Selenium-Enriched Microalgae: A Novel Bioactive Strategy for Immune System Enhancement

Abstract

Background and Objective: Selenium-enriched microalgae represent a promising functional food and nutraceutical resource, offering a symbiotic blend of bioactive molecules and essential micronutrients. Selenium, a trace element, is vital for immune system control, antioxidant defense systems, and cell homeostasis. Microalgae possess high nutritional value and rapid growth capabilities, allowing them to bioaccumulate selenium in the form of organic compounds like selenomethionine and selenocysteine. These organic forms have increased bioavailability and reduced toxicity compared to inorganic selenium supplements. This review examines the latest advancements in the cultivation, selenium-enrichment strategies, and biological effects of selenium enriched microalgae, with a particular focus on their immunomodulatory properties. Results and Conclusion: The review highlights the potential applications of selenium enriched microalgae in disease prevention, immunotherapy, and functional food development. Also, the immunomodulatory effects of selenium enriched microalgae have been illustrated. Selenium and Se-enriched microalgae, due to their effective roles in enhancing immune function, reducing inflammation, and providing highly bioavailable forms of Se, hold strong potential for applications in food biotechnology and nutritional supplements. Despite existing challenges in optimizing production and clarifying mechanisms of action, the future outlook is highly promising given the growing demand for functional foods and natural health-promoting solutions. Keywords: Antioxidant, Bioavailability, Chlorella vulgaris, functional foods, Immunomodulation, Selenium-enriched microalgae, Selenoproteins, Spirulina, Sustainable biotechnology. Introduction   Selenium (Se) is an essential element for the proper functioning of immune system cells, including macro-phages, natural killer (NK) cells, neutrophils, and T lymphocytes. This element plays an essential role in reducing oxidative stress, inflammation, and preventing the spread of infectious diseases, especially when its serum concentration is adequately increased through the diet [1, 2]. It is involved in modulating immune and reproductive function. This micronutrient induces the production of selenoproteins, helping to protect cells against reactive oxygen species (ROS) [3]. Se primarily functions in the body as selenoproteins, which enhance the regulation of the human immune system in various ways. Immunity is one aspect of human health is influenced by Se levels and the expression of selenoproteins in the body [4]. Although the exact physiological role of Se is unknown, it has been determined that it exhibits most of its effects by being incorporated into selenoproteins. One such class of proteins is iodothyronine deiodinases, which are essential in the maturation of thyroid hormones T3 and T4. These hormones control body weight, development, and metabolism. A deficiency in Se may therefore disrupt the process of maturation of T3 and T4, leading to stunted growth. [5]. Dietary supplements containing l-selenometh-ionine are a source of Se for nutritional purposes that is easily absorbed and utilized by the body, approved for adult use at a dosage level of less than 250 mcg per day [6,7]. Seafood and meat are regarded as the primary sources of Se for humans, providing more than 70% of daily Se intake. In contrast, fruits and vegetables contribute only a small fraction to this consumption. Se is crucial because of its antioxidant and chemoprotective roles at low levels, offering protection against various cancers, heart diseases, and type 2 diabetes. Therefore, food systems must produce sufficient amounts of this essential trace element to ensure a daily intake of at least 40 micrograms, which supports the optimal expression of Se enzymes, and potentially up to 300 micrograms per day to lower cancer risk [8]. The availability of Se varies significantly depending on the geographical region. In some places, soil Se concen-trations are limited or reduced [9]. It is reported that approximately 1 billion people worldwide are affected by Se deficiency, meaning that diets are limited to products harvested from these regions or are nutritionally unbalanced and may result in Se deficiency [10]. The global population is rapidly increasing, leading to the emergence of new diseases and placing a significant strain on healthcare systems. Therefore, it is more important than ever to research immune modulators, including Se, to tune the immune system to help combat new pathogens in a changing world [11, 12]. Essential minerals for optimal health are provided by edible plants, which are collected from the environment, namely, soils and aquatic sediments. The consumption of Se-rich plants makes this element bioavailable to humans [9]. Soils used in agriculture can be challenging to meet daily Se requirements through diet if they are low in Se. Se mineral salts are mainly used in food supplements and animal feed, and this supplementation method with sodium selenite or selenate also has disadvantages [13-15]. Today, organic Se-enriched foods, mainly plants, animals, and microorganisms enriched with Se, have been widely developed to address the problem [13]. In recent years, the production of Se-enriched microalgae has attracted much interest as an efficient and accessible