Algae are one of the major types of microscopic organisms that can be seen with our naked eyes and are commonly found in our surroundings. Algae belong to a group of predominantly aquatic photosynthetic organisms of the simplest types. Algae do not possess true roots, stems, leaves, or leaf-like organs, as they lack a well-defined body structure. They are often placed in the division Thallophyta along with fungi. However, algae show many differences from fungi. Such as they differ from fungi regarding the presence of the photosynthetic pigment chlorophyll and also in their mode of nutrition. The most common types of algae are autotrophic in nature, which means they synthesise their food by themselves. However, heterotrophic and holozoic modes of nutrition are also widely seen in algae. 

The general characteristics of algae include: 

1. They are mostly chlorophyll-containing autotrophic thalloid plants with a well-determined thallus organisation, so they are photosynthetic organisms. 
2. They occur in a variety of habitats, but the majority of them are aquatic in nature and are found in conditions where there is adequate moisture present. 
3. The body of the algae does not differentiate into various types of tissue systems. 
4. They are eukaryotic organisms. 

Algae have mostly unicellular sex organs without a jacket of sterile cells around them. If jacket cells are present, they have different initials, and this mechanism leads to the progressive complexity in reproduction. Therefore, such organisms are either unicellular or multicellular in nature on the basis of cellular complexity. 

The reproduction in algae takes place by vegetative, sexual and asexual methods. However, at the time of reproduction, the embryo is not formed after the gametic union. One of the distinguishable features of algae is that they show distinct alternation of generations.

Algae are present in the whole universe as they are found in a variety of habitats such as seawater, freshwater, snow, rocks, over the plant body, surface of an animal body, etc. Out of these habitats, aquatic forms are most common. Due to the diversity of habitats, algae are present in large numbers in our surroundings and play an important role. 

Examples of Algae

On the basis of habitat, algae may be classified into three groups:

1. Aquatic Algae

Aquatic algae are found both in freshwater as well as in saline water of the sea. 

1. Freshwater forms occur where there is low or no salinity, such as ponds, rivers, lakes, pools, etc. Examples of freshwater algae include Oedogonium, Ulothrix, Cladophora, and Chara. These species of algae are found in slow-running water, while species like Volvox, Hydrodictyon, Chlamydomonas, and Spirogyra are found in stationary water. 

1. Marine forms of algae occur in the high salinity of seawater. They include the members of Phaeophyceae (Sargassum, Fucus, Ectocarpus) and Rhodophyceae. 

The aquatic algae again can be free-floating or attached to a substratum with the help of an attachment disc, known as holdfasts. Free-floating algae are Spirogyra, Volvox, Chlamydomonas, while the attached algae are Ulothrix and Oedogonium. Such free-floating algae form colonies in association with other similar organisms on the surface of the water, which are called water blooms or phytoplanktons. 

2. Terrestrial Algae

Varieties of algae are found in terrestrial habitats like soil, rocks and logs. The forms of algae that are found on the surface of the soil are known as saprophytes. Examples include Vaucheria, Euglena, Botrydium, etc. The forms of blue-green algae, which occur under the surface of the soil, are called cryptophytes. Examples include Anabaena, and Nostoc. 

3. Unusual Habitats Algae

These types of algae are found in uncommon habitats:

• Some algae have the ability to withstand a high concentration of salts and are called halophytes algae. Examples include Stephanoptera, Dunaliella, Chlamydomonas, etc. 
• Some algae have the ability to grow in support of others and are called epiphytic algae. Examples include Oedogonium, Cladophora, Bulbochaete, etc. 
• Some algae have the ability to grow on the body of animals and are called Epizoic algae. Examples include Cladophora crispata growing on the body of a snail, Stigeoclonium growing on the gills of fishes. 
• Some algae have the ability to grow on the tissues of animals and are called Endozoic algae. Examples include Zoochlorella is found in the tissue of Hydra Viridis.  

Algae also grow as parasites on many plants and animals. An example includes Cephaleuros virescens, a member of the Chlorophyceae family that causes red rust in tea, coffee, guava, mango leaves. They are known as parasitic algae. 

Identification of Algae under Microscope

Algae can be identified by using a microscope. A whole procedure is carried out in order to determine and identify algae:

• Collect specimens from any reference site where there is an excessive amount of algae present. 
• Bring the specimen in the laboratory for analysis and identification. 
• Put a very thin drop of the collected specimen on a microscope slide.
• Gently cover the slide with the help of a coverslip, and observe it under a microscope. 

The range of the microscope should be 40X or 100X magnification. Under microscopic magnification, we can see some roots or indistinguishable leaves, observe its texture on rubbing with hands, if it feels like silky and smooth substances, it must be green algae. It contains chlorophyll pigment a, b and xanthophylls. It is mostly unicellular. If the colour of the algae is yellow and the texture is a little bit dusty or rough, it must be blue-green algae. If the colour of the algae is red and the texture is neither silky nor dusty, it must be red algae. The chlorophyll pigment is a, b and phycobilins. It is mainly multicellular. 

Conclusion 

Algae may be defined as a group of mostly aquatic chlorophyll-containing thalloid plants. They lack true roots, stems, and leaves. Algae are either unicellular or multicellular. Based on the type of habitats, they are classified into aquatic algae, terrestrial algae, and other unusual habitats including halophytes, epiphytes, epizoic, endozoic, and parasitic algae. Algae are also classified with the help of a microscope on the basis of their colour, texture, chlorophyll pigment, and cellular complexities.