Introduction

Algae, encompassing both microalgae and macroalgae, have gained significant attention in recent decades as a promising biomass resource for sustainable production of bioactive molecules and renewable energy. Microalgae are unicellular photosynthetic organisms capable of rapid growth and high productivity, while macroalgae (seaweeds) are multicellular marine plants rich in polysaccharides, proteins, pigments, and other bioactive compounds. The ability of algae to grow in diverse habitats, including freshwater, marine, and wastewater systems, along with their high photosynthetic efficiency and carbon sequestration potential, makes them highly suitable for integrated biorefinery approaches aimed at circular bioeconomy and sustainability.

Algal biomass is an exceptional resource due to its multifaceted applications. It serves as a source of high-value bioactive compounds such as proteins, peptides, polysaccharides, lipids, pigments, antioxidants, vitamins, and secondary metabolites [1]. These bioactive molecules have extensive applications in pharmaceuticals, nutraceuticals, cosmetics, and functional foods, offering antimicrobial, anticancer, anti-inflammatory, and antioxidant properties [16]. Simultaneously, algal biomass can be converted into various forms of renewable energy, including biodiesel, bioethanol, biogas, and hydrogen, thereby addressing global energy demand while reducing reliance on fossil fuels and mitigating greenhouse gas emissions.

Recent advances in algal biotechnology have led to the development of innovative cultivation and bioprocessing technologies that enhance biomass yield and metabolite productivity. Open pond systems, raceway ponds, and photobioreactors (PBRs) have been optimized for light, nutrient supply, gas exchange, and mixing efficiency. Hybrid cultivation systems integrating open ponds with controlled photobioreactors further maximize biomass production while minimizing contamination risks. Strain selection and improvement, either through conventional breeding or genetic engineering, have resulted in algae with superior growth rates, enhanced lipid content, and increased production of bioactive compounds [2], downstream processing and extraction technologies have seen remarkable innovations. Green extraction methods, such as supercritical fluid extraction, ultrasound-assisted extraction, microwave-assisted extraction, and enzymatic hydrolysis, offer eco-friendly and efficient recovery of high-value compounds while minimizing chemical usage [3]. These methods ensure the preservation of bioactive functionality and improve the yield of target molecules, making algal biorefineries economically and environmentally viable.

Integrated biorefineries represent the next step in algal biotechnology, where the simultaneous production of bioactive molecules and renewable energy is optimized through sequential fractionation and valorization of the biomass. By adopting a zero-waste approach, residual biomass can be utilized for animal feed, biofertilizers, or bioplastics, promoting a circular bioeconomy [4]. This integrated approach not only enhances resource efficiency but also improves the economic feasibility of algal-based technologies, which is essential for large-scale commercialization. The tremendous potential, challenges such as high production costs, contamination, energy-intensive harvesting, and downstream processing remain significant hurdles for industrial-scale implementation. Research efforts are increasingly focused on cost reduction, process optimization, strain improvement, and the development of robust, scalable, and sustainable biorefinery systems.

This review critically examines the recent technological innovations in algal biorefineries, with particular emphasis on sustainable production of bioactive molecules and renewable energy. It highlights advances in cultivation systems, bioprocessing techniques, extraction methods, and integrated biorefinery strategies, as well as their applications in medicine, nutrition, and energy production [5], challenges, opportunities, and future perspectives for commercial implementation of algal biorefineries are discussed to provide a comprehensive understanding of this rapidly evolving field.

2. Algal Cultivation Technologies

Efficient and sustainable biomass production is the foundation of any algal biorefinery. Cultivation technologies are continually evolving to enhance growth rates, optimize nutrient utilization, minimize contamination, and improve environmental control. Innovations in this area are critical to ensure consistent biomass supply for downstream production of bioactive molecules and renewable energy.

2.1 Open Pond Systems

Open pond systems, particularly raceway ponds, remain the most widely adopted and cost-effective method for large-scale algal cultivation. These systems offer low capital investment and simplicity of operation, making them suitable for bulk biomass production. Recent technological advances in open pond systems include automated monitoring of pH, dissolved oxygen, temperature, and nutrient concentrations, which allows real-time optimization of growth conditions. The use of computational fluid dynamics (CFD) for pond design enhances mixing and light distribution, minimizing dead zones and improving photosynthetic efficiency. Additionally, the integration of nutrient management strategies, such as staged nutrient feeding and CO₂ supplementation, has been shown to increase biomass productivity while reducing contamination by undesirable microorganisms [6]. Efforts to combine open pond cultivation with wastewater treatment have also gained attention, leveraging nutrient-rich effluents to support algal growth while contributing to environmental remediation.

