Algae: Characteristics, Types and Its Classification

Algae are the green slimy blanket which covers the rock surface or the top of the ponds or a poorly kept aquarium that have the ability to conduct photosynthesis. The algae belong to the subphylum Thallophyta of the kingdom Protista in modern classification of organisms. Latin ‘alga’ means seaweed. These are eukaryotic organisms, autotrophic in nature as have chlorophyll in their cells. During photosynthesis, they produce oxygen with help of light energy from the sun and generate carbohydrates.

They possess many types of life cycles and their size range from microscopic Micromonas species to giant kelps. In this case, kelps reach up to 60 meters (200 feet) in length.

More than 30000 species of algae have been identified. Most algae are aquatic but some grow in semi-aquatic and terrestrial environments. Many algae live as endophytes in plant or animal tissue and many grow on plant or animal as epiphytes. Some of them make asymbiotic relationship with fungi and exist as lichen. Green algae, brown algae, red algae, golden-yellow algae are main types of algae. The study of algae is known as Phycology.

Characteristics of Algae

  • Algae are unicellular, colonial or large multi-cellular organisms.
  • The multi-cellular algae develop specialized tissues but they lack the true stems, leaves, or roots.
  • Most algae are aquatic but some are semi-aquatic and terrestrial.
  • Cells contain photosynthetic chlorophyll and other pigments.
  • Algae cell wall composed of cellulose and pectin.
  • With a few exceptions, most algae are autotrophic; they do not have vascular tissues.
  • Most algae store carbohydrate as reserve food, few members contains alcohol, fat or oil as reserve food.
  • They reproduce through vegetative, asexual and sexual methods. Asexual reproduction occurs by fragmentation or producing spores. Sexual reproduction can be isogamous, anisogamous or oogamous types.
  • Gametangia (reproductive organ) always single celled, if multi-celled, do not cover with sterile cell layer.
  • Zygote develops by either mitosis or meiosis cell division. Zygote never form embryo. 
  • Meiosis cell division is seen in different stages of life cycle.

Classification of Algae

image of Algae classification at a glance

Outline of Algal Classification

Division-1: Chlorophyta (Green Algae)

  • This group contains about 7000 species, among them, most occur in freshwater and some others in marine environment.
  • They contain pigments like chlorophyll a and chlorophyll b.
  • They also possess accessory pigments like carotenoids and xanthophylls.
  • Cell wall consists of cellulose, hemicellulose, and calcium carbonate in some species.
  • They store food as starch inside the chloroplast. 
  • Mitochondria contain flattened cristae.
  • They contain two or more flagella which do not bear tubular hairs (mastigonemes).

Class-1: Chlorophyceae

  • They have unicellular, colonial, filamentous, or multicellular body.
  • They contain pigments like chlorophyll a, chlorophyll b and beta-carotene.
  • They bear one or more storage bodies, known as pyrenoids located in the chloroplast.
  • The cell wall is rigid, composed of cellulose and pectose.
  • Asexual reproduction occurs by zoospores, aplanospores, hepnospores, akinetes, Palmella stage, etc.
  • Sexual reproduction is anisogamous, isogamous, or oogamous types.
  • Most of them inhabit in freshwater environment.

Examplses:  Chlorella, Chlamydomonas, Oedogonium,  Dunaliella,  Volvox, etc.

This class includes the following orders:

Order-1: Dunaliellales (e.g. Dunaliella)

Order-2: Chlamydomonadales (e.g. Volvox, Chlamydomonas)

Order-3: Chlorococcales

Order-3: Oedogoniales (e.g. Oedogonium)

Order-4: Sphaeropleales

Order-5: Chaetophorales

Order-6: Microsporales

Order-7: Tetrasporales (e.g. Tetraspora)

Class-2: Charophyceae

  • They are also known as stoneworts" and "brittleworts".
  • They are commonly found in freshwater environment.
  • They have unicellular, filamentous, colonial, or multicellular and plantlike bodies.
  • Many species bear flagellated cells.
  • They can store starch in characteristic plastids.

Examples: Stonewort (Chara), filamentous (Spirogyra) and desmids.

