**4. Algae**

Cyanobacteria (blue–green algae) are Gram-negative oxygenic photosynthetic autotrophs that are among the most successful and oldest living organisms on the planet [26, 27]. The majority of oxygen in the early atmosphere originated from cyanobacteria's oxygenic photosynthesis [27]. They are important primary producers on a global scale and play important roles in nitrogen, carbon, and oxygen biogeochemical cycles (30% of the annual oxygen production on earth) [28, 29].

They are the organisms that deliver oxygen to the earth and hence played an important part in the evolution of life. Some cyanobacteria have a unique biological mechanism (which combines N2-fixation and oxygenic photosynthesis) and can be used as a model to research significant biological activities or capabilities. Unicellular, colonial, filamentous, and branched filamentous forms are all included [30]. They are broken down into five pieces [31].

Cyanobacteria are also responsible for the origin of eukaryotic plant life on the planet, as the chloroplast of eukaryotic cells is descended from a cyanobacterial predecessor. It is a filamentous Cyan bacterium that belongs to the Cyanophyta class of algae. *Spirulina* was deemed "the best for tomorrow" by the United Nations (UN) world food conference, and it has gained appeal as a nutritional supplement in recent years [32].

The utilization of cyanobacteria as a nontraditional food and protein source appears to be promising [33–35]. Extremophyles are cyanobacteria that live in severe settings, such as *Spirulina* (alkalophilic), Extremophyle mass cultures are expected to be free of microbial contamination due to their high needs, avoiding a serious problem in outdoor cultures [34].

Pigments, such as chlorophyll a, carotenoids, and phycobiliproteins, are abundant in cyanobacteria. *Spirulina* phycocyanin buffer extract is utilized in eye shadow, eyeliner, and lipsticks. Because the product is water-insoluble, it does not fade or irritate the skin when exposed to water or sweat [36]. Cyanobacteria manufacture carbohydrates, particularly the compatible solutes glucosyl glycerol, trehalose, and sucrose, under various osmotic conditions.

In both animals and people, cyanobacteria can help lower cholesterol levels. When a high cholesterol meal was supplemented with cyanobacteria, the levels of total cholesterol, low-density lipoprotein, and very low-density lipoprotein cholesterol in rat serum were lowered. Mollusks, fish, and crabs feed on cyanobacteria. It has been found that the cyanobacterium *Spirulina* not only increases protein content but also improves the color of fish flesh. Cyanobacteria, in conjunction with bacteria, perform a crucial function in regulating the water body's O2 and CO2 balance, supporting aquaculture. Cyanobacteria assist in the removal of phosphate and nitrogen from polluted water while also producing biomass.

Cyanobacteria are vulnerable to unexpected physical and chemical changes in environmental factors, such as light, salinity, temperature, and nutrient constraint, in their native habitat. *Spirulina* is prokaryotic cyanobacteria that are spirally coiled or filamentous and have a lot of similarities morphologically (as shown in **Figures 1** and **2**) [37]. The loosely coiled trichomes of varied width with cross-walls, visible in light microscopy, are the most distinctive feature of *Spirulina* [31]. The morphology of these related strains has traditionally been used to distinguish them: helix type, distribution of pores in the cell wall, appearance of septa under light microscopy, and trichome diameter and fragmentation type [38].

The cyanobacterium *Spirulina* platensis is cultivated commercially as a possible source of proteins and medicines. Diatoms, dinoflagellates, green and yellow–brown flagellates, and blue–green algae are all examples of phytoplankton.

These groups, as photosynthetic organisms, play a critical role in ocean productivity and form the foundation of the marine food chain. *Spirulina* and *Chlorella* are two alga genera that require special attention because of their value as human food and *in vitro* and/or *in vivo* antioxidant capacity. These algae can be widely farmed to produce

**Figure 1.** *Spirulina in natural shape.*

**Figure 2.** *Spiral shaped of Spirulina.*

#### *Microalgae and Fish Nutrition DOI: http://dx.doi.org/10.5772/intechopen.105028*

a protein-rich material for alimentary (diet supplementation) or industrial usage (blue pigments, emulsifiers, thickening, and gelling agent) [39, 40].

