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The international community’s attention has lately risen in a major way to the emerging market of dietary fibers and other phytochemical supplements, which are considered to be safe and have significant functional impacts. As an important example, microalgae are considered as future superfoods, as mentioned by Torres-Tiji et al. (1). They have several important nutritional phytochemicals such as phycocyanin (a novel protein in Spirulina), exopolysaccharides, polyphenols, and flavonoids.

In addition, many studies have reported that microalgae have high antiradical activity coming from their unique chemical structure (2). Furthermore, scientists are trying to find instant and non-invasive techniques to explore the scientific principles of microalgae phytochemicals’ functional principles, especially bioactive macromolecules and micromolcules (3). These phytochemicals increase the media antioxidant, anticancer, and other physicochemical potentials (4–9).

The microalgae health-related key components like carbohydrates (especially prebiotics), lipids, and proteins are precursors of essential pathways and are novel sources of bioactive molecules. As an example, microalgae polysaccharides, provide many intriguing aspects in the food science and nutrition area as a viable alternative in food technology. They are widely used in biopolymer-based materials, like alginate polysaccharides that is among the most versatile biodegradable polymers by microorganisms (10). Also, Phaeodactylum tricornutum produces up to 40% eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) of its fatty acids, as a functional bioactive molecule (1). Additionally, microalgae are recently used as a novel proteins source. These proteins include commercially available phycocyanin novel food protein supplements produced by Spirulina (A. platensis; or Cyanobacteria) (11).

As their potential uses as functional food, microalgae prebiotics have novel characteristics distinguished from the other edible sources against the digestion by the gastrointestinal tract. Therefore, they help to enhance the growth of the health-related beneficial bacterial microorganisms called probiotics in the lower part of the gastrointestinal tract or the colon (12). Meanwhile, it could be used as a symbiotic (probiotics and prebiotics) functional food, for improving their positive health effects. In which, there are a large number of probiotic bacteria in dairy products that proved their efficiency when used in a mixture with the prebiotic fibers for improving the gut health in the different cultures traditional foods, like the fermented Sobya and Kefir (5, 6). Also, this recently explored prebiotics from microalgae confirmed their potentials to support probiotic beneficial bacterial growth in the host gut upon consumption (13). The most promising polysaccharides (PS) from microalgae (like exopolysaccharides, alginates, and carrageenans) are barely fermented by the colonic microbiota, therefore they act as efficient prebiotics. Moreover, the microalgae prebiotic functionality are significantly affected by industrial processes impacts on its chemical composition (14). Currently, chemical and chromatographic methods are used for the evaluation and analysis of microalgae chemical composition. However, these methods are destructive and are chemical dependencies that affect their structure (15). On the other hand, some potential novel examples for detecting single-cell microalgae macromolecules functionality could be through its electrochemical charge that is a very important parameter for identifying and establishing a reference method for its characterization based on electronegativity fingerprint (16–18).

The aims of this review was to present and highlight the potential uses of microalgae, as alternative prebiotic edible sources for human consumption. Furthermore, the advanced non-destructive techniques were summarized for facilitating the tracking of these functional molecules’ during its cultivation process.

Microalgae species have the potential to be a sustainable high nutritious sources. In which, several microalgae species are generally recognized as safe (GRAS) (1), like Arthrospira platensis, and Scenedesmus obliquus. They are well known in United States, China, and Japan market (2, 19). Microalgae are unicellular cells that are considered eukaryotic, non-flowering, and photosynthetic aquatic plants sometimes. These microorganisms can act independently or with some other formulations to improve human health (2, 20).

Several large-scale aquaponics and Hydroponics plant systems have developed and used microalgal technologies to reuse the organic and inorganic molecules in their aquatic culture (21). Microalgae biotechnology have several advantages compared to the other microorganisms. For instance, they could change their metabolic pathways in the production of new novel functional compounds which open the way to use these kind of safe source instead of other known sources like bacteria for the production of novel molecules (22). Furthermore, its high biosorption ability of the essential elements like Mn (II) (e.g., Thalassiosira pseudonana) through its active hybrid membrane system performance let it be used in several advanced pharmaceutical field for optimizing their production of prebiotics like exopolysaccharides (EPS) (23), as anticancer, anti-inflammatory, and immunomodulatory agent (24).

