Algae Definition, Characteristics, Types, and Examples

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Algae are commonly thought of as slimy, green films formed in stagnant waters (freshwater and marine). An Alga (plural− algae) can be microscopic or macroscopic spanning a few feet in length depending on species.

Though algae are considered culprits spoiling the beauty of transparent freshwater, they are the main source of atmospheric oxygen supporting many life forms on earth. Algae are a large and diverse group. Understanding them requires tons of knowledge in terms of their characteristics, occurrence, and life cycle. The study of algae is called phycology and people carrying out extensive studies on algae are called phycologists.

What are Algae?

Algae are unicellular or multicellular autotrophic eukaryotes with photosynthetic ability. Algal cells have membrane-bound organelles like chloroplasts, mitochondria, and nucleus. They are ubiquitous inhabiting different zones of the globe.

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Examples of Algae

Euglenoids, Diatoms, kelps, Laminaria, Spirogyra, Volvox, Chara, Fucus, Micromonas, Noctaluca, Chilomonas, Gracillaria, and Chlamydomonas are some commonly known algae.

Characteristics of Algae

Algae may be unicellular or macroscopic, multicellular leaf-like Gaint kelps also. They have unique characteristics necessary for survival in their stipulated living conditions.


  • Majority of the algal species belong to aquatic habitats, both freshwater, and marine.

  • Algae survive in different conditions of temperature, and type of water.

  • Type of water relates to the extent of salinity, pH, depth, turbidness (whether muddy or clear), and chemical composition of water (presence of pollutants).

  • They can grow on moist rocks, and submerged surfaces also.


  • Algae bear a simple morphology, unlike plants.

  • Unicellular algae are either motile or non-motile, organising themselves as colonies or filaments.

  • Multicellular algae-like kelps have body structures defined for specific functions. Few body parts resemble leaves but algae lack true organs like roots, stems, and leaves.

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Interaction with environment

Some algae live independently (suspended on water or attached to the substrate). While some species live in a symbiotic relationship with fungi, coral reefs, and sponges.

Mode of nutrition

  • Most algae are photoautotrophs credited for the presence of chlorophyll (photosynthetic pigment).

  • Very few exceptions are the facultative and obligate heterotrophic algae. They require carbon substrates from the surrounding for their survival. Some consider that algae show mixotrophy (autotrophy and heterotrophy).


It can be vegetative, asexual, and sexual reproduction.

  • Vegetative reproduction is by mitosis and fragmentation. The broken part regenerates into a complete body in fragmentation.

  • Asexual reproduction is by spore formation. The cytoplasm of mature cells divides and forms spores. Spores develop into new individuals upon the advent of favorable conditions.

  • Gametes carry on sexual reproduction. Male and female gametes fuse to form a zygote. Sometimes, female gametes directly develop into zygotes. This is parthenogenesis.

Classification of Algae

Algal species are subdivided into different phyla based on their pigments.

  • Chlorophyta (Green algae)

    • Pigments are chlorophyll a and b along with 𝛂 and β carotenes. They are either unicellular, colonies or multicellular forms also

  • Rhodophyta (Red algae)

    • Pigments are chlorophyll a and d along with phycoerythrin and phycocyanin. They appear in red color due to the pigment phycoerythrin

  • Phaeophyta (Brown algae)

    • Brown algae contain chlorophyll a and c along with fucoxanthin pigments. Mostly seaweeds and kelps belong to this category.

Although blue-green algae are photosynthetic and share habitat with algae they lack membrane-bound cell organelles and are prokaryotes.

Types of Algae

Algae are of different types based on their inhabitation.

  • Cryophilic algae− grow in freezing temperatures prevailed by snow and ice.

  • Thermophilic algae− as the name suggests, grow at high temperatures near hot springs.

  • Epizoic algae− live on the body of aquatic animals like turtles.

  • Edaphic algae− grow in soil.

  • Epilithic algae− grow on rocks.

  • Endolithic algae− inhabit coral reefs. Some call it a symbiotic relationship.

  • Corticolous algae− grow on moist tree trunks.

Some unicellular algae possess motile structures like flagella for swimming. They float on water surfaces and are called planktons.

Chemical composition of Algae

Algal cells are enclosed by a cell wall and have membrane-bound organelles. They contain a wide range of pigments like chlorophyll, phycocyanins, phycoerythrins, carotenes, and fucoxanthins. Cell wall composition is markedly different in algae. Algal cell wall components are cellulose, alginate, carrageenan, agarose, and glycoproteins like galactans and mannans. Proteins, carbohydrates, nucleic acids (DNA since eukaryotes), and lipids are some other biomolecules in algae.

Difference between normal plants and algae

Like many complex multicellular plants, algae perform photosynthesis accounting for the presence of chlorophyll. However, they differ from plants in lacking well defined vascular system, true stems, and leaves.