2.2 Photobioreactors

Photobioreactors (PBRs) provide a closed cultivation environment that allows precise control over light, temperature, gas exchange, and CO₂ supplementation. This control minimizes contamination and enables year-round cultivation of high-value algal strains. Technological innovations in PBR design, such as tubular, flat-panel, vertical column, and hybrid configurations, have increased light penetration and volumetric productivity. Advanced PBRs also incorporate features like LED-based illumination, automated pH and dissolved oxygen control, and sensor-driven nutrient delivery, which together optimize biomass yield and metabolite accumulation [7], photobioreactors facilitate the cultivation of genetically engineered or high-value algal strains that are sensitive to environmental fluctuations, ensuring consistent production of target bioactive compounds.

2.3 Hybrid Systems

Hybrid cultivation systems integrate the advantages of open ponds and photobioreactors, balancing cost-effectiveness with productivity and environmental control. In such systems, initial biomass propagation may occur in controlled PBRs to ensure contamination-free cultures, followed by scaling up in open ponds for bulk production. Hybrid systems also allow the co-utilization of wastewater, industrial effluents, and nutrient-rich agricultural runoff to supply essential minerals while simultaneously treating waste streams [8]. This integration not only reduces production costs but also enhances the sustainability of algal biomass production, making it a viable approach for large-scale biorefinery applications.

3. Bioactive Molecules from Algal Biomass

Algae are a rich source of structurally diverse bioactive compounds, which exhibit therapeutic, nutritional, and industrial significance. Innovations in extraction and purification technologies have greatly enhanced the yield, stability, and functionality of these biomolecules, thereby increasing their commercial value.

3.1 Proteins and Peptides

Algal proteins are highly nutritious, containing all essential amino acids, and can serve as an alternative protein source in food, feed, and nutraceutical industries. Bioactive peptides derived from algal proteins demonstrate antihypertensive, antioxidant, antimicrobial, and immunomodulatory activities. Recent advancements in extraction techniques, including enzymatic hydrolysis, ultrasound-assisted extraction, and membrane-based separation, have improved protein yield and preserved bioactivity [17]. Optimization of these processes allows selective recovery of peptides with specific functional properties, expanding their potential applications in therapeutic formulations.

3.2 Polysaccharides

Polysaccharides from algae, including carrageenan, alginate, ulvan, and fucoidan, are highly valued for their bioactivity and industrial utility. These molecules exhibit antioxidant, antiviral, anti-inflammatory, and immunomodulatory properties, making them important for pharmaceutical, food, and cosmetic applications [9]. Modern extraction techniques such as microwave-assisted extraction, ionic liquid-based extraction, and enzyme-assisted methods improve polysaccharide purity, yield, and bioactivity, while reducing chemical usage and energy consumption. Recent research also focuses on fractionation and structural modification of polysaccharides to enhance their solubility and functional properties for specific applications.

3.3 Lipids and Fatty Acids

Algal lipids, particularly polyunsaturated fatty acids (PUFAs) including omega-3 fatty acids (EPA and DHA), have significant nutraceutical and pharmaceutical value. Lipid-rich algae are also a promising feedstock for biodiesel production. Strategies such as nutrient stress, metabolic engineering, and cultivation under controlled light and temperature conditions have been applied to enhance lipid accumulation. Recent developments in lipid extraction, including supercritical CO₂ extraction, solvent-free methods, and green solvents, have improved efficiency and environmental sustainability, allowing simultaneous production of high-value lipids and biofuels [18].

3.4 Pigments and Secondary Metabolites

Algal pigments such as carotenoids, chlorophylls, and phycobiliproteins serve as natural colorants, antioxidants, and therapeutic agents. Secondary metabolites including polyphenols and flavonoids also demonstrate potent antimicrobial, anti-inflammatory, and anticancer properties. Technological innovations in extraction and purification, such as supercritical fluid extraction, membrane-assisted separation, and green solvent technologies, have enhanced yield and stability while reducing environmental impact [10]. Optimization of cultivation conditions, such as light intensity, nutrient availability, and stress induction, further increases the production of these bioactive metabolites, enabling their industrial-scale application in medicine, food, and cosmetics.