Table 1 presents the edible marine microalgae carbohydrate, oil, and protein contents, in which it can be observed that Scenedesmus sp. contains up to 41%, Porphyridium contains up to 57%, and Chlorella contains up to 26% of carbohydrates that contains many potential prebiotic bioactive. On the other hand, Spirulina Maxima contains up to 71% protein and Phaeodactylum sp. contains up to 57% of oil. Thus, microalgae, in general, could be used as perfect sustainable alternative food that have health benefits for humans (25). And therefore, Qiao et al. (26) reported that these naturally existing and very-fast growing unicellular microorganisms’ phytochemicals have been applied in the medical area to increase oxygen local level in tumor regions (Figure 1).

As an example, microalgae have high concentrations of the polyunsaturated fatty acids (PUFAs), which are category of fatty acids containing more than one double bond in the backbone and which have high bioactivity against cholesterol levels in the human body (27, 28) (Table 2).

Also, microalgae can scavenge a wild range of Reactive Oxygen Species (ROS) through their high phenolic and flavonoid content. For instance, our recent study on four microalgae species confirmed their high potential as antioxidant agents (Table 1). An innovative way of in situ oxygen-generation by an engineered C. vulgaris to overcome hypoxia in wounds was reported by Qiao et al. (26), whereby the engineered live C. vulgaris added to hypoxic tumor areas has both increased local oxygen levels and re-sensitized resistant cancer cells via microalgae-mediated photosynthesis under red-light beams. This engineered- C. vulgaris was achieved by adding an engineered red blood cell membrane (RBCM) to the C. vulgaris surface which reduces macrophage uptake and systemic clearance of the C. vulgaris and was termed as “RBCM-Algae.” The significant effect of C. vulgaris was reported through its in situ generation of O₂ in the human’s system using a natural photosynthation. In which, the increased oxygen level on the tumor areas by the RBCM-Algae resulted in a remarkably radiotherapeutic efficacy. Besides, the chlorophyll from the C. vulgaris was observed to produce ROS during laser irradiation adding to improved tumor cells’ apoptosis (26).

Furthermore, it was demonstrated that the engineered- C. vulgaris when injected into the body by either intravenous or intratumoral, could efficiently arrive at the tumor spot and improve oxyhemoglobin, as it shows an extended life span with increased stability during in vivo circulation, which exhibits excellent biocompatibility with tissue cells (26). Furthermore, when compared to most normal tissues that were equally inoculated with the RBCM-Algae, such as the brain, heart, and kidney tissues, the tumor tissues showed the highest uptake of the RBCM-Algae (Figure 2). Therefore, the microalgae functional components could give it a high selectivity when they are used as a prebiotic functional source in the human body (26).

In another recent practical study of the health-related impacts of microalgae and their functional components by Chen et al. (29), an oxygen-producing-patch filled with gel beads containing active Synechococcus elongatus was reported to have efficiently supplied more than 100 times sufficient oxygen than the available conventional ways of supplying oxygen, such as hyperbaric oxygen (HBO), topical dissolved oxygen (TDO), and topical gaseous oxygen (TGO) (Figure 3). For example, based on the level of oxygen penetration to the dermis, TDO and TGO had recorded > 700 and only 300 μm of human skin, respectively, demonstrating TDO as the best available method to supply oxygen to the chronic diabetic wounds. In which, its mechanism is based on producing continuous oxygen by the photosynthesis process, which can effectively penetrate the skin to stimulate aerobic metabolism and angiogenesis in hypoxic tissues (30). Based on these excellent qualities demonstrated by S. elongatus gel-wound dressing, the hypothesized microalgae-hydrogel patch could be applied to diabetic chronic wounds as it showed increased wound oxygenation, and angiogenesis, making it a novel medical innovation for the diabetic foot ulcers (Figure 3) (29, 31).

