Microalgae exhibit a vast diversity, with thousands of species identified so far. They can be classified into different taxonomic groups based on their cellular characteristics, pigments, and other morphological features [4, 8, 12, 15, 16]. Some of the major taxonomic groups of microalgae include:

(1)

Bacillariophyceae: Diatoms are unicellular algae with intricate siliceous cell walls, known as frustules. They are highly diverse and contribute significantly to global oxygen production.

Thus, there is a vast number of species of microalgae which can be further exploited.

Microalgae possess several characteristics that make them attractive for various applications, including their role as natural antioxidant factories. Microalgae are photosynthetic organisms, utilizing sunlight to produce energy-rich compounds such as carbohydrates and lipids. During photosynthesis, they also produce antioxidants to protect themselves from ROS generated as byproducts of their metabolic processes [17, 18]. Many microalgae species have rapid growth rates, allowing for efficient biomass production in a relatively short time. This characteristic makes them suitable for large-scale cultivation and commercial use. Microalgae are also rich in proteins, essential fatty acids, vitamins, minerals, and other bioactive compounds. They have a huge potential to serve as a sustainable source of high-quality nutrition for humans and animals [19]. However, due to biochemical variation, there is a need to select the species by their growth potential, bio-chemical profiling, economic output, and sustainability.

Microalgae like Dunaliella (Figure 2a), Chlorella (Figure 2b), Haematococcus (Figure 2c), Scenedesmus (Figure 2d), and Trentepohlia (Figure 2e) (Chlorophyta), and Euglena (Euglenophyceae), produce a range of antioxidants, such as carotenoids (e.g., β-carotene, lutein, astaxanthin), tocopherols (vitamin E), and phycobiliproteins, produced by Porphyridium (Figure 2f) (Rhodophyta). These antioxidants can help neutralize free radicals and protect cells from oxidative damage, making them valuable for various health and cosmetic applications. In recent years, researchers and industries have been exploring microalgae’s potential as natural antioxidant factories, harnessing their ability to produce valuable antioxidants for various applications in food, pharmaceuticals, and cosmetics, among others [13]. Additionally, ongoing research aims to improve cultivation techniques and optimize antioxidant production to make microalgae-based products more economically viable and sustainable [20]. Microalgae play a crucial role in carbon fixation and oxygen production, contributing to global carbon and oxygen cycles. Additionally, they have the potential to be utilized in wastewater treatment and carbon capture technologies [21, 22].

Thus, as above mentioned, microalgae have a wide range of compounds, some of which have an innate antioxidant activity. These compounds play a significant role in protecting cells from oxidative stress and damage caused by ROS. Microalgae are rich in carotenoids, which are responsible for their characteristic colors (green, yellow, orange, red). Carotenoids, such as β-carotene, astaxanthin, lutein, and zeaxanthin, are potent antioxidants that can neutralize free radicals and protect cells from oxidative damage (see Table 1) [13, 20, 23].

Chlorophylls are essential pigments for photosynthesis in microalgae. Besides their role in photosynthesis, they also possess antioxidant properties, helping to scavenge free radicals and reduce oxidative stress [24]. Phycobiliproteins are water-soluble pigments found in some groups of microalgae, like Anabaena (Figure 3a), Nostoc (Figure 3b), and Tolypothrix (Figure 3c) (Cyanophyceae), and red microalga like Porphyridium. These pigments, including phycocyanin (Figure 4a) and phycoerythrin (Figure 4b), have strong antioxidant activities and can protect cells from oxidative damage [25, 26].

Some microalgae species like Chlorella, Scenedesmus, Trentepohlia, and Euglena (Figure 5a) (Euglenophyceae), produce polyphenolic compounds, including flavonoids and phenolic acids, which are well-known for their antioxidant properties. These compounds can neutralize free radicals and exhibit various health benefits [6, 13]. Microalgae like Dunaliella and Porphyridium can also contain vitamin E compounds, such as tocopherols and tocotrienols, which act as antioxidants to protect cellular membranes from lipid peroxidation [27]. Some microalgae, as is the case with the green alga Dunaliella and the blue-green Spirulina/Arthrospira (Figure 5b), produce antioxidant enzymes like superoxide dismutase (SOD), which catalyze the breakdown of super-oxide radicals, reducing oxidative stress in cells. Microalgae also contain glutathione (GSH), a tripeptide antioxidant that helps in maintaining cellular redox balance by scavenging free radicals [28].