This class includes single order:

Order-1:  Charales

Class-3: Pleurastrophyceae

  • They inhabit in both freshwater and marine habitats.
  • They include coccoid, sarcinoid, and filamentous algae.
  • They reproduce asexually by autospores or by biflagellate, flattened zoospores.
  • Sexual reproduction is unknown among the representatives of Pleurastrophyceae.
  • They possess a counter-clockwise orientation of the basal bodies and an unusual mitotic spindle.

Examples: Marine flagellate (Tetraselmis).

Class-4: Prasinophyceae (Micromonadophyceae)

  • They have both unicellular and colonial forms.
  • They possess one, two, four, or eight flagella, with or without cell walls, scales, thecae, or loricae.
  • They are mostly motile and photosynthetic algae containing pigments like chlorophyll a, and chlorophyll b.
  • They are mainly marine algae, but some are also found in brackish- and freshwater habitats.
  • A few algae inhabit benthic region, with both coccoid and colonial forms.
  • There are about 180 known species under 13 genera.

Examples: Micromonas, Ostreococcus, Pyramimonas, etc.

Class-5: Ulvophyceae

  • They are marine algae with a variety of shapes.
  • The body consists of a few cells with thin sheaths, long filaments.
  • They possess two or more apical flagella, if present.
  • They perform photosynthesis due to presence of chlorophyll in their body.
  • Alternation of generations occurs in their life cycle.

Examples:  Sea lettuce (Ulva), Acetabularia, Caulerpa, Monostroma, etc.

Division-2: Chromophyta

  • They contain chlorophyll a and chlorophyll c.
  • They also contain accessory pigments such as Carentoids, xanthophylls.
  • They store foods as oils or carbohydrates laminarin.
  • If present, flagellum possesses hair like projection.  
  • Mitochondria contain tubular cristae.
  • Mucous organelles are common.

Class-1: Bacillariophyceae (Diatoms)

  • They primarily inhabit in freshwater, marine, and soil environments.
  • There are about 12,000 to 15,000 living species.
  • The cell contains a silica cell wall which is known as frustules. In this case, frustule is made up of two valves called thecae.
  • They contain  pigments like  chlorophylls a and chlorophyll c with accessory pigments such as  beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.

Examples: Cyclotella, Thalassiosira ,  Bacillaria, Navicula, Nitzschia, etc.

Class-2: Chrysophyceae (Golden Algae)

  • They are commonly known as golden algae.
  • They inhabit in freshwater environment.
  • They are unicellular or colonial organisms.
  • The photosynthetic pigments are chlorophyll a, chlorophyll c and fucoxanthin.
  • They store energy as carbohydrate and oil droplets.
  • The cell contains silica deposition vesicles.
  • They possess apical flagella which is unequal in length.
  • They perform haplontic type life cycle.
  • Reproductions occur through binary fission, sporogenesis, etc.
  • This class contains about 1200 known species.

Examples: Chrysamoeba,  Lagynion, Chrysocapsa, Ochromonas, etc

Class-3: Dictyochophyceae

  • They are a small group of unicellular heterokont algae.  
  • They contain golden-brown chloroplast.
  • They have a long and wing like flagella.
  • They are predominantly marine algae.
  • Bases of flagella attach directly to nucleus.
  • This class contains only 25 described species.
This class includes the following orders:

Order-1: Pedinellales

Order-2: Olisthodiscales

Order-3: Sulcochrysidales

Order-4: Pelagomonadales

Order-5: Sarcinochrysidales

Order-6: Florenciellales 

Examples: Apedinella,  Mesopedinella, Parapedinella, Actinomonas, Pteridomonas, Dictyocha,  Pseudopedinella, Pedinella, etc

Class-4: Eustigmatophyceae

  • They are mostly small and pale green unicellular coccoid algae.
  • They inhabit in soil, freshwater and marine environments.
  • The cell is non-motile which is enclosed by cellulosic cell wall.
  • They contain one or more yellow-green chloroplasts with pigments like chlorophyll a,  violaxanthin and β-carotene.
  • The flagellate cell bears one or two flagella.
  • This class includes about 41 described species.

