmotrophy when light is low


  • Dinobryon divergens
  • Heterokontophyta


They can grow as phototrophs as well as heterotrophs


  • Fragilidium subglobosum
  • Dinophyta.


Their primary mode of nutrition is also phototrophy, but phagotrophy and/or osmotrophy can also provide essential substances for growth.


  • Euglena gracilis
  • Euglenophyta


  • Many algae are capable of movement.
  • Movement is accomplished by means of flagellar action and by extrusion of mucilage.
  • There are also peristaltic and amoeba-like algal movements.
  • Within algal cells, movement of the cytoplasm, plastids, and nucleus also occurs.


  • Algae may reproduce either asexually or sexually.
  • Asexual reproduction in algae includes formation of unicellular spores that germinate without fusing with other cells, fragmentation of filamentous forms and cell division by splitting.
  • In sexual reproduction, parent organisms produce gametes, which then fuse to form a zygote.
  • Zygotes may either transform into new filaments or produce haploid spores by meiotic division.
  • Algae exhibit different types of life cycles. Some algal life cycles are characterized by an alteration of generations similar to that of plants.
  • Two phases occur in algal reproduction
  1. Sporophyte (usually diploid)
  2. Gametophyte (usually haploid).
  • The sporophyte form produces diploid spores by meiosis
  • These spores divide mitotically to form haploid gamete-producing bodies called gametophytes
  • Gametophyte and sporophyte may be structurally identical or dissimilar, depending on the algal group.


  • Spirogyra is unicellular green algae that grows in long, filamentous colonies and appear to be a multicellular organism.
  • Even though it is technically unicellular but its colonial nature classifies its life cycle as haplontic (life cycle in which the main form is haploid and the diploid zygote is formed only briefly).
  • In the haploid vegetative cells of the colony, the chloroplasts are organized in spirals, containing darkened regions called pyrenoids where carbon fixation happens.
  • Each haploid cell in the filament is an individual, which carries out sexual reproduction between colonies.
  • When two colonies of complementary mating type of Spirogyra meet, sexual reproduction occurs.
  • conjugation tube extends from each cell in one colony into the cells of the other colony and induces the formation of a tube on the cells of the other colony.
  • The conjugation tubes from both the colonies fuse together.
  • The gametes of one cell move through the conjugation tube and fuse with the gametes of the complementary cell, forming a diploid zygote.
  • The zygote appears as a large, egg-like structure contained within the complementary cell.
  • It comprises a thick wall that provides resistance against desiccation and cold, to the colonies of Spirogyra.
  • The other colony acts as a filament of empty cells that is broken down by some decomposers.
  • When conditions are favorable, the zygote undergoes meiosis to produce another vegetative colony of haploid cells.


  • Various species of algae have been a part of human diet since ancient times.
  • Aquatic ecosystem contains an enormous variety of algae which constitute the fundamental link of many diverse food chains
  • Various selected algae are grown in recreational areas like lakes and streams along with fish.
  • Carrageenan, an algal derivative, is useful in making sauces, syrups, toothpastes, and cosmetics.
  • Algin, derived from giant kelp, is also widely utilized in industries.
  • Algal Species like Chlamydomonas, Scene­desmus, Chlorella and Euglena are used in sewage treatment plants.
  • They provide the oxygen necessary for rapid decomposition of the sewage by bacteria through photosynthesis.
  • Many species of algae extract calcium from water, both fresh and salt water and deposit it in the form of calcium carbonate, in their cell walls or gelatinous sheaths.
  • Algae like seaweeds play significant role in fertilizing the soil
  • Algae are also used in making medicines.


  • Microalgae in marine or brackish waters are harmful and alter the ecosystems badly.
  • Marine food resources e.g. aquacultures, are affected badly by algal blooms.
  • Blue-green algal toxins pose massive threats through drinking water, which are carcinogenic and may cause critical damage.
  • They produce extremely dangerous toxins that sicken and kill humans and animals
  • Algal blooms produce dead zones in the water, by covering the water surface and preventing sunlight from entering the water.
  • Moreover, they raise treatment costs for drinking water and adversely affect the industries that depend on water.


  • Arihant’s handbook of biology. Plant Kingdom. Page no: 24-42.
  • NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.
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