The antioxidant properties of these bioactive compounds make microalgae potential candidates for various applications in the nutraceutical, pharmaceutical, and cosmetic industries. Extracts and supplements derived from microalgae are being explored for their health-promoting effects and their potential to combat oxidative stress-related diseases [6]. It’s important to note that the antioxidant potential of microalgae can vary depending on the species, growing conditions, and extraction methods. Researchers continue to study and explore microalgae to harness their antioxidant capabilities and unlock their potential as natural antioxidant factories [29].

The purpose of these assays is to assess the ability of microalgae-derived antioxidants to scavenge free radicals and reduce oxidative damage [13, 20]. The main assess tests used are described below.

DPPH (2,2-diphenyl-1-picrylhydrazyl) assay is one of the most widely used methods for evaluating antioxidant activity. DPPH is a stable free radical that appears purple in solution. When an antioxidant is added to the DPPH solution, it reduces the free radical, resulting in a color change from purple to yellow. The degree of discoloration indicates the scavenging ability of the antioxidant [57, 58].

ABTS [2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] is another stable free radical that is used to assess the antioxidant capacity of compounds. The ABTS assay measures the reduction of the ABTS radical, which is accompanied by a color change. The decrease in absorbance at a specific wavelength indicates the antioxidant activity of the tested compound [58].

The ferric reducing antioxidant power (FRAP) assay evaluates the ability of antioxidants to reduce ferric ions (Fe³⁺) to ferrous ions (Fe²⁺) in the presence of a reducing agent. The reduction is measured spectrophotometrically and provides information on the antioxidant potential of the test sample [59].

The total antioxidant capacity (TAC) assay measures the overall antioxidant capacity of a sample. It involves generating free radicals in the presence of the antioxidant, and the decrease in free radicals is determined by using a suitable marker. The TAC assay provides a comprehensive evaluation of the cumulative antioxidant activity of the tested compound [60].

Oxygen radical absorbance capacity (ORAC) assay is one method to measure antioxidant capacity. The fluorescence decay rate is monitored, and the antioxidant capacity is calculated based on the degree of inhibition of the fluorescence decay [61].

Lipid peroxidation inhibition assay is an assay that assesses the ability of antioxidants to prevent lipid peroxidation, a process that damages cell membranes and other lipids. The inhibition of lipid peroxidation is determined by measuring the formation of thiobarbituric acid reactive substances (TBARS) or other lipid peroxidation markers [62].

SOD assay is an enzyme that plays a crucial role in the defense against oxidative stress by converting superoxide radicals into hydrogen peroxide and oxygen. The SOD assay measures the inhibition of superoxide-mediated reduction of a tetrazolium dye, providing insights into the antioxidant activity of the test compound [63].

These cell-free assays help researchers identify microalgae-derived antioxidants with strong free radical scavenging and antioxidant potential. However, it’s essential to validate the results from cell-free assays with further studies, including in vivo experiments and clinical trials, to determine their effectiveness and safety in living systems before potential applications in the pharmaceutical or nutraceutical industries [64].

Microalgae-derived antioxidants exhibit their protective effects against oxidative stress through various mechanisms of action. These mechanisms are attributed to the specific chemical compounds present in the microalgae, which act as antioxidants or promote the antioxidant defense system within cells. Some of the key mechanisms of action of microalgae-derived antioxidants are described below.

Microalgae-derived antioxidants, such as carotenoids (e.g., astaxanthin, β-carotene) and phycobiliproteins (e.g., phycocyanin, phycoerythrin), can directly neutralize free radicals and ROS. They donate electrons to ROS, thereby stabilizing and reducing their damaging effects on cellular components like lipids, proteins, and DNA. Some microalgae-derived antioxidants possess metal-chelating properties, meaning they can bind to transition metal ions (e.g., iron, copper) that contribute to the generation of ROS through Fenton reactions. By chelating these metals, antioxidants prevent ROS formation and reduce oxidative damage [65–67].

