MICROALGAE: CHARACTERISTICS, CLASSIFICATION AND APPLICATIONS - BIOLOGY - 2025

The microalgae are eukaryotic organisms, photoautotrophs, ie obtain energy from light and synthesize their own food. They contain chlorophyll and other accessory pigments that give them great photosynthetic efficiency.

They are unicellular, colonial -when they are established as aggregates- and filamentous (solitary or colonial). They are part of phytoplankton, along with cyanobacteria (prokaryotes). Phytoplankton is the set of photosynthetic, aquatic microorganisms that float passively or have reduced mobility.

Figure 1. Volvox (spherical) Source: Frank Fox, via Wikimedia Commons

Microalgae are found from the terrestrial Ecuador to the polar regions and are recognized as a source of biomolecules and metabolites of great economic importance. They are a direct source of food, medicine, forage, fertilizers and fuel, and are even indicators of contamination.

characteristics

Producers who use sunlight as an energy source

Most microalgae are green in color because they contain chlorophyll (tetrapyrrolic plant pigment), a photoreceptor of light energy that allows photosynthesis to be carried out.

However, some microalgae have red or brown coloration, because they contain xanthophylls (yellow carotenoid pigments), which mask the green color.

Habitats

They inhabit various sweet and salty, natural and artificial aquatic environments (such as swimming pools and fish tanks). Some are capable of growing in the soil, in acidic habitats and within porous (endolytic) rocks, in very dry and very cold places.

Classification

Microalgae represent a highly heterogeneous group, because it is polyphyletic, that is, it groups species descendants of different ancestors.

To classify these microorganisms, various characteristics have been used, among which are: the nature of their chlorophylls and their energy reserve substances, the structure of the cell wall and the type of mobility they present.

Nature of its chlorophylls

Most of the algae present chlorophyll type A and a few present another type of chlorophyll derived from it.

Many are obligate phototrophs and do not grow in the dark. However, some grow in the dark and catabolize simple sugars and organic acids in the absence of light.

For example, some flagellates and chlorophytes can use acetate as a source of carbon and energy. Others assimilate simple compounds in the presence of light (photoheterotrophy), without using them as an energy source.

Carbon polymers as an energy reserve

As a product of the photosynthetic process, microalgae produce a wide variety of carbon polymers that serve as an energy reserve.

For example, the microalgae of the Chlorophyta division generate reserve starch (α-1,4-D-glucose), very similar to the starches of higher plants.

Cell wall structure

The walls of microalgae have a considerable variety of structures and chemical composition. The wall can be made up of cellulose fibers, usually with the addition of xylan, pectin, mannan, alginic acids or fucinic acid.

In some calcareous or coralline algae, the cell wall shows calcium carbonate deposition, while others have chitin.

Diatoms, on the other hand, have silicon in their cell wall, to which polysaccharides and proteins are added, forming shells of bilateral or radial symmetry (frustules). These shells remain intact for a long time, forming fossils.

Euglenoid microalgae, unlike the previous ones, lack a cell wall.

Type of mobility

Microalgae can have flagella (like Euglena and dinoflagellates), but never have cilia. On the other hand, some microalgae show immobility in their vegetative phase, however, their gametes can be mobile.

Biotechnological applications

Human and animal feeding

In the 1950s, German scientists began to grow microalgae in bulk to obtain lipids and proteins that would replace conventional animal and plant proteins, in order to cover the consumption of livestock and humans.

Recently, the massive cultivation of microalgae has been projected as one of the possibilities to combat world hunger and malnutrition.

Microalgae have unusual nutrient concentrations, which are higher than those observed in any higher plant species. A daily gram of microalgae is an alternative to supplement a deficient diet.

Advantages of its use as food

Among the advantages of using microalgae as food, we have the following:

• High speed of microalgal growth (they present a yield 20 times higher than soybeans per unit area).
• It generates benefits measured in the "hematological profile" and in the "intellectual status" of the consumer, when consuming small daily doses as a nutritional supplement.
• High protein content compared to other natural foods.
• High concentration of vitamins and minerals: the ingestion of 1 to 3 grams per day of microalgae by-products, provides appreciable amounts of beta-carotene (provitamin A), vitamins E and B complex, iron and trace elements.
• Highly energizing nutritional source (compared to ginseng and pollen collected by bees).
• They are recommended for high intensity training.
• Due to its concentration, low weight and ease of transport, the dry extract of microalgae is suitable as a non-perishable food to store in anticipation of emergency situations.