Importance and uses of Algae

  • Marine environments are devoid of trees and hence algae are the main nutrient for marine herbivores like zooplankton, dugongs, turtles, and some species of fish.

  • Algae are a great source of atmospheric oxygen. They contribute about 30 to 50% of oxygen, supporting other life forms on earth.

  • Many red and brown algae are consumed as food. They are popular and cultivated on commercial scale.

  • Alginates, agar, and carrageenans extracted from red and brown algae have great demand in the food industry for their gelation, colloidal and emulsifying properties.

  • Agar has a great market for its use in biotechnology and tissue culture labs.

  • Dried seaweeds are rich in nitrogen and used as plant fertilisers.

  • Algae are highly sensitive to water quality (pH and composition). They are bioindicators of environmental pollution.

  • Diatoms are a source of silica for glass, paints, and toothpaste industries. .

  • Natural gas and crude oils are formed from algae that once lived on sea beds. Biofuel extracted from algae gained popularity these days.

Difference between algae and fungi

  • Algae and fungi are multicellular eukaryotes lacking vascular tissues.

  • Fungi are saprophytes. They depend on dead and decaying organic material for nutrients while algae are autotrophs.

  • Fungi have a chitinous cell wall and are devoid of chlorophyll, they are entirely different from algae. However, both share a symbiotic relationship and live as lichens.

  • Fungi provide protection to algae and in return algae (usually green algae) provide nutrients to fungi.

Life cycle of Algae

Alternation of generations between haploid (gametophyte stage) and diploid (sporophyte) stages is seen in algae. Four patterns of the life cycle are observed.

  • Haplontic lifecycle

    • The plant body is haploid. Gametes (produced by mitosis) fuse and form a diploid zygote.

    • The zygote undergoes meiosis and form meiospores that develop into new algae.

    • The sporophytic stage is confined to the diploid zygote only.

  • Diplontic lifecycle

    • The sporophytic plant body is diploid. Haploid gametes fuse to form the zygote.

    • Zygote directly develops into a sporophytic body.

  • Diplohaplontic lifecycle

    Haploid and diploid stages have equal dominance in the lifecycle. Haploid gametophyte reproduces sexually while diploid sporophyte reproduces asexually.

  • Isomorphic − The gametophyte and sporophyte look similar. Gametes fuse to form a diploid zygote. Zygote either develops into sporophyte body or undergoes meiosis to form zoospores. Zoospores develop into the new gametophyte.
  • Heteromorphic − Gametophyte and sporophyte have dissimilar morphology. Gametes fuse to form a diploid zygote. The zygote develops into a sporophyte that bears zoosporangia to produce zoospores by meiosis. Zoospores develop into a gametophyte.
  • Triphasic life cycle

    Life cycle alternate between three generations.

  • Haplobiontic − Dominant stage is gametophyte. The life cycle has two haploid and one diploid generation.
  • Diplobiontic
  • − Dominant stage is gametophyte. The life cycle has two haploid and one diploid generation.


Algae are photosynthetic unicellular or multicellular organisms that live as colonies or multicellular organised bodies in aquatic systems. They are classified into Green algae, red algae, and brown algae based on the pigments present. They are ubiquitous and inhabit a wide range of environmental conditions. Algae lack true stems and leaves that are possessed by green plants. Algae have great ecological importance with respect to photosynthetic ability in producing oxygen. They are a source of nutrition for many aquatic species and are the base of aquatic food chain. They are cultivated on a commercial scale for substances like Alginates, agar, and carrageenans. Algae are bioindicators of organic pollution in water since they are sensitive to minute changes in acidity, turbidity, and composition of water.


Q1. Do algae have a root system?

Ans. Algae lack true roots. Instead of roots, algae have hold-fast organs that act as an anchor and hold immotile algae to a firm substrate.

Q2. What are kelp forests?

Ans. Kelps are multicellular brown algae. They inhabit shallow waters near shore. Dense and thick grown kelps are breeding spots for several small invertebrates and fishes. Carnivores like seals and sea lions dive into kelps for nutrition creating an ecosystem.

Q3. Are algae harmful to humans?

Ans. Algae are not harmful. Some algae produce harmful toxins which have negative effects on humans. Direct exposure to these toxins results in fever, diarrhea, and skin rashes.

Q4. Is it possible to produce biofuel from algae?

Ans. The energy-rich oil extracted from algae is converted to various types of fuel by following different steps. The process varies for each species.

Q5. What happens if giant kelp is cut off?

Ans. Multicellular algae have the property of regeneration. The broken part can regenerate into a new body if environmental conditions are favorable. Otherwise, it degenerates and decomposes if not eaten by herbivores.

Updated on 13-Oct-2022 11:19:47