method for producing organic Se [16, 17]. Microscopic algae, or microalgae, grow in freshwater and marine environments and have the capability to convert sunlight energy into chemical form. The two most notable ones are Arthrospira platensis (Spirulina) and Chlorella vulgaris. Spirulina is blue-green algae and a nutritional powerhouse containing a plethora of vitamins and poly-unsaturated fatty acids and plays critical roles in nearly every activity of human life [18, 19]. Microalgae represent promising sources of bioactive compounds suitable for pharmaceutical and food uses [20]. Humans have been eating algae as a food crop and dietary supplement for thousands of years. Algae also serve as a natural carbon sequester, counteracting global warming and reducing the pressure on arable land and freshwater resources for conventional food crops. For the best nutritional com-position of algae, there is a need to stress their cultivation keeping in view many environmental parameters such as pH, intensity of light, availability of nutrients, availability of CO2, temperature, and mixing conditions [21]. Micro-algae, especially Chlorella vulgaris, are well-known as excellent sources of protein, balanced amino acids, and essential vitamins and micronutrients. Due to their high nutritional content, these microalgae are commonly con-sumed by humans. Chlorella vulgaris can absorb inorganic Se salts and convert them into protein-based compounds such as selenomethionine, selenocysteine, and methyl selenocysteine [16]. Thus, the aim of this review is to draw on scientific studies used to explore how Se-enriched microalgae modulate the immune system, providing updates on the recent advancements in this research area. This review is distinct from others due to its up-to-date and broad inclusion of all research drawn, thus giving the reader a com-prehensive overview of Se-enriched microalgae immune-modulatory effects. Types of Se-enriched microalgae Se-enriched biomass (Se-Chlorella) can serve as both a dietary supplement and an antioxidant [16]. Food products derived from Chlorella include green tea powder, soups, noodles, bread, biscuits, ice cream, and soy sauce. Raw Chlorella is commonly offered in tablets, capsules, powders, granules, and drinks. Noodles containing 1.5% Chlorella extract are exceptionally nutritious. Adding Chlorella powder to barley bread not only enhances its nutritional content but also improves its appearance and flavor. Additionally, Chlorella is used as a food additive to enhance the taste and quality of pasta, wine, and fermented foods. With vitamins C, K, A, and E, Chlorella is valuable for pharmaceuticals, animal feed, food additives, aquaculture, and cosmetics [22]. Chlorella can develop under conditions of light, carbon dioxide, water, and a minimum amount of nutrients, thus its culture is easy. The life cycle of the microalga is simple while its metabolic processes are as complicated as those of superior plants, which makes it capable of synthesizing high amounts of proteins, carotenoids, vitamins, and minerals. Being so, it is a popular source of nutritious food. Among numerous species of Chlorella, Chlorella vulgaris, Chlorella sp., and Chlorella pyrenoidosa are the most utilized in industrial production and scientific research [23]. Se exerts its biological effects via selenoproteins. Various microalgae, such as Chlamydomonas, Volvox, Ostreococcus, Micromonas, and Emiliania, have been identified as possessing selenoproteins [24]. Spirulina, a type of microalgae, possesses antioxidant properties and can be fortified with Se during its growth process. Research revealed a hierarchy in the distribution of Se and the expression of selenoproteins based on the form of supplementation. Supplementing with sodium selenite enhanced glutathione peroxidases (GPx) activities and selenoprotein expression, whereas Se-enriched spirulina was more effective in restoring Se levels [5]. In 1992, the World Health Organization listed it as a future food. Spirulina is used as a dietary supplement due to its high protein value, vitamins, β-carotene, and phycocyanin-like pigments. It contains anti-inflammatory, anti-cancer, and antioxidant properties. In vivo and in vitro studies have demonstrated that spirulina supplementation can reduce markers of oxidative stress and enhance the activity of antioxidant enzymes. Also, as it matures, spirulina may be enriched with elements like Se, which is incorporated into organic molecules like selenomethionine and seleno-cysteine [5]. In recent years, although there has been a notable rise in algal production, it remains insufficient to satisfy the high commercial demand for biomass [21]. The dried biomass of microalgae is an excellent option for food use because of its rich nutritional content, economical processing expenses, and various health advantages [25]. Table 1 provides a summary of the types of Se-enriched microalgae. Biotechnological ethods for seleni-um enrichment (cultivation methods and genetic modifications) The ideal harvesting process for microalgae should maintain the integrity of the composition of the biomass and facilitate effective recovery of the target product. Traditional harvesting processes involve filtration, centrifugation, flotation, flocculation, electroflocculation, sedimentation, electrolytic treatment, electrophoresis, and magnetic separation. Incorporating an additional step of chemical or biological coagulation