Examples: Eustigmatos, Botryochloropsis,Pseudocharaciopsis, Ellipsoidion , Pseudellipsoidion, Nannochloropsis,Pseudostaurastrum,   etc.

Class-5: Phaeophyceae (Brown Algae or Brown Seaweeds)

  • They are also known as brown algae or brown seaweeds.
  • They are mostly marine with unicellular or multicllular body.
  • The photosynthetic pigments are chlorophyll a and chlorophyll c.
  • They also possess accessory pigments like beta carotene, fucoxanthin, lutein, violaxanthin and diaanthin.
  • They store foods as laminarin, maninitol and oils.
  • Sexual reproduction is anisogamous, isogamous or oogamous types.
  • The male gametes possess heterokont flagella.
  • They perform alternation of generation with haplobiontic or diplobiontic life cycles.
  • This class contains about 1500 described species.

Examples: Ascophyllum, Ectocarpus, Laminaria,  Fucus, Nereocystis, Macrocystis,  Pelagophycus,  Postelsia, Pelvetia,  Sargassum, etc.

Class-6: Prymnesiophyceae (Haptophyceae)

  • The cells have typically two slightly unequal flagella.
  • The cell possesses hair like appendages known as haptonema between two flagella.
  • They are predominantly marine algae.
  • This class contains about 762 described species.
  • The exoskeleton consists of calcareous plates called coccoliths.
  • The thylakoids are stacked in triplets and it contains chlorophyll a and chlorophyll c.
  • They store food as chrysolaminarin.

Examples:  Chrysochromulina, Emiliania, Phaeocystis, Prymnesium, etc.

This class includes the following orders:

Order-1: Rappemonadales

Order-2: Pavlovales

Order-3: Discoasterales

Order-4: Phaeocystales

Order-5: Prymnesiales

Order-6: Isochrysidales

Order-7: Eiffellithales

Order-8: Stephanolithiales

Order-9: Zygodiscales

Order-10: Syracosphaerales

Order-11: Watznaueriales

Order-12: Arkhangelskiales

Order-13: Podorhabdales

Order-14: Coccolithales

Class-7: Raphidophyceae (Chloromonadophyceae)

  • They are unicellular algae with large cells which range from 50 to 100 μm.
  • They possess a pair of flagella.
  • The cell contains numerous ellipsoid chloroplasts.
  • They are photosynthetic organisms which contain pigments like chlorophylls a and c1 and c2.
  • The cells also contain accessory pigments such as β-carotene and diadinoxanthin.
  • They inhabit in both in freshwater or marine environments.
  • This class contains more than 50 described species.

Examples: Gonyostomum, Vacuolaria, Merotricha,  Chattonella, Chlorinimonas,  Haramonas, Psammamonas,Fibrocapsa, Heterosigma, and Viridilobus, etc.

This class includes the following orders:

Order-1: Commatiida

Order-2: Actinophyrida

Order-3: Raphidomonadales

Class-8: Synurophyceae

  • They are the most important photosynthetic stramenopile algae.
  • They inhabit in freshwater ecosystem.
  • They are motile organisms which contain two parallel heterokont flagella.
  • The cell is covered by siliceous scales.
  • Asexual and sexual reproduction occur. In this case, sexual reproduction is isogamous type.
  • This class contains about 200 described species.

Examples: Mallomonas, Synura

This class includes the following orders:

Order-1: Chloramoebales

Order-2: Heterogloeales

Order-3: Ochromonadales

Order-4: Synurales

Class-9: Xanthophyceae (Yellow-Green Algae)

  • They are commonly known as yellow green algae.
  • They contain yellow green chromatophores.
  • Photosynthetic pigments are chlorophyll a, chlorophyll e,  xanthophyll or carotenoids.
  • They store energy as oil and leucosin.
  • The cell wall is composed of pectic compounds.
  • They do not contain pyrenoids.
  • They perform sexual reproduction which is isogamous type.
  • The zoospores possess unequal flagella. 
  • Most of them inhabit in freshwater environments.
  • This class contains about 600 described species.