Microalgae-derived antioxidants may stimulate the activity of endogenous antioxidant enzymes, such as SOD, catalase (CAT), and GSH peroxidase (POD; GPX). These enzymes work together to neutralize ROS and maintain redox balance in cells. Certain microalgae-derived antioxidants, like omega-3 fatty acids and polyphenols, have anti-inflammatory properties. By reducing inflammation, they indirectly contribute to the reduction of oxidative stress, as inflammation is closely linked to the generation of ROS [1, 68].

Various microalgae-derived antioxidants can activate the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. Nrf2 is a transcription factor that regulates the expression of various antioxidant and detoxification genes, enhancing the cellular defense against oxidative stress. Microalgae-derived antioxidants may protect mitochondria, the cellular powerhouses, from oxidative damage. Preserving mitochondrial function helps maintain cellular energy production and reduces the release of ROS from malfunctioning mitochondria [69].

Autophagy is a cellular process that removes damaged components and helps maintain cellular homeostasis. Certain microalgae-derived antioxidants have been found to regulate autophagy, which can contribute to cellular protection against oxidative stress. Oxidative stress can damage DNA, leading to mutations and potentially harmful consequences. Microalgae-derived antioxidants may aid in DNA repair mechanisms, preventing long-term damage and maintaining genetic integrity [70].

It’s important to note that the specific mechanisms of action can vary depending on the type of microalgae, the particular antioxidant compounds present, and the biological context in which they are applied. Additionally, the interactions among different antioxidants and their combined effects may further enhance their overall antioxidant capacity. Further research and studies are needed to fully elucidate the precise mechanisms of action of microalgae-derived antioxidants and their potential therapeutic applications [6, 13, 20].

Animal studies provide valuable insights into the potential health benefits, safety, and mechanisms of action of these antioxidants. Below are listed some of the common types of animal models used in such research [5, 8, 20].

Mice and rats are commonly used in animal studies due to their small size, short reproductive cycle, and genetic similarity to humans. These models allow researchers to investigate various health aspects, such as antioxidant activity, anti-inflammatory effects, lipid metabolism, and immune response, after administering microalgae-derived antioxidants through diet or other means [71, 72].

Zebrafish and other fish species are used as models to study the effects of microalgae-derived antioxidants on aquatic organisms. Fish models can provide insights into oxidative stress, neuroprotection, cardiovascular health, and other relevant parameters in aquatic environments [73].

Pigs are anatomically and physiologically more similar to humans than rodents, making them valuable models for studying nutritional interventions and health outcomes. Pigs can help researchers understand the impact of microalgae-derived antioxidants on gut health, lipid metabolism, and overall systemic effects [74].

Non-human primates, such as monkeys, share significant genetic and physiological similarities with humans. While their use is less common due to ethical considerations and cost, primate models offer valuable data regarding the potential effects of microalgae-derived antioxidants on human health [75].

Birds, such as quails or chickens, are also used as models in some studies to investigate the health effects of microalgae-derived antioxidants. These models can provide insights into antioxidant activity, egg quality, and other relevant parameters [76].

Some key health benefits that have been studied in animal models regarding microalgae-derived antioxidants include animal studies that can confirm the antioxidant capacity of microalgae-derived compounds and their ability to reduce oxidative stress in various tissues and organs. Microalgae-derived antioxidants may modulate inflammatory responses in animals, potentially providing benefits for inflammatory conditions [5, 6, 20].

Animal models can be used to evaluate the impact of microalgae-derived antioxidants on blood pressure, lipid profiles, and other cardiovascular parameters. Also, research on animal models helps understand the potential neuroprotective effects of microalgae-derived antioxidants in conditions like Alzheimer’s, Parkinson’s, or stroke [13, 77]. Animal studies can provide insights into the effects of microalgae-derived antioxidants on liver function and hepatic health. Research on animals may reveal the impact of microalgae-derived antioxidants on immune function and overall immune system support [27, 78].

It’s important to note that while animal studies provide valuable preliminary data, further research, including human clinical trials, is essential to validate the health benefits and safety of microalgae-derived antioxidants for human consumption. Animal studies serve as an important steppingstone in the research process, helping to identify promising compounds and potential health effects that warrant further investigation in human subjects [5, 6, 77].

Microalgae-derived antioxidants have been studied for their potential protective effects against oxidative damage, inflammation, and chronic diseases. These protective effects are attributed to their ability to scavenge free radicals and reduce oxidative stress, as well as their anti-inflammatory properties. Here are some of the chronic diseases and health conditions where microalgae-derived antioxidants have shown promise in animal and in vitro studies [79].