or flocculation into these processes can enhance the efficiency of the process as well as reduce operating expenses. Centrifugation is widely utilized for microalgae harvesting due to its effectiveness and speed in cell recovery. Centrifugation involves centrifugal force to separate microalgal biomass from the culture medium, from which excess water may easily be drained. Centrifugation can achieve up to 98% yield, but its greatest flaws are high energy demand and the potential for cellular structure breakdown during the process [23]. Microalgae are highly efficient at fixing carbon dioxide during their growth, so they do not require arable land for cultivation. Microalgae biomass production involves three main steps: cultivation, harvesting, and processing. Among these, harvesting is particularly challenging due to its high costs, and it has a direct impact on the processing stage [26]. The harvesting stage of microalgae is crucial for its overall production. Research has shown that harvesting accounts for 20–30% of the total production cost. Key challenges in the harvesting and dewatering process include the small size of the cells (<30 µm), their low concentration and dilute presence in the culture medium (<1 g.l-1), the highly electronegative nature of the cell membrane surface, and the relatively fast growth rate of the algae. As a result, the energy required for the harvesting process exceeds the energy content of the microalgal biomass itself [26]. Coagulation using polyaluminum chloride (Al2O3) and hydrophilic polytetrafluoroethylene membranes has been used as a sustainable technology for harvesting microalgae. Transparent exopolymer particles that are also produced by microalgae have been used to reduce the fouling of membranes in microfiltration. The particles prevent membrane fouling when solutions are collected through the use of the filtration-based method, hence improving filtration efficiency [26]. Microalgae possess the ability to absorb inorganic Se and combine it with amino acids to create Se amino acids, including selenomethionine, selenocystine, which are advantageous for the health of both humans and animals. Furthermore, Se can be added from a primary source to cultivate Se-enriched microalgae in domestic wastewater, which can act as a nutrient base for microalgae growth. Low levels of Se may stimulate microalgae growth. Turbidity significantly increased over the incubation period, indicating that microalgae growing in domestic wastewater treatment systems can tolerate such high Se concentrations. The removal process facilitated by microalgae in HRAP involves biomass assimilation, as microalgae can use these substances to produce cellular components, including proteins, nucleic acids, and carbohydrates [27]. A post talks about various methods of cultivation, starting with open systems using open ponds for growing microalgae. They rely on natural water and sunlight but are prone to environmental stresses and the risk of contamination. It also talks about closed systems, which use photobioreactors or bioreactors that yield closed environ-ments for the growth of microalgae. These closed systems are efficient and less likely to cause contamination compared to open operations. In addition to this, advanced techniques like mutagenesis and genetic alteration are also discussed, which are used in order to enhance some characteristics of microalgae to maximize biomass production and quality. The article also emphasizes the necessity of specific environmental conditions, such as pH, temperature, and light intensity, in order for maximum growth and production. Altering these can lead to increased production of Se and other secondary products. Lastly, nutrient-rich conditions are suggested as an alternate approach to enhance microalgae production. Nevertheless, it is essential to maintain adequate levels of nutrients so as not to compete with Se absorption. These culture processes enhance microalgae productivity across various industries [16]. One advantage of cultivating microalgae in a mixotrophic manner is the increased biomass production [28]. Today, with advancements in microalgae harvesting, methods such as bioflocculation, electroflocculation, bio-electroflocculation, ultrasound/hydrodynamic techniques, magnetic nanoparticle flocculation, and phototaxis-based harvesting are employed [29]. Table 2 shows biotechnological methods for Se enrichment. Analysis of bioavailability of selenium in microalgae The way Se is absorbed differs among animal species and is affected by various factors, including physiological traits, functional status, the number of intestinal contents, the chemical forms of Se, the duration of Se’s stay in the intestine, and the methods of Se administration. Se is mainly found in the liver, kidneys, heart, and pancreas, with muscles, bones, and blood having the following highest levels, while fat tissues contain the least amount. Typically, in normal circumstances, the Se that animals metabolize is mainly eliminated through urine and feces. However, when Se is consumed in excessive amounts, breathing also becomes a significant pathway for its excretion. Moreover, Se can be removed from the body through hair and sweat [30]. The bioavailability of Se in microalgae, indicating how much Se can be absorbed in the gastrointestinal tract, was assessed in a study. Grinding the microalgae, their cell walls are broken down, facilitating the release