Examples: Botrydium,  Tribonema, Bumilleriopsis,  Vaucheria, etc.

Division-3: Cryptophyta

  • They are unicellular photosynthetic flagellated algae.
  • They inhabit in both freshwater and marine environments.
  • The cell is flattened shape with 10–50 μm in length.
  • The cell bears typically two unequal flagella.

Class-1: Cryptophyceae

  • They inhabit in both marine and freshwater habitats.
  • The photosynthetic pigments are chlorophyll a, chlorophyll c and phycobiliprotein.
  • The cell body is asymmetric with dorsi-ventral sides.
  • The cell possesses two anteriorly directed flagella with tubular hairs on one or both flagella.
  • They store foods as pyrenoids outside of chloroplasts.
  • The mitochondria contain flattened cristae.
  • The cell is covered with periplast with often elaborately decorated sheet or scale.
  • This class contains about 200 described species.

Examples: Cryptomonas, Chilomonas,  Falcomonas,  Rhinomonas, Plagioselmis,  Teleaulax, etc.

Division-4: Rhodophyta (Red Algae)

  • This group contains about 6,000 described species.
  • Most of them are photosynthetic marine organisms but few are parasitic.
  • Photosynthetic species contain pigments like chlorophyll a  and chlorophyll d.
  • They also contain phycocyanin carotenoids, xanthophyll, and phycobilins and phycoerythrin (phycobiliproteins) as accessory pigments.
  • The cell wall consists of cellulose and polysaccharides such as agar and carrageenin.
  • They store energy as a specialized polysaccharide, known as floridean starch outside chloroplast.
  • They do not contain flagella; mitochondria with flattened cristae.

Examples: Palmaria, Polysiphonia, Bangia, Corallina, Gelidium Chondrus,   Kappaphycus, Gracilaria,  Porphyra, Rhodymenia, etc.

Division-5: Dinoflagellata

  • They are mostly unicellular flagellated algae.
  • They are heterotrophic or autotrophic (photosynthetic) organisms.
  • Photosynthetic forms contain chlorophyll a, and chlorophyll c with accessory pigments like   peridinin or fucoxanthin.
  • The flagellum does not contain tubular hairs.
  • Mitochondria possess tubular cristae.
  • This phylum contains more than 1,500 described species. 

Class-1: Dinophyceae

  • They are also is known as dark yellow to brown algae.
  • They store food materials as starch and oil.
  • The body contains characteristics nucleus with condensed and banded chromosomes.
  • They live as free living, symbiotic or parasitic forms.
  • Often, they are commonly known as sea water planktons.
  • The motile form possesses two different flagella.

Examples: Dinophysis, Alexandrium,  Gonyaulax, Ceratium,  Noctiluca, Gymnodinium,  Polykrikos, Peridinium,  etc.

This class includes the following orders:

Order-1: Haplozoonales

Order-2: Akashiwales

Order-3: Blastodiniales

Order-4: Apodiniales

Order-5: Dinotrichales

Order-6: Phytodiniales

Order-7: Brachidiniales

Order-8: Ptychodiscales

Order-9: Amphilothales

Order-10: Actiniscales

Order-11: Gymnodiniales

Order-12: Prorocentrales

Order-13: Nannoceratopsiales

Order-14: Dinophysales

Order-15: Gonyaulacales

Order-16: Thoracosphaerales

Order-17: Peridiniales

Division-6: Euglenophyta

  • This phylum contains about 800 species. Most of them inhabit in freshwater environment.
  • They lack a true cell wall, and the body is bounded by a proteinaceous cell covering which is known as a pellicle.
  • They possess one to three flagella for locomotion.
  • They store carbohydrate as paramylon.
  • They are both photosynthetic and heterotrophic organisms.
  • The primary photosynthetic pigments are chlorophyll a and chlorophyll b.
  • They also contain carotenoids and xanthophylls as accessory pigments.  
  • They feed on organic material suspended in the water.

