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Algae are a diverse group of undifferentiated aquatic organisms that contain chlorophyll. They range in size from microscopic organisms to giant kelp.

They are important as primary producers and provide oxygen for other aquatic life.

Algae are plant-like organisms that are photosynthetic and aquatic, with simple reproductive structures, but do not have true roots, stems, leaves, vascular tissue like other plants.

They are important source of crude oil and are also sources of food and several pharmaceutical and industrial products.

Characteristics of Algae

Algae are chlorophyll-bearing, aquatic autotrophic plant like organisms. They can be unicellular or multicellular.

Although the multicellular forms can have large structures, they still lack vascular tissues.

Algae can reproduce sexually, or asexually, or by a combination of both processes through alternation of generations.

Both gametophyte and sporophyte, when present in the life cycle are independent.

Figure 1. Microscopic view of cells of Algae
Figure 1. Microscopic view of cells of Algae

Habitat

Most algae live in aquatic habitats. They can thrive in freshwater (e.g. river, lakes, or ponds) or in saltwater (oceans, sea) covering a wide range of temperature, oxygen or carbon dioxide concentrations, acidity, and turbidity.

They are symbiotic partners with fungi i.e., lichens (figure 2) and with animals (e.g., in corals and in some protozoans and Cnidaria).

Figure 2. Lichen on the trunk of a tree
Figure 2. Lichen on the trunk of a tree

Chlamydomonas, Volvox, Ulothrix, Chara, Spirogyra, Nostoc, Oscillatoria etc. are common fresh water algae.

Figure 3. Volvox colony
Figure 3. Volvox colony

Sargassum (Figure 4), Laminaria, and Ectocarpus are common marine forms.

Figure 4. Sargassum algae to float in shallow sandy sea
Figure 4. Sargassum algae to float in shallow sandy sea

The algae that grow on the surface of the soil are known as saprophytes.

Many blue-green algae grow under the surface of the soil, and are called cryptophytes. For example, Oscillatoria sancta, Vaucheria geminata, Chlorella lichina etc.

Figure 5. Classification of Algae
Figure 5. Classification of Algae

Euglenophyta

Euglenophyta is a part of the kingdom Protista, consisting of mostly unicellular aquatic algae. Euglenophyta species live in freshwater. They use flagella to move.

Some euglenoids contain chloroplast and manufacture their own food, whereas others are heterotrophic and can ingest or absorb their food.

They reproduce by longitudinal cell division. Euglena belongs to this group.

Figure 5. Anatomy of Euglena
Figure 5. Anatomy of Euglena

Chrysophyta

Chrysophyta (Golden algae) includes unicellular organisms of the kingdom Protista consisting of golden, or golden-brown, algae (class Chrysophyceae), and yellow-green algae (class Xanthophyceae), and diatoms (class Bacillariophyceae).

Some species are colorless, but most of them are photosynthetic. Their cell wall is composed of cellulose or hemicellulose often with siliceous scales.

Chrysophytes store their food as oils and can be used for biofuel (Figure 6). Asexual reproduction is by the formation of motile and non-motile spores and by longitudinal cell division.

Sexual reproduction is rare.

Figure 6. Yellow seaweed on the forest lake
Figure 6. Yellow seaweed on the forest lake

Pyrrophyta

Pyrrophyta (Fire/flame colored algae, dinoflagellates) comprises many algal species of varying shapes and sizes. Most are marine, though some live in freshwater habitats.

Dinoflagellate refers to the forward-spiraling swimming motion of these organisms (Figure 7).

They are free-swimming unicellular eukaryotic microorganisms with two flagella, a nucleus with condensed chromosomes, chloroplasts, mitochondria, and golgi bodies.

They reproduce  primarily by longitudinal cell division.

Figure 7. Dinoflagellate under microscope
Figure 7. Dinoflagellate under microscope

Chlorophyta

Chlorophyta (Green algae) consist of about 7,000 species, most of which occur in fresh water (Figure 8). Examples are Chlamydomonas (Figure 9) and Chlorella.

Some species are unicellular, while others are multicellular.

The photosynthetic pigments of green algae are chlorophylls a and b, and other accessory pigments such as carotenoids and xanthophylls.

Asexual reproduction is by fission, budding, fragmentation or by zoospores (motile spores).

Sexual reproduction is very common and can be isogamous (gametes both motile and same size), anisogamous (they are both motile and are of varied sizes–female is bigger), or oogamous (female non-motile and egg-like and male motile).

Figure 8. Green seaweed
Figure 8. Green seaweed
Figure 9. Chlamydomonas
Figure 9. Chlamydomonas

Rhodophyta

Rhodophyta (Red algae) includes about 4,000 species of mostly marine algae that range from microscopic to macroscopic in size.