4. Renewable Energy Production from Algal Biomass

Algal biomass offers a versatile platform for the production of multiple forms of renewable energy, addressing the dual challenges of energy security and environmental sustainability. Its high growth rate, ability to fix atmospheric CO₂, and capacity to accumulate carbohydrates, lipids, and proteins make it an attractive feedstock for biofuel production. Recent technological innovations have improved the efficiency, yield, and economic feasibility of bioenergy production from algae.

4.1 Bioethanol

Bioethanol production from algae primarily relies on carbohydrate-rich species. The polysaccharides in algal cell walls can be hydrolyzed into fermentable sugars, which are then converted to ethanol via microbial fermentation. Innovative pre-treatment techniques, including enzymatic hydrolysis, acid-assisted saccharification, and combined hydrothermal methods, have significantly enhanced sugar release while minimizing inhibitor formation [11]. Genetic engineering of fermenting microorganisms and algal strains has further improved conversion efficiency, allowing high ethanol yields. Integrating algal bioethanol production with wastewater treatment also provides an environmentally sustainable route by utilizing nutrient-rich effluents while producing clean fuel.

4.2 Biodiesel

Algal lipids, especially triacylglycerols (TAGs), are ideal feedstocks for biodiesel production due to their high energy content and favorable fatty acid profiles. Recent technological advancements have focused on optimizing lipid extraction and transesterification processes. Green solvents, supercritical CO₂ extraction, and solvent-free techniques reduce environmental impact while maintaining high lipid recovery. In situ transesterification, which combines lipid extraction and esterification in a single step, further reduces processing time and cost [12]. Additionally, metabolic engineering and nutrient manipulation strategies have been employed to enhance lipid accumulation in microalgae, making biodiesel production more economically viable at large scales.

4.3 Biogas and Hydrogen

Algal biomass is an excellent substrate for anaerobic digestion, producing methane-rich biogas through microbial decomposition. Pretreatment strategies such as thermal, enzymatic, and chemical hydrolysis improve the biodegradability of algal biomass, increasing biogas yield. Moreover, specific algal strains can be engineered or subjected to stress conditions to produce molecular hydrogen via photobiological or dark fermentation processes [13]. Hydrogen generated from algae is a clean, high-energy fuel with zero carbon emissions, offering potential for sustainable energy systems. Combined production of biogas and hydrogen from residual algal biomass enhances overall energy recovery, improving the efficiency of algal-based bioenergy systems.

5. Integrated Algal Biorefinery Approaches

Integrated algal biorefineries represent a holistic strategy to maximize resource utilization, economic feasibility, and environmental sustainability by producing multiple high-value products from a single biomass feedstock. These approaches adopt a zero-waste philosophy, ensuring that every component of the algal biomass is valorized.

Key approaches in integrated algal biorefineries include:

• Sequential Extraction of Bioactive Compounds: Lipids, proteins, polysaccharides, pigments, and secondary metabolites can be extracted in a stepwise manner, preserving their bioactivity and enabling the co-production of pharmaceuticals, nutraceuticals, and functional foods.
• Co-production of Biofuels and High-Value Molecules: Algal biorefineries can simultaneously produce bioethanol, biodiesel, biogas, and hydrogen, along with bioactive compounds such as antioxidants, omega-3 fatty acids, and pigments. This co-production strategy improves the overall economic viability of the process [15].
• Coupling with Wastewater Treatment: Algae can utilize nutrient-rich industrial or municipal wastewater as a growth medium, reducing the cost of cultivation while treating effluents. This integration contributes to sustainable waste management and circular bioeconomy principles.
• Utilization of Residual Biomass: Post-extraction residual biomass can be converted into value-added products such as biofertilizers, soil amendments, animal feed, or bioplastics, ensuring minimal waste and maximizing resource efficiency.
• Process Intensification and Automation: Innovations in reactor design, real-time monitoring, and process automation allow high-density algal cultivation and optimized product recovery, further enhancing productivity and scalability.

Integrated biorefineries not only improve economic feasibility but also minimize environmental impact, supporting sustainable production and resource-efficient use of algal biomass [14]. The combining bioactive molecule extraction with renewable energy production and wastewater remediation, these systems align with the principles of green chemistry, circular economy, and sustainable development goals.