Class-1: Euglenophyceae

  • They are commonly known as pure green algae.
  • Most of them inhabit in freshwater environments.
  • The body is covered by  flexible pellicle which is formed of protein.
  • The reserve food materials are carbohydrates or starch.
  • The body has two definite ends such as anterior and posterior ends.
  • The body possesses large and prominent nucleus and a contractile vacuole for performing osmo-regulation.
  • Autotrophic or heterotrophic nutrition occurs.
  • The body contains two apically or laterally placed flagella which lack tubular hairs.
  • Reproduction is isogamous type.
  • They contain single lobed chloroplast with central pyrenoids. In this case, photosynthetic pigments are chlorophylls a and chlorophyll b.
  • Mitochondria contain paddle-shaped cristae.
  • This class contains about 1000 known species.

Examples: Colacium, Euglena, Eutreptiella, Phacus, etc.

Types of Algae

Based on colors, algae are divided into the following major four groups:

Blue Green Algae

They belong to the class Cyanophyceae under the phylum Cyanophyta. They inhabit in freshwater or in a wide variety of moist soils of a terrestrial environment. They also form a symbiotic relationship with plants or lichen-forming fungi. They contain pigments like chlorophyll 'a', 'b', and phycobilins and they appear in blue green color. They are also known as Cynabacteria.

Green Algae

They may be either unicellular or multicellular algae which belong to the class Chlorophyceae  under the phylum Chlorophyta. They contain pigments like chlorophyll a, chlorophyll b, carotenoids, and xanthophylls.

Examples: Chlamydomonas, Spirogyra, and Chara

Red Algae

Red algae belong to the class Rhodophyceae under the phylum Rhodophyta which is one of the largest phylum of algae. This phylum contains over 7000 recognized species. Among them, 6,793 species are found in the Class Florideophyceae. Most of the algae are multicellular and marine algae (seaweeds). Red algae contain pigments like chlorophyll a, chlorophyll d, carotenoids, xanthophylls, and phycobilins. About 5% of the red algae inhabit in freshwater environments. They are used as a stabilizer in milk products.

Examples:  Porphyra, Gracilaria, and Gelidium.

Brown Algae

These types of algae belong to the class Phaeophyceae under the phylum Phaeophyta. They contain pigments like chlorophyll a, chlorophyll c, carotenoids and fucoxanthin. They form a large group of multicellular algae. Majority of them inhabit in marine environments. Over 1500 known species of brown algae are available worldwide.

Examples:  Dictyota, Laminaria, Sargassum, etc.

Based on morphology, algae can be divided into several types. Some are filamentous forms having cells arranged in chains like strings of beads. Some filamentous are un-branched such as Spirogyra while others are branched and bushlike such as Stigeoclonium.

Algae are almost ever-present throughout the world. Ecologically, they can be grouped into the following types by their habitats.

  1. Planktonic algae: They are microscopic and grow suspended in the water.
  2. Neustonic algae: They grow on the water surface and can be either microscopic or macroscopic.
  3. Cryophilic algae: They can occur in snow and ice.
  4. Thermophilic algae: They can live in hot springs.
  5. Edaphic algae: They can inhabit on or in soil.
  6. Epizoic algae: They grow on the body of animals, such as turtles and sloths.
  7. Epiphytic algae: They can grow on other algae, fungi, and land plants.
  8. Corticolous algae: They can grow on the bark of trees.
  9. Epilithic algae: They inhabit on rock surfaces.
  10. Endolithic algae: They inhabit in porous rocks or coral.
  11. Chasmolithic algae: They grow in rock fissures.
  12. Endosymbionts algae: Some algae live inside other organisms, and this situation is known as endosymbionts. In this case, endophytic endosymbionts live in plants, fungi, or other algae.

Concluding Remarks

Algae are living organisms which are distributed throughout the world. They have different sizes, shapes, and colors. They can inhabit in freshwater and marine environments. They also grow on the body surfaces of other organisms such as turtles and polar bears, on rocks or in the soils, or under or inside porous rocks, such as limestone and sandstone.   The algae have great important because they produce much oxygen on the earth for animals and human beings. Nutritionally, they contain several healthy elements such as proteins, fats, carbohydrates, and vitamins A, B, C, and E. They also contain a number of important minerals such as iron, potassium, calcium, magnesium, zinc and manganese.

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