Microalgae is an important source of prebiotics that defined as potential native and modified forms of polysaccharides (PS) such as xylooligosaccharides (XOS), galacto-oligosaccharides (GOS), alginate oligosaccharide (ALGOS), neoagaro-oligosaccharides (NAOS), galactans, arabinoxylans, and β-glucans (Table 3). These microalgae PS are against the digestion process by the metabolic enzymes in the human gut. Therefore, they could be used as dietary prebiotics and able to augment the growth of probiotics bacterial in the gut and colon (32, 33). de Medeiros et al. (34) used the prebiotic score (P. Score) technique to investigate the different tolerance abilities of microalgae against the digested through a method of an in vitro colonic fermentation. In that study, they mentioned that A platensis P. Score was 6.93 ± 0.05 and C. vulgaris was 2.54 ± 0.02 which were significantly high compared to the control with -1.35 ± 0.04.

Prebiotic was described by the Association of Probiotics and Prebiotics (ISAPP) as a non-digestible carbohydrate group, such as short and long-chain β-fructans [Fructooligosaccharide (FOS) and inulin], and GOS that beneficially affects the human by stimulating the growth of probiotics bacteria (like Lactobacillus spp., Bifidobacterium spp., Streptococcus spp., Lactococcus spp., and Saccharomyces spp.), and release some essential metabolites that could improve the human gut health (e.g., enhance the gut permeability through the released butyric acid) (48). These beneficial compounds are classified based on their resistance to the acidic pH and hydrolyze enzymes, absorption in the gastrointestinal tract, and fermentation potential by the intestinal microbiota (49). For instance, oligosaccharide carbohydrates (OSCs) are a good prebiotic family-like Fructans that consists of inulin and FOS or oligofructose. Their structure is a linear chain of fructose with β(2→1) linkage (50). In which, the specific structure of these molecules like, the length of fructans chain, is play a key role in their bacterial selection that can ferment them to the beneficial secondary metabolites (51).

Figure 4 presents the systematic metabolic pathways in unicellular microalgae for the synthesis of prebiotics and their potential benefits as functional prebiotics for human health (52). In microalgae, PS synthesis occurs in the Golgi apparatus whereas it is cytoplasmic for Cyanobacteria (53). The main stages of EPS synthesis in microalgae and cyanobacteria are the production of activated monomer sugars, then their assembly by enzymes like glycosyltransferases that can produce polymers from these monomers in the extracellular compartment, as the meaning of EPS expression (54).

In this section, we will discuss in brief the recent commonly used methods that could be applicable for microalgae prebiotic extraction. These techniques have benefits and limitations. Figure 6A shows the method and common protocol for the extraction and purification of exopolysaccharides from microalgae.

Microalgae macromolecule analysis has been practiced for decades. A promising strategy for microalgae chemical assessment is using bioinformatics to provide a fast prediction tool for its carbohydrates, proteins, and even lipids. Nowadays, a potential limitation is the lack of reference structures of some microalgae proteins, and thus a wide range of analytical methods should be used for building a strong database based on these different analytical destructive or non-destructive techniques combined with chemometrics and other informatics methods.

In conclusion, microalgae chemical composition plays an important role in its novelty applications as alternative nutritious prebiotic sources. The advancement of commonly used extraction methods and the analytical techniques for measuring microalgae species’ chemical composition could increase their applicability and potential uses in the field of bioactive prebiotic application and functional evaluations. Which, enhances their application in pharmaceuticals, functional food, and all related scientific fields. These advancements could include the discovery of new microalgae bioactive prebiotics, as well as the study of the real-time functional activity of microalgae exopolysaccharides, oligosaccharides during microalgal cultivation, as a critical approach to be developed for microalgae biotechnology of these functional compounds. Furthermore, nanoprobe microelectrodes with gold and other nanoparticles such as silver have become one of the most important approaches to verifying the functionality of various microalgae (for example, their antioxidant activity) as well as evaluating their physiological status. As a result, both conventional and spectroscopic methods could be used to validate the health benefits of microalgae health functional potential and their bioactive components benefits.

MG: conceptualization, studied the literature, drafted, and edited the manuscript. MT, YZ, and FF: helped in the writing process. XL and BC: helped in the writing process and the project administration. YH: supervision, writing, and reviewing. All authors edited, proofread, and accepted the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.