Due to a wide spectrum of vital elements, such as vitamins, minerals, and proteins, *Spirulina*'s chemical makeup suggests that it has a high nutritional value [41]. Microalgae play a vital function in aquaculture as a source of zooplankton for fish and larvae to eat [42]. *Spirulina* may boost carotenoid and pigment levels, according to a study by Lu et al. [43].

Furthermore, the usage of *Spirulina* meal in the animal feed business is growing [44, 45]. Aquaculture of macro- and microalgae is a lucrative global business. Macroalgae are grown for both their hydrocolloids and their nourishment.

In the commercial rearing of many aquatic species, microalgae are an essential food source and feed supplement. Algae are the natural food source for these creatures, therefore their relevance in aquaculture is unsurprising, only a few algae species contain components that have antioxidant properties. It has been reported that including *Spirulina* in ayu's feed produces in better flavor, firmer flesh, and brighter skin color. Other research has found that a 5% dietary *Spirulina* supplement reduces muscle lipids and improves the flavor and texture of striped bass jack.

*Spirulina* has been identified as a potential protein source for fish feed. Earlier research looked into how adding dry *Spirulina* powder to a diet changes the taste and quality of fish. *Spirulina* supplementation in freshwater fish feed has been shown to improve growth and promote gonad development and maturation, according to other studies.

Antioxidants from marine organisms, including alga extracts from several species, were studied. Many algae species have been shown to be powerful antioxidants. Due to a wide spectrum of vital elements, such as vitamins, minerals, and proteins, *Spirulina*'s chemical makeup suggests that it has a high nutritional value. Aztecs have been collecting and using *Spirulina* (now known as Arthrospira) [34].

Externally, *Spirulina* is used as a poultice to treat certain disorders. The International Association of Applied Microbiology designated *Spirulina* as a "great future food source" in 1967. While no microbe ever delivered on its promise of inexpensive protein, *Spirulina* continues to spur research and production, owing to its claimed nutritional benefits [46].

The ability of this microbe to use ammonia as a nitrogen source at high alkaline pH values may be due to a comparatively high cytoplasmic pH (4.2–8.5). *Spirulina* contains a higher percentage of high-quality protein (59–65%) than other regularly used plant sources such as dried soybeans (35%), peanuts (25%), or cereals (8–10%). Due to the absence of cellulose in its cell walls (as is the case for eukaryotic green microalgae, such as *Chlorella*, Ankistrodesmus, Selenastrum, Scenedesmus), *Spirulina* has a unique value: after 18 hours, more than 85% of its protein has been digested and assimilated.

*Spirulina* is also a common ingredient in ornamental fish feed, such as carp, because it improves coloration. Algal carotenoids may also operate as a growth factor, which could lead to yet another use for algae in aquaculture diets. Because of its incredible ability to generate high-quality concentrated food, cyanobacteria, particularly *Spirulina*, is being developed as the "Food of the Future." *Spirulina* is said to offer a full protein content of 65–70%, with all essential amino acids in perfect balance.

When the necessary circumstances for producing *Spirulina* can be attained, culturing this organism is not difficult. *Spirulina*, on the other hand, has a high protein and vitamin content despite its low protein output by an order of magnitude. 20 g dried *Spirulina* offers 100% of the recommended daily allowance of vitamin B12, 70% of the recommended daily allowance of thiamine, 50% of the recommended daily allowance of riboflavin, and 12% of the recommended daily allowance of niacin. *Spirulina* also has a high level of p-carotene (provitamin A) and important unsaturated fatty acids, which are both beneficial nutritionally. For the feeding of artificially grown clams, a semi-commercial concept on Cape Cod, USA, uses three different and relatively pure algae cultures in unheated water. The three species employed enable seasonal changes in growing conditions to be compensated for. After that, the algae is diluted with seawater and circulated through the hatchery beds, where the clams filter feed to get the protein source. Algae has also been discovered to give a growth factor to the larvae's culture media, improving their survival and growth.

The following are some of the primary advantages of using *Spirulina* in aquaculture, according to their promotional literature:


Due to the unpopularity of artificial dyes, one of the key areas of research into the aquacu1tural relevance of *Spirulina* has been the color improvement potential. *Spirulina* is used to improve the color of ornamental koi carp, trout, salmon, and shrimp, sweet smelt, red tilapia, and the striped jack [48].

