Algae are a diverse group of aquatic eukaryotic microorganisms that are capable of photosynthesis, and belonging to the kingdom Protista. Their sizes range from tiny species of Micromonas to giant kelps reaching up to 200 feet in length.

Algae has many different types of life cycles, and some algae are easily recognizable to most people; for example, pond scum, algae blooms, phytoplankton, and seaweed.

The pigments in algae are more varied than those of plants. Their cells have features not seen in animals or plants. Algae are known producers of oxygen and are a source of food to aquatic life.

They are also a source of crude oil, food for humans, and pharmaceutical and industrial raw materials, which makes them economically important.

Although algae is an umbrella term for different organisms capable of producing oxygen via photosynthesis, the organisms are not necessarily closely related.

However, certain distinguishing features unite them while at the same time differentiating algae from other photosynthetic organisms like terrestrial plants.

According to the journal Current Biology, many algae are unicellular organisms. They occur in a variety of forms and sizes capable of living as single microscopic cells or in colonies. They can also take a leafy appearance as seen in seaweeds.

Also, algae are as differentiated as; they lack stems, true roots, and a vascular system for the transport of nutrients and water throughout their bodies.

Although cyanobacteria – also known as blue-green algae – are informally grouped with algae, they differ from the true algae. Cyanobacteria lack the photosynthetic pigment known as chlorophyll. They also lack cell walls made of cellulose and do not store energy as starch.

Algae are typically found in diverse aquatic environments. They can thrive in both saltwater bodies and freshwater rivers and lakes. They are also capable of withstanding a range of acidity, temperatures, turbidity, and oxygen concentrations.

For instance, the giant kelp has been found in depths of more than 200 meters below polar ice sheets. Also, according to the Journal of Biological Research, the alga Dunaliella salina has been observed in hypersaline environments like the Dead Sea.

Algae has also been able to survive in terrestrial environments. They have been found in hot springs, tree trunks, snowbanks, and animal fur.

They could occur as independent cells or as a colony. They are also capable of forming symbiotic relationships with non-photosynthetic organisms, such as fungi (lichens), sponges, and mollusks.

Generally, algae are capable of utilizing the process of photosynthesis to manufacture their food. They can get nutrients by using radiant energy from the sun and carbon dioxide in other to produce oxygen and carbohydrates.

Algae are typically autotrophic organisms. However, several species of heterotrophic algae obtain nutrients from external sources. These species have evolved, developing means by which they can acquire nutrients from organic sources containing carbohydrates, fats, and proteins.

Examples include Galdieria sulphuraria, Oxyrrhis marina, Polykrikos kodoidii, and Stoeckeria algicida. Algae can reproduce through sexual and asexual methods.

The sexual reproduction of algae involves the joining of gametes from individual parent cells via meiosis, while the asexual reproduction of algae involves the production of a motile spore.

Algae are also capable of reproducing via vegetative means, which involves the division of cells – also known as mitosis.

Types of Algae

Golden-brown algae (Chrysophyta)

This family of algae accounts for over 1000 species of unicellular algae. They occur in both marine and freshwater, although they are more common in marine habitats.

They possess cellulose cell walls that are heavily imbued with silica, making it quite rigid and decay-resistant. Golden-brown algae store energy in oil droplets and also in carbohydrates known as leucosin.

They are capable of movement by using one or two flagella. They possess chlorophyll, which enables photosynthesis. They also include other pigments such as xanthophylls and carotenoids.

Green Algae (Chlorophyta)

Green algae number up to 7,000 species, most of which are often found in freshwater and some marine habitats.

They are mostly microscopic except for a few that are macroscopic and multicellular. The cells of the green algae are made up of mostly cellulose, with calcium carbonate being incorporated as well in some species.

They store food as starch and possess flagella for movement. They also possess chlorophyll for photosynthesis, as well as carotenoids and xanthophylls.

Examples of green algae include Chlorella and Chlamydomonas. Other examples include Volvox, Gonium, and Cladophora, all of which occur in colonies.

Euglenoids (Euglenophyta)

These are about 800 species of unicellular algae. They are the least-algae like of the algae family. They are freshwater dwelling organisms that lack a true cell wall and possess a protein covering called the pellicle.

Euglenoids have up to three flagella for locomotion and they store energy as paramylon. The pigments present in euglenophytes include chlorophyll, carotenoids, and xanthophylls.

These organisms usually possess chloroplasts and are photosynthetic. However, some species of euglenoids are heterotrophic, feeding on organic matter suspended in water.

Fire Algae (Pyrrophyta)

Pyrrophytta accounts for about 1,110 species of unicellular algae. They are mostly found in marine habitats and sometimes in freshwater.

These organisms possess two flagella and have a cell wall made up of glucose. They store energy as starch and contain pigments like chlorophyll, xanthophylls, and carotenoids.

An abundance of fire algae can cause ‘red tides’, a phenomenon in which the color of the water body is changed to red. These red tides are said to be toxic to marine animals due to the presence of chemicals that are synthesized by the algae.

Some species are also capable of bioluminescence, which, when seen at night, causes the surface of the water body to look as if it were on fire.

Red Algae (Rhodophyta)

Red algae number about 4,000 species of mostly seawater algae. Examples of these species coralline algae, such as Porolithon spp., filamentous species like Plenosporum spp., and thalloid species such as the Irish moss.

The cells of the red algae are constructed with cellulose and polysaccharides like agar and carrageenan. The size of these algae range from microscopic to macroscopic, and they store energy as a polysaccharide known as floridean starch.

The photosynthetic pigment of these species is chlorophyll, and the accompanying pigments are xanthophylls, phycobilins, and carotenoids.

Yellow-green Algae (Xanthophyta)

Yellow-green algae are about 450 species that majorly occur in freshwater habitat. They occur as unicellular organisms or in small colonies.

Their cell walls are constructed with pectic and cellulose compounds and sometimes contain silica. These species store energy as leucosin and possess two or more flagella for movement.

The primary photosynthetic pigment of xanthophytes is chlorophyll, and the accessory pigments are xanthophyll and carotenoids.

Brown Algae (Phaeophyta)

There are about 1,500 species of brown algae, most of which occur in marine habitats. They are especially ubiquitous in cool waters.

Brown algae species are very large and can reach lengths of more than ten meters. Their cell walls consist of cellulose and polysaccharides known as alginic acid. The tissues of brown algae species are typically more differentiated in complexity than other species.

They contain air bladders for buoyancy, anchors that secure the algae to the substrate, and wide blades with large surface area for nutrient absorption and photosynthesis. They also possess specialized cells for reproduction.

Examples of brown algae include kelps, sargassum weed, rock weeds, and giant kelps.