Oxidative stress plays a significant role in the development of cardiovascular diseases, including atherosclerosis and hypertension. Microalgae-derived antioxidants, such as astaxanthin and omega-3 fatty acids, have demonstrated potential in reducing oxidative damage to blood vessels, lowering blood pressure, and improving lipid profiles in animal models [80].

Chronic inflammation and oxidative stress are implicated in neurodegenerative conditions like Alzheimer’s and Parkinson’s diseases. Studies on microalgae-derived antioxidants, particularly astaxanthin and phycocyanin, have shown neuroprotective effects and the potential to mitigate neuroinflammation in animal models [81, 82].

The liver is susceptible to oxidative damage due to its role in detoxification processes. Microalgae-derived antioxidants, such as Spirulina and Chlorella extracts, have demonstrated hepatoprotective effects in animal models, reducing liver injury induced by toxins and oxidative stress [83, 84].

Oxidative stress and inflammation are involved in the development of insulin resistance and metabolic syndrome. Certain microalgae-derived compounds, such as fucoxanthin from brown algae, have shown potential in improving insulin sensitivity and reducing markers of inflammation in animal studies related to diabetes and metabolic syndrome [85]. Oxidative stress is also associated with DNA damage and the promotion of cancer development. Microalgae-derived antioxidants, such as carotenoids and polyphenols, have exhibited anti-cancer properties in animal and cell culture studies by reducing oxidative damage and inhibiting tumor cell growth [86]. Some microalgae-derived compounds, such as fucoxanthin, have been studied for their potential in reducing body weight and adipose tissue accumulation in animal models of obesity [87, 88].

Microalgae-derived antioxidants like astaxanthin and β-carotene have shown promise in protecting the skin from UV-induced oxidative damage and promoting skin health in animal studies. Microalgae-derived compounds, particularly polysaccharides and phycobiliproteins, have also been investigated for their potential prebiotic and anti-inflammatory effects on gut health [27, 80].

It’s important to emphasize that while animal and in vitro studies provide valuable insights into the potential health benefits of microalgae-derived antioxidants, human clinical trials are necessary to establish their efficacy and safety for therapeutic use [74, 89]. The bioavailability, dosage, and long-term effects in humans need to be thoroughly investigated before these compounds can be recommended for the prevention or treatment of chronic diseases. Additionally, individual responses to antioxidants may vary based on genetic factors, diet, and lifestyle, further highlighting the need for human studies to validate their effects on specific health conditions [90].

Microalgae-derived antioxidants, such as astaxanthin and β-carotene (Figure 1), have been studied for their ability to scavenge free radicals induced by UV radiation. By reducing UV-induced oxidative stress, these antioxidants may help protect the skin from sunburn, premature aging (wrinkles, fine lines), and other UV-related damage. Certain microalgae-derived compounds, like phycocyanin and polysaccharides, have shown potential in promoting collagen synthesis and cellular regeneration, contributing to improved skin elasticity and texture. Some microalgae extracts contain polysaccharides that help retain moisture in the skin, improving hydration and reducing dryness [20, 92].

Some microalgae-based antioxidants possess anti-inflammatory properties, which can be beneficial for skin conditions characterized by inflammation, such as acne, eczema, and psoriasis. By reducing inflammation, these antioxidants may help soothe irritated skin and promote its overall health [93].

Microalgae-based antioxidants, particularly polysaccharides and phycobiliproteins extracted from Dunaliella, Chlorella, and Haematococcus (Chlorophyta), and Arthrospira/Spirulina (Cyanophyceae) have been investigated for their immunomodulatory effects. They may help enhance immune function by stimulating the activity of immune cells, such as macrophages and lymphocytes, thereby supporting the body’s defense against infections and diseases. By reducing inflammation, microalgae-derived antioxidants may help regulate the immune response and prevent excessive immune reactions that can lead to chronic inflammatory conditions [6, 29, 93].

Microalgae-based antioxidants can neutralize free radicals and reduce oxidative stress, which is a major contributor to cellular aging. By protecting cells from oxidative damage, these antioxidants may help maintain cellular health and delay the aging process. Some microalgae-derived compounds have also been shown to promote cell repair and regeneration processes, which are essential for maintaining tissue health and function [6, 94, 95].