The high production costs of pure-culture-produced biomass have hampered the use of *Spirulina* as a protein and pigment source in aquaculture. As a result, the algae are either utilized as a beginning feed for larvae or as a specialty diet for adults (e.g., for color enhancement in ornamental fish). Given *Spirulina*'s nutritionally complete nature, it appears that if production costs could be kept to a low, this cyanobacterium might provide a novel feed source for aquaculture creatures.

*Spirulina* was named "The Best for Tomorrow" by the United Nations World Food Conference, and it has gained appeal as a food supplement in recent years. *Spirulina*, planktonic blue–green microalgae, has been proposed as a future food source that is both acceptable and safe. Due to its antioxidant, anti-inflammatory, antimetastasis, and blood cholesterol-lowering properties, it has recently been considered a source for possible medicines. *Spirulina*, for example, increased interferon production and natural killer cell activity when given orally [49].

Aquacultural systems based on microalgae and their animal consumers, which can be considered an indirect use of microalgae in human food, have been far more successful thus far, however, the uptake of microalgal biomass by commercially

#### *Microalgae and Fish Nutrition DOI: http://dx.doi.org/10.5772/intechopen.105028*

important filter-feeders is very promising from an energetic standpoint. Microalgae are the biological starting point for energy transfer in most aquatic ecosystems and are, hence, the foundation of many aquaculture operations' food chains.

*Spirulina* is also used in fish farming, primarily for colored fishes [50], as a good source of antioxidant pigments, such as carotenoids, lutein, astaxanthin, zeaxanthin, and others, for intracellular protection of fish larvae against various diseases as well as the bright coloration of fishes [50]. *Spirulina* supplementation has been shown to prevent ischemic brain damage [51].

Despite its widespread distribution and economic importance, little is known about the feeding ecology of the common carp in natural settings. The influence of this cyprinid species on macrophytes has been well described, as has the functional anatomy of its feeding mechanism.

However, the majority of diet research has been conducted in fish culture ponds. The risk of consuming *Spirulina* was evaluated, and after a subchronic therapy, mice showed no harmful effects. *Spirulina* maxima oil extract or defatted fraction feeding reduced carbon tetrachloride-induced fatty liver growth in rats, showing a hepatoprotective activity. This lower plant group contains a large range of vitamins, colors, and practically all-important nutrients, including PUFA (polyunsaturated fatty acid), and is also a good source of proteins and carbs. A lot of algae have been validated over time due to their remarkable impact on fish development and vitality, but only about 40 genera have gained widespread use in aquaculture.

Furthermore, some *Spirulina* species lack a cell wall, resulting in enhanced digestion and absorption. A number of studies have previously reported that dietary inclusion of *Spirulina* improves fish growth [13]. The ability of *Spirulina* to act as an antiviral, anticancer, hypercholesterolemia, and health improvement agent is receiving interest as a nutraceutical and a possible pharmaceutical source. When *Spirulina* alga is fed to young prawns and fingerlings, the fish have good coloring, a low death rate, and a high growth rate. These studies have also discovered an increase in the amounts of linoleic acid, GLA, protein, and an improved color in the meat of the fish when compared to fish fed on standard instant feeds [52].

The use of plant products as protein sources in fish meals has a lot of potential for aquaculture around the world. *Spirulina* is multicellular, filamentous blue–green algae that have grown in popularity in the health food sector and is increasingly being included in people's diets. Because muscle protein deposition is the primary cause of growth in fish, the flow of amino acids (A.A) from diet to developing biomass must be maintained. Fish require a variety of essential elements, including protein, fat, carbohydrate, vitamins, and minerals, although these requirements differ depending on the species. In comparison to the basal diet, 1–10% *Spirulina* supplementation boosted growth rate (up to 1.5 times), survival rate, and feed efficiency. There was also evidence of illness resistance to bacterial infection.

Bermejo et al., [53] found that the biliproteins found in *Spirulina*, such as phycocyanin, are responsible for the majority of the antioxidant capacities of this microalga's protean extract, and they suggested that *Spirulina* could be used to make a natural dietary antioxidant supplement or added to healthy food products like cereals, fruit bars, or drinks to prevent some chronic diseases involving free radicals.

