What is Alage? In 1753 Linnaeus first, introduce the term Algae, it is a Latin word (seaweeds) which means the Hepaticae. Algae are the most prominent group of Thallophyta which is the primitive and most simplistic division of the plant kingdom. Where Thallos means a sprout and phyton means a plant.

What is Algae?

• Algae is a large and diverse group of photosynthetic eukaryotic organisms, which contains both unicellular and multicellular organisms. They contain chlorophyll as their primary photosynthetic pigment and they absent a sterile covering of cells around their8 reproductive cells. 
• For example, Chlorella and the diatoms are unicellular whereas giant kelp, a large brown alga is multicellular.

Characteristics of Algae

• Most of them are aquatic and autotrophic.
• They are either unicellular or multicellular organisms.
• They mostly Inhabitat where adequate moisture is present.
• Some of them lack different cell and tissue types, such as stomata, xylem, and phloem, which are present in land plants.
• Algae can Inhabitat in both seawater and freshwater. For example, seaweeds are known as the most complex and largest marine algae, whereas Charophyta is the most complex form of freshwater algae.
• Algae contain chlorophyll for their photosynthesis.
• They lack the sterile covering of cells around their reproductive cells.
• The algae group is consists of both unicellular and multicellular organisms.
• Green algae contain endosymbiotic cyanobacteria derived primary chloroplasts.
• Diatoms and brown algae have endosymbiotic red alga derived secondary chloroplasts.
• They reproduced by both asexual and sexual forms.
• Many algae get their organic carbon from either by osmotrophy, mixotrophy, or phagotrophy.
• Some algae form symbiotic relationships with other organisms.
• The vascu­lar tissue is lacking in multicellular complex and they exhibit little differentiation of tissues.
• Algae contain cell walls which is composed of glycoproteins and polysaccharides such as carrageenan, cellulose, hemicellulose, and pectins.

The occurrence of Algae

Algae are present all over the earth, they are present everywhere which means they are ubiquitous. 

Algae can be found in freshwater, marine water, on soil, on a rock, as epiphytes or parasites on plants and animals, in hot springs, in the desert, on permanent snow-fields, etc. Most of the algae are found in aquatic environments.

Based on the habitat the algae is classified into different groups such as;

1. Aquatic algae.
2. Terrestrial algae
3. Algae of remarkable habitats.

1. Aquatic Algae:

• These types of algae can be found in either freshwater or marine. Freshwater has a low salinity of  10 ppm whereas the Marin water has a salinity of 33-40%.
• Freshwater algae can be found in ponds, lakes, tanks, ditch­es, etc.
• The algae which are suspended on the upper part of freshwater is known as planktonic whereas at the benthic algae are known as bottom-dwellers.
• The neustonic algae are grown at the air-water interface.

Example:

• Freshwater Algae: Chlamydomonas, Volvox, Ulothrix, Chara, Oedogonium, Spirogyra, Nostoc, Oscillatoria, etc.
• Marin water:  Sargassum, Laminaria, Ectocarpus, Polysiphonia, Caulerpa, Bangia, Padina, etc.

2. Terrestrial algae

• Terrestrial algae are growing on soils,’ rocks, logs, etc.
• Those algae that are growing on the soil surface are called saprophytes and those are growing under the soil is known as cryptophytes.
• Terrestrial algae are classified into different groups based on the types of habitat such as;
  • Endedaphic: They Inhabitat in soli.
  • Epidaphic: They inhabitant on the soil surface.
  • Hypolithic: They grow on the lower surface of the stones on the soil.
  • Chasmolithic: They grow on rock fissures.
  • Rndolithic algae: They penetrate the rock.

Example: Osci­llatoria Sancta, Vaucheria geminata, Chlorella lichina, Euglena sp., Fritschiella sp., and Phormidium sp.

3. Algae of remarkable habitats.

Some algae are grown on uncommon habitat they are included in this group;

• Halophytic Algae (or Eurhaline)
  • Halophytic Algae Inhabitat in highly concentrated salt lakes. 
  • Example: Chlamydomonas ehrenbergli, Dunaliella and Stephanoptera sp.
• Symbiotic Algae
  • These are grown in association with other organisms such as fungi, bryophytes, gymnosperms or angiosperms, etc.
  • When algae grow in association with fungi is known as lichen.
  • Example: Nostoc, Gloeocapsa, Rivularia; the members of Cyanophyceae and Chlorella, Cytococcus, Pleurococcus; the members of Chlorophyceae.
• Cryophytic Algae
  • Cryophytic Algae Inhabitat on ice or snow and are responsible for attractive colors to snow-covered mountains.
  • Example:  Haemotococcous nivalis is responsible for the red color on alpine and arctic mountains, Chlamydomonas yellowstonensis responsible for green snow in Europe, Scotiella nivalis and Raphidonema brevirostri responsible for black colouration of snow, whereas Ancyclonema nordenskioldii  generate brownish purple colouration.
• Thermophytes or Thermal Algae
  • Thermophytes or Thermal Algae grow in hot water springs where the temperature is around 50- 70°C.
  • Example: blue-greens such as Oscillatoria brevis, Synechococcus elongates, Heterohormogonium sp. are grown in such hot springs.
•  Lithophytes
  • Lithophytes Inhabitat on the moist surface of stones and rocks.
  • Example: Nostoc,. Gloeocapsa, Enteromofpha, Batrachospermum etc.
• Epiphytic Algae

Epiphytic Algae Inhabitat on other plants including other algal members such as;

• Algae on Algae
  • Ptilota plumosa and Rhodymenia pseudopalmatta grow on Laminaria hyperborean
  • Diatoms grow on Oedogonium, Spirogyra etc.
• Algae on Angiosperms
  • Cocconis, Achnanthes etc. grow epiphytically on Lemna.
  • Trentepohlia grows on the barks of different angiosperms plants 
• Algae on Bryophytes
  •  Nostoc, Oscillatoria, diatoms Inhabitat on different bryophytes.
• Epizoic Algae
  • Epizoic Algae Inhabitat on animals like fish, snail etc.
  • Example: Stigeoclonium are grow in the gills of fishes.
• Endozoic Algae
  • Endozoic Algae Inhabitat within the animal tissues.
  • Example:  Zoochlorella sp. is grown in Hydra viridis.
• Parasitic Algae
  • Parasitic Algae are grown parasitically on various plants and animals.
  • Example: Cephaleuros (Chlorophyceae) Inhabitat on leaves of tea (Camellia sinensis), coffee (Coffea arabica), Rhododendron, Magnolia and pepper (Piper nigrum), Rhodochytrium (Chlorophyceae) Inhabitat on ragweed (Ambrosia) leaves, Phyllosiphon (Chlorophyceae) Inhabitat on the leaves of Arisarum vulgare, Ceratocolax (Rhodophyceae) Inhabitat in Phyllophora thallus.
• Psammon
  • They are mainly Inhabitat on sandy beaches.
  • Example: Vaucheria, Phormidium etc.

Further Reading

• What is bacteria? Types, Structure, Shapes, Morphology, Nutrition, Growth, Habitat, Reproduction, Examples.
• Prions: Definition, Structure, Function, Disease, Prevention, Control, Treatment, Transmission.
• Viroids Definition, Infection, Structure, Examples, Diagnostic, Origin, Disease.
• Laboratory Diagnosis of Viral Infections