Exopolysaccharides from microalgae: Production in a biorefinery framework and potential applications

https://doi.org/10.1016/j.biteb.2022.101006Get rights and content

Highlights

  • Different biological activities are observed in microalgal EPS.

  • The production of microalgal EPS is limited by extraction methods costs.

  • Microalgal EPS production using the biorefinery concept can reduce costs.

  • Microalgal EPS can be used in producing nanostructures.

Abstract

Microalgae have significant biotechnological importance, with great interest by the industries in the production of compounds of microalgal origin. Besides intracellular biomolecules, microalgae can excrete several substances, mainly exopolysaccharides (EPS). These compounds have been highlighted because of their bioactivities and potential applications in the pharmaceutical, food, and cosmetic fields. However, microalgal EPS are underexplored compared to the competitive production of EPS by other organisms. The aim of this review was to address aspects of EPS production from microalgae, focusing on strategies to improve EPS production viability on a biorefinery framework and production of nanobased structures/compounds for applications in the wastewater treatment and stabilization of nanoemulsions and nanoparticles with antimicrobial, antioxidant and/or anticancer properties. Furthermore, this review will address the main challenges and future prospects for market investments in this field.

Introduction

Microalgae exopolysaccharides (EPS) are increasingly attracting interest owing to their physicochemical, functional, and industrial applications, and the growing market demand for natural polysaccharides. These bioactive compounds can be beneficial to health, biodegradable and versatile in product development (Santos and Amorim, 2018). EPS have biochemical properties with great potential for application in the biotechnology area because of their anticancer, antibacterial, antioxidant, and antiviral activities (Sed et al., 2017).

EPS are defined as extracellular polysaccharides that can be excreted by microalgae in the environment around them or be linked to the cell walls of these microorganisms. Some of the functions of EPS, include protecting the microalgal cells from biotic and abiotic stresses, mainly dehydration or toxic substances (Zhang et al., 2019b). Furthermore, EPS are related to the production of biofilms and cell adhesion and interaction (Liu et al., 2016). Many microalgae, especially the reds and cyanobacteria, produce EPS and can excrete large amounts of this compound (around 20 g L−1) (Michaud, 2018). Around 90,000 tons/year of algal polysaccharides are used in the pharmaceutical, cosmetic, and hydrocolloid industries. However, there is no saturation of this market and can integrate new products, for example, biopolymers and functional foods, obtained from microalgal EPS (Kraan, 2012; Michaud, 2018).

The cost of EPS can be reduced considerably through microalgae biomass production technologies and treatments under the biorefinery concept (Michaud, 2018). Biorefineries allow the sustainable processing of the biomass in various products (chemicals, biofuels, food, and feed) and bioenergy (Mitra and Mishra, 2019). Some strategies can be applied in biorefineries to integrate with a sustainable economy (circular bioeconomy), such as reducing costs and increasing yields by using flue gases and/or wastewater as nutrient sources. Furthermore, microalgal biomass can be fully used to obtain various compounds and coproducts with high value (Bongiovani et al., 2020; Rosero-Chasoy et al., 2021).

An option to increase the market value and potential applications of EPS are the use of nanotechnology techniques. The nanometer scale can bring to EPS different physicochemical properties and these structures can interact with cells and tissues at the molecular level (Dutta and Das, 2021). An EPS product can act more specifically as an bioactive compound though nanoencapsulation and its biodisponibility can also be increased (Roychowdhury et al., 2021). EPS as its antioxidant and anti-inflammatory characteristics, can be applied for the development of nanostructured scaffolds mixed with polymers that can be used as a smart dressing that speeds up the healing process (Morais et al., 2014).

In this context, this review article aims to address aspects related to EPS production from microalgae, focusing on strategies to improve production efficiency, environmentally friendly extraction techniques, and potential applications of these biocompounds. The application of nanobased structures and products with EPS in pharmaceutical, cosmeceutical, water treatment, and food industries will be also addressed. In addition, the integrated EPS obtention with the coproduction of compounds with a high value-added under the biorefinery concept, aiming to minimize costs and overcome the challenges of large-scale production will be encompassed.

Section snippets

Microalgal sources of exopolysaccharides: characteristics, cultivation strategies, and extraction methods

Microalgal EPS are mainly heteropolysaccharides formed from xylose, glucose, and galactose, and considerable amounts of monosaccharides such as fucose, methylated sugars, rhamnose, and iduronic acid (Michaud, 2018). In addition, they may contain non-sugar substituents, such as pyruvate, proteins, and sulfate (Table 1) (Delattre et al., 2016; Pereira et al., 2009). In general, their structure is complex and can contain from 9 to 12 different monosaccharides (Delattre et al., 2016). Although most

Production of exopolysaccharides in a biorefinery framework

Microalgal EPS are a by-product that can be obtained under the biorefinery concept, because after the biomass concentration they can be removed from the supernatants without the generation of residues. In addition, polysaccharides synthesized by most microalgal species are composite heteropolymers. Based on this, the bioactive compounds have a wide spectrum of molecular weights and ions that result in high value-added applications in several areas, for example pharmaceuticals, cosmetics,

Biological activity

Studies have reported different biological activities of EPS produced by microalgae (Table 1). Their antioxidant, anti-inflammatory, antitumor, and antimicrobial properties make microalgal EPS promising for pharmaceutical, cosmetic, and food applications.

EPS containing uronic acids and charged groups (pyruvate and sulfate) are characterized by an anionic nature and have been associated with several biological activities (De Philippis and Vincenzini, 1998; Sun et al., 2009; Tannin-Spitz et al.,

Current challenges and future prospects

Microalgal EPS production is limited by two main factors: production costs and reduced knowledge about these compounds produced by microalgae (Patel et al., 2013). The biomass production by the photosynthetic pathway is what generally increases the cost of production. Despite the high yield of polysaccharides, the amount of biomass produced does not make the product competitive compared to EPS extracted from plants or macroalgae (Delattre et al., 2016).

The alternatives to overcome this issue

Conclusion

Microalgae can produce EPS with structurally diverse and biological properties that vary according to species, cultivation, and extraction conditions. Moreover, the potential for expanding the use of these compounds was demonstrated with the production of nanostructures. However, although EPS are used industrially, microalgae represent only a small portion of the market. For greater industrial use of microalgal EPS, technological stages of production must be optimized to reduce large-scale

CRediT authorship contribution statement

M.G. Morais: Conceptualization, Writing – review & editing, Supervision. T.D. Santos: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. L. Moraes: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. B.S. Vaz: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. E.G. Morais: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. J.A.V. Costa:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES) – Finance Code 001, National Council for Scientific and Technological Development (CNPq), Ministry of Science, Technology and Innovation (MCTI), Research Support Foundation of the State of Rio Grande do Sul (FAPERGS), and Project CAPES-PRint FURG for their support of this study.

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