Furthermore, *Spirulina* is gaining popularity due to its bioactive components, which have antioxidant properties [51]. Supplementing with live *Spirulina* enhanced fish growth and feed utilization, which could be related to improved feed intake and nutrient digestibility. *Spirulina*, on the other hand, contains a number of nutrients, including vitamins and minerals, that may aid in the development of growth.

Increased fish appetite may have resulted in higher feed intake and improved growth in *Spirulina*-enriched diets, leading to better feed intake and growth. Changes in protein and lipid content in the fish body, on the other hand, could be linked to changes in their synthesis, muscle deposition rate, and/or different growth rates. Additionally, it works as an immunomodulator [54]. *Spirulina* platensis is more extensively dispersed and found primarily in Africa, Asia, and South America. Several studies have been undertaken using dried *Spirulina* as a feed supplement [55]. *Spirulina* has been shown to have an accessible energy content of 2.50–3.29 kcal/gram and a phosphorous availability of 41%.

*Spirulina* typically contains only trace amounts of zinc (21–40 g/g), although it is easily enhanced [56] (Azina®: 6000 g Zn/g). There are simple methods for obtaining zinc-rich *Spirulina* [46]. Magnesium is abundant in *Spirulina*, and its bioavailability is excellent [57]. *Spirulina* has been designated a national food in China [58].

It is said to have been consumed as food in Mexico during the Aztec civilization 400 years ago. It is still eaten by the Kanembu tribe in the Republic of Chad's Lake Chad region, where it is sold as dried bread known as "dihe." [59].

*Chlorella* is a freshwater single-celled microalga with a grassy odor. Its distinctive emerald-green hue and lovely grass odor are attributed to its high chlorophyll concentration, which is the highest of any known plant. The name "*Chlorella*" comes from the Latin words "chlor" which means "green" and "ella" which means "little." Its size ranges from 2 to 8 microns, making it only visible through a microscope. It is about the same size as a human red blood cell, but the shape is different: *Chlorella* is spherical, whereas human red blood cells are disc-shaped. *Chlorella* reproduces quickly, dividing into four new cells every 17–24 hours. This exceptional ability to reproduce indicates a high level of "qi," or life energy [60].

*Chlorella* spp. is being investigated as a potential source of a wide range of nutrients (carotenoids, vitamins, minerals) that are widely used in the healthy food industry, as well as in animal feed and aquaculture Gastric ulcers, wounds, constipation, anemia, hypertension, diabetes, newborn malnutrition, and neurosis are all problems that *Chlorella* spp. can help. Glycolipids and phospholipids are also thought to have antiatherogenic and antihypercholesterolemic properties, whereas glycoproteins, peptides, and nucleotides have antitumor properties. However, a beta-1,3-glucan, which is a strong immunostimulator, a free-radical scavenger, and a blood lipid reducer, appears to be the most important component in *Chlorella* spp. [61]. These groups, as photosynthetic organisms, play a critical role in ocean productivity and form the foundation of the marine food chain. Spirulina and *Chlorella* are two alga genera that require special attention because of their importance as human meals and *in vitro* and *in vivo* antioxidant capacity. These algae can be widely grown to produce a protein-rich material for alimentary (diet supplementation) or industrial usage (blue pigments, emulsifiers, thickening, and gelling agent) [39, 40].

At technical medium, *C. vulgaris* grew satisfactorily. Up to 10% phyto-s, 57.63% crude protein, 5.84% fat, 6.44 mg/gram beta-carotene, 4.12 mg/gram vitamin C, and 1.32 mg/gram vitamin E *Chlorella vulgaris* has the potential to be a natural and ASUH feed additive, and Phyto-s can be employed for mass production nutrition [62]. *C. vulgaris* is a spherical, unicellular microalga that grows in fresh water and has a diameter of 2–10 M. It grows quickly under ideal conditions and is resistant to invaders and the harsh climate. In the aqueous medium, light and CO2 are the bare minimum conditions for algae formation. Their development is expedited and targets the synthesis of a specific set of compounds by changing the medium and changing the circumstances [63].