They primarily live in marine environments, though some species live in freshwater habitats.

Along with chlorophyll, they also contain accessory photosynthetic pigments such as, phycoerythrin, phycocyanin and allophycocyanin.

Red algae are ecologically significant as primary producers (Figure 10). They play a significant role in the formation and maintenance of coral reefs.

Red algae lack flagella and centrioles. Some red algae are important foods. These rhodophytes are commonly known as seaweeds.

Red algae are distinctive from other eukaryotic algae in that they lack flagella in their vegetative cells, spores, and gametes. They reproduce both asexually and sexually.

Methods of asexual reproduction include sporulation and fragmentation. The male gametophytes produce male non-flagellated gametes called spermatia.

Female gametophytes produce carpogonial branch, that produces a terminal carpogonium (oogonium, an egg-bearing structure).

Figure 10. Red algae (seaweed) bushes on flat sea bottom of coarse sand
Figure 10. Red algae (seaweed) bushes on flat sea bottom of coarse sand

Uses of Red Algae

  • The most valuable of all algae is Porphyra. Porphyra is used as a wrapper for sushi or may be eaten mixed with rice and fish and in salads.
  • It is very rich in vitamins B and C as well as minerals, including iodine.
  • Agar, a gelatin-like substance prepared primarily from Gracilaria and Gelidium species, is important as a culture medium for bacteria and fungi.
  • Agar is also used in canning industry as a protective agent against the unwanted effects of metals.
  • It is also a source of agarose, which is widely used in recombinant DNA (deoxyribonucleic acid) technology for gel electrophoresis.
  • Carrageenan (produced by Eucheuma, Kappaphycus, and Chondus) is used in toothpaste, cosmetics, and food, such as ice cream and chocolate milk.
  • It is a great adjuvant in the treatment of urinary tract infections, asthma, stomach disorders, skin diseases.
  • Algae benefits also extend to inflammation and pain relief.
Figure 11. Agar-agar in powdered form
Figure 11. Agar-agar in powdered form

Phaeophyta

Phaeophyta are commonly known as brown algae (Figure 12), due to the presence of a golden-brown xanthophyll pigment, fucoxanthin.

They live in marine environment, especially abundant in cool waters.

Phaeophyta is a large group of algae consisting of 240 genera and over 1,500 species, such as, Sargassum and Laminaria.

They range from simple microscopic filament (Ectocarpus) to largest alga (Macrocystis pyrifera).

Body is immobile, multicellular, and highly differentiated both externally and internally. They reproduce by all the three means: vegetative, asexual, and sexual.

Vegetative reproduction takes place by fragmentation. Special reproductive branches, propagules, develop to new plants after detachment. Sexual reproduction ranges from isogamy to oogamy.

Figure 12. Brown algae
Figure 12. Brown algae

Xanthophyta

Xanthophyta or Heterokontae are commonly known as yellow-green algae. They live primarily in freshwater, though some are found in marine waters, in damp soil, or on tree trunks.

Chlorophyll a and chlorophyll e are present, but not chlorophyll b. The food storage products are oil and fat. Vegetative reproduction is by accidental breaking of the body.

Asexual reproduction in most species is usually by zoospores. Sexual reproduction is isogamous (Botrydium) and oogamous (Vaucheria).

Table showing difference between plant and algae

Features

PlantAlgae
HabitatPlants are generally terrestrial; however, they can also live in water such as water liliesAlgae are mostly aquatic although a few are found on land, snow, rocks, marine animals or even on the fur of some rainforest animals such as sloth
Body differentiationBody is well differentiated into roots, stem, leaves, flowers, fruitsNot differentiated into various parts as plants
Cell organizationMulticellularUnicellular, multi-cellular or even colonial
Vascular systemWell developed; vascular systems that allow dispersing of nutrients to the entire plantAbsent, therefore each cell absorbs its own nutrients independently of the other cells
LocomotionSessileDrift with water currents and actively mobile as dinoflagellates, which whip themselves through water with their flagella
Photosynthetic pigmentsChlorophyllChlorophyll, carotenoid, and phycobilin
FeedingProduce their own food by photosynthesis (autotrophs)Some are autotrophs, while the others are heterotrophs
ReproductionComplex process. Often the reproduction process requires vectors such as wind, birds, bees, insects or other small animals for pollinationAsexual reproduction by spore formation, fragmentation; whereas sexual reproduction by isogamy or anisogamy

External References:

https://www.britannica.com/science/algae

https://www.discovermagazine.com/planet-earth/20-things-you-didnt-know-about-algae

https://www.encyclopedia.com/plants-and-animals/microbes-algae-and-fungi/moneran-and-protistan/algae

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