It’s important to note that while in vitro and animal studies have provided promising results regarding the potential benefits of microalgae-based antioxidants for skin health, immune system modulation, and cellular rejuvenation, human clinical trials are necessary to validate these effects in humans [96]. Additionally, individual responses to antioxidants may vary, and factors like dosage, formulation, and application method need to be carefully considered in potential therapeutic applications. Always consult with a healthcare professional or dermatologist before using any microalgae-based antioxidant products or supplements for specific health concerns [96].

The sustainable production and cultivation of microalgae involve the use of various cultivation systems, with photobioreactors and open ponds being the two primary methods for large-scale production [97].

Photobioreactors (Figure 6) are enclosed systems designed to optimize microalgae growth by controlling various environmental factors. These closed cultivation systems offer several advantages. Photobioreactors allow precise control of temperature, light intensity, pH, and nutrient concentration, which helps maximize microalgae growth and productivity [97, 98].

The closed nature of photobioreactors minimizes the risk of contamination from external sources, such as competing microorganisms or pollutants. So, the controlled environment in photobioreactors can lead to higher microalgae biomass and better quality of target compounds. Photobioreactors enable year-round cultivation, making them suitable for locations with varying climatic conditions [99].

However, photobioreactors also have some limitations since the construction and maintenance of photobioreactors can be more expensive compared to open pond systems. Operating a photobioreactor requires energy input for controlling various parameters, leading to higher operational costs. Scaling up photobioreactors to very large volumes can be challenging due to engineering and cost constraints [100].

Open ponds (Figure 7a) are large, shallow basins where microalgae are cultivated under natural sunlight. They are more straightforward and cost-effective compared to photobioreactors and are commonly used for large-scale microalgae cultivation [101].

Open ponds are relatively simple to construct and operate, resulting in lower capital and energy expenses. Expanding the cultivation area in open ponds is more feasible than scaling up photobioreactors. Open ponds use natural sunlight, which is abundant and does not require additional energy input. The open raceway pond system (Figure 7b and c) has a very simple structure consisting of a closed raceway and paddle wheel. However, open systems are susceptible to contamination from competing microorganisms, which can affect microalgae growth and product purity. Also, the productivity of open ponds can be influenced by weather conditions like temperature, sunlight availability, and rainfall. In regions with severe winters or extreme climates, open ponds may not be suitable for year-round cultivation [102].

To optimize the advantages and mitigate the limitations of both photobioreactors and open ponds, some large-scale microalgae cultivation facilities use hybrid systems. These hybrid systems combine the advantages of controlled conditions in photobioreactors with the scalability and lower cost of open ponds. For instance, microalgae can be initially grown in photobioreactors and then transferred to open ponds for further cultivation and biomass expansion [103].

Overall, the choice between photobioreactors and open ponds depends on factors such as the specific microalgae species, intended application, location, available resources, and economic considerations. Sustainable cultivation practices, efficient resource utilization, and continuous advancements in cultivation technology play crucial roles in the successful large-scale production of microalgae for various applications, including food, feed, biofuels, pharmaceuticals, and more [104].

While the potential applications of microalgae-derived compounds in preventive and therapeutic strategies are exciting, several challenges need to be addressed for successful translation from research to practical use. For example, ensuring sufficient bioavailability of microalgae-derived compounds in the human body remains a challenge, as their absorption, metabolism, and distribution can vary significantly [118].

Standardizing the cultivation, harvesting, and processing of microalgae is essential to maintain consistent quality and ensure the presence of bioactive compounds in the final products. Microalgae-derived compounds must go through rigorous testing and regulatory approval processes to ensure their safety and efficacy before they can be used in medical or pharmaceutical applications [119].

Making microalgae-derived compounds economically viable for large-scale production and commercialization is a significant hurdle that needs to be addressed. Raising public awareness and acceptance of microalgae-based products as preventive and therapeutic agents will be crucial for their successful integration into healthcare and nutrition practices [120].

Despite these challenges, ongoing research and advancements in biotechnology, along with a deeper understanding of microalgae biology and biochemistry, are likely to drive the realization of the potential applications of microalgae-derived compounds in preventive and therapeutic strategies, improving human health and well-being in the future [121].