Figure 2. Arthrospira is a widely used and mass-cultured cyanobacterium. Source: Joan Simon, cropped by Perdita (English Wikipedia User), via Wikimedia Commons

Aquaculture

Microalgae are used as food in aquaculture due to their high protein content (from 40 to 65% in dry weight) and their ability to increase the color of salmonids and crustaceans with their pigments.

For example, it is used as food for bivalves in all their growth stages; for larval stages of some species of crustaceans and for early stages of some species of fish.

Pigments in the food industry

Some microalgae pigments are used as additives in forages to increase the pigmentation of chicken meat and egg yolks, as well as to increase the fertility of cattle.

These pigments are also used as colorants in products such as margarine, mayonnaise, orange juice, ice cream, cheese, and bakery products.

Figure 3. Tubular photobioreactors, used to obtain high-value compounds from microalgae. Source: IGV Biotech, from Wikimedia Commons

Human and veterinary medicine

In the field of human and veterinary medicine, the potential of microalgae is recognized, because:

• They reduce the risk of various types of cancer, heart and ophthalmic diseases (thanks to their lutein content).
• They help prevent and treat coronary heart disease, platelet aggregation, abnormal cholesterol levels, and are also highly promising for the treatment of certain mental illnesses (due to their omega-3 content).
• They have antimutagenic action, stimulating the immune system, reducing hypertension and detoxifying.
• They have anticoagulant and bactericidal action.
• They increase the bioavailability of iron.
• Medicines based on therapeutic and preventive microalgae have been generated for ulcerative colitis, gastritis and anemia, among other conditions.

Figure 4. Flat photobioreactor: used to obtain high added value microalgae by-products and in experimentation. Source: IGV Biotech, from Wikimedia Commons

Fertilizers

Microalgae are used as biofertilizers and soil conditioners. These photoautotrophic microorganisms quickly cover the disturbed or burned soils, reducing the risk of erosion.

Some species favor nitrogen fixation, and have made it possible, for example, to grow rice on flooded land for centuries, without the addition of fertilizers. Other species are used to replace lime in compost.

Cosmetics

Microalgae derivatives have been used in the formulation of enriched toothpastes, which eliminate the bacteria that cause dental caries.

Creams that include such derivatives have also been developed for their antioxidant and ultraviolet-protective properties.

Figure 5. Maintenance of microalgae in banks or strains. Source: CSIRO

Sewage treatment

Microalgae are applied in processes of transformation of organic matter from wastewater, generating biomass and treated water for irrigation. In this process, the microalgae provide the necessary oxygen to aerobic bacteria, degrading organic pollutants.

Pollution indicators

Given the ecological importance of microalgae as primary producers of aquatic environments, they are indicator organisms of environmental pollution.

In addition, they have great tolerance to heavy metals such as copper, cadmium and lead, as well as chlorinated hydrocarbons, which is why they can be indicators of the presence of these metals.

Biogas

Some species (for example, Chlorella and Spirulina), have been used to purify biogas, since they consume carbon dioxide as a source of inorganic carbon, in addition to simultaneously controlling the pH of the medium.

Biofuels

Microalgae biosynthesize a wide range of commercially interesting bioenergetic by-products, such as fats, oils, sugars, and functional bioactive compounds.

Figure 6. Carousel-type microalgae cultivators, used in mass cultivation of microalgae for the cosmetic and food industry. Source: JanB46, from Wikimedia Commons

Many species are rich in lipids and hydrocarbons suitable for direct use as high-energy liquid biofuels, at levels higher than those present in terrestrial plants, and also have potential as substitutes for refinery products of fossil fuels. This is not surprising, considering that most of the oil is believed to have originated from microalgae.

One species, Botryococcus braunii, in particular, has been extensively studied. The oil yield from microalgae is forecast to be up to 100 times that of land crops, from 7,500-24,000 liters of oil per acre per year, compared to rapeseed and palm, at 738 and 3690 liters, respectively.

References

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