of Se from the biomass during digestion. Under gastrointestinal conditions, 49% of Se from raw microalgae and 63% from ground Se-enriched microalgae were solubilized, indicating their potential bioavailability. A similar level of Se bio-availability (approximately 49%) has been observed in Se-enriched Chlorella vulgaris. Additionally, it's important to highlight that the bioavailability of Se in Se-rich yeast widely recognized as a leading organic Se supplement is generally greater than that found in microalgae cultivated in HRAP-Se. Furthermore, the type of Se also influences absorption rates [27]. The Se in food sources is mainly in organic forms, including selenocysteine and selenomethionine, which are more easily absorbed than inorganic forms like selenite or selenate. There is a significant risk of losing dietary Se during processing and cooking; for instance, the refining of grains can diminish Se levels by 50 to 75%, while boiling can lead to an additional reduction of 45%. Microalgae that utilize Sec, such as Nannochloropsis oceania, can enhance the organic Se content and can thus be used as Se-enriched food or feed additives [24]. Organic Se compounds have higher bioavailability and usability than their inorganic counterparts. Studies also indicate that Se-enriched plant products usually provide greater bioavailability than animal-based products. Moreover, adding Se to proteins can not only increase their Se content but also improve their functional qualities, including emulsion stability and gelling properties, along with enhancing their bioavailability [31]. In numerous microalgal species, selenite may exhibit greater toxicity compared to selenate, while the opposite can be true for other species. Due to its high solubility, selenate is more readily bioavailable to aquatic organisms than selenite, indicating that selenate might be the predominant dissolved form. Prior research has shown that selenate does not pose toxicity risks for some microalgal species [8]. Applications in food biotechnology development and utilization of function-al foods, fortified foods, beverages, and dietary supplements Microalgae cultivation is one factor that can enhance world food security and reduce the environmental impact of rising agricultural food production. The quality and safety of microalgae product, in turn, are both a function of proper cultivation, harvesting, and processing protocols since foreign organic and inorganic material occur as a result of environmental contamination or process operations when dealing with microalgae biomass [28]. Functional food production with microalgal biomass will see growth in the coming years as a result of growing demand for healthy foods globally. Microalgae incor-poration in food products is the next wave of evolution of the functional food industry, given that there is continuous need to search for new raw materials for use in creating new foodstuffs. Nevertheless, some of the major questions need to be answered to facilitate the successful development of microalgal-derived food products. Some of the other questions that belong to this category are how microalgal biomass interacts with the other components of the food matrix, the effect of microalgal addition on the sensory attributes of food, particularly color and flavor, textural and rheological modification of food products with microalgal addition, and changes and stability of microalgal constituents under various processing conditions [32]. Understanding the mechanisms and potential for Se accumulation in plants and animals is one of the most critical directions in developing nutritional science and supplementation [33]. To create algae-based food products and by-products, the first essential step is enhancing our knowledge of the biochemical makeup and digestibility of microalgae. In line with this, it’s important to assess more microalgal species for novel food approval, ultimately supporting the broader inclusion of these microorganisms in human diets over time [34]. Numerous studies have demonstrated that microalgae species, particularly from the Chlorella and Spirulina genera, are well-suited for direct use in their natural forms. This adaptability has led to their widespread application in the food industry, where they enhance the nutritional content of products like noodles, cookies, energy bars, and juices [23]. In the food and beverage industry, as well as within the cosmetics industry, the demand for spirulina has risen due to its fat-reducing, antioxidant, and anti-inflammatory properties [35]. An article discusses the production of beverages such as "selenized kawas," made from fennel grains soaked and germinated in solutions containing Na₂SeO₃. These beverages are considered a source of Se. Regarding Se in beer, yeasts (S. cerevisiae) can convert inorganic Se (Na₂SeO₃) into bioactive organic forms that are utilized in selenized beers. This process enhances the Se content in the beverages [36]. In a study, Chlorella vulgaris was blended into spreadable processed cheese, leading to increased magnesium, potassium, Se, Zn, iron, and antioxidant capacity, with greater enhancements observed at higher formulation levels. Microalgae were also incorporated into various snacks, breads, isotonic drinks, and yogurts; however, these products did not attract consumer interest due to their unappealing green color and distinct flavor. On the other hand, a dehydrated soup prepared with S.