7. Conclusion

Recent advancements in algal biorefinery technologies have greatly enhanced the potential for sustainable production of bioactive compounds and renewable energy. Innovations in cultivation systems, including open ponds, photobioreactors, and hybrid approaches, have improved biomass productivity while optimizing resource utilization. The development of efficient extraction and downstream processing techniques has facilitated the recovery of high-value biomolecules such as proteins, polysaccharides, lipids, pigments, and secondary metabolites, supporting diverse applications in pharmaceuticals, nutraceuticals, and functional foods.Integrated biorefinery strategies, combining biofuel production with bioactive compound extraction and wastewater treatment, ensure maximum resource efficiency, reduced environmental impact, and improved economic feasibility. These approaches align with circular economy principles by valorizing all components of algal biomass and minimizing waste generation, metabolic engineering, process intensification, and techno-economic evaluation will be critical to scale up algal biorefineries. By addressing current challenges related to cost, process efficiency, and commercialization, algae have the potential to play a pivotal role in the global bio-based economy, providing renewable energy solutions and high-value bioactive molecules to meet increasing environmental, industrial, and healthcare demands.

References

1. Bhatia, L., Bachheti, R. K., Garlapati, V. K., & Chandel, A. K. (2022). Third-generation biorefineries: a sustainable platform for food, clean energy, and nutraceuticals production. Biomass conversion and biorefinery, 12(9), 4215-4230.
2. Michalak, I., &Chojnacka, K. (2015). Algae as production systems of bioactive compounds. Engineering in Life Sciences, 15(2), 160-176.
3. Chandra, R., Iqbal, H. M., Vishal, G., Lee, H. S., &Nagra, S. (2019). Algal biorefinery: a sustainable approach to valorize algal-based biomass towards multiple product recovery. Bioresource technology, 278, 346-359.
4. Yadav, K., Vasistha, S., Nawkarkar, P., Kumar, S., & Rai, M. P. (2022). Algal biorefinery culminating multiple value-added products: recent advances, emerging trends, opportunities, and challenges. 3 Biotech, 12(10), 244.
5. Feng, S., Kang, K., Salaudeen, S., Ahmadi, A., He, Q. S., & Hu, Y. (2022). Recent advances in algae-derived biofuels and bioactive compounds. Industrial & Engineering Chemistry Research, 61(3), 1232-1249.
6. Zhu, L. (2015). Biorefinery as a promising approach to promote microalgae industry: An innovative framework. Renewable and sustainable energy reviews, 41, 1376-1384.
7. Moreno-Garcia, L., Adjallé, K., Barnabé, S., & Raghavan, G. S. V. (2017). Microalgae biomass production for a biorefinery system: recent advances and the way towards sustainability. Renewable and Sustainable Energy Reviews, 76, 493-506.
8. Hamid Nour, A., Mokaizh, A. A. B., Alazaiza, M. Y., Bashir, M. J., Mustafa, S. E., &Baarimah, A. O. (2024). Innovative strategies for microalgae-based bioproduct extraction in biorefineries: Current trends and future solutions integrating wastewater treatment. Sustainability, 16(23), 10565.
9. Olguín, Eugenia J., Gloria Sánchez-Galván, Imilla I. Arias-Olguín, Francisco J. Melo, Ricardo E. González-Portela, Lourdes Cruz, Roberto De Philippis, and Alessandra Adessi. “Microalgae-based biorefineries: challenges and future trends to produce carbohydrate enriched biomass, high-added value products and bioactive compounds.” Biology 11, no. 8 (2022): 1146.
10. Jacob-Lopes, E., Maroneze, M. M., Deprá, M. C., Sartori, R. B., Dias, R. R., &Zepka, L. Q. (2019). Bioactive food compounds from microalgae: An innovative framework on industrial biorefineries. Current Opinion in Food Science, 25, 1-7.
11. Martins, R., Sales, H., Pontes, R., Nunes, J., & Gouveia, I. (2023). Food wastes and microalgae as sources of bioactive compounds and pigments in a modern biorefinery: a review. Antioxidants, 12(2), 328.
12. Eppink, M. H., Olivieri, G., Reith, H., van den Berg, C., Barbosa, M. J., &Wijffels, R. H. (2017). From current algae products to future biorefinery practices: a review. Biorefineries, 99-123.
13. Salami, R., Kordi, M., Bolouri, P., Delangiz, N., &AsgariLajayer, B. (2021). Algae-based biorefinery as a sustainable renewable resource. Circular economy and sustainability, 1(4), 1349-1365.
14. Vernès, L., Li, Y., Chemat, F., &Abert-Vian, M. (2019). Biorefinery concept as a key for sustainable future to green chemistry—the case of microalgae. In Plant Based “Green Chemistry 2.0” Moving from Evolutionary to Revolutionary (pp. 15-50). Singapore: Springer Singapore.
15. Fabris, Michele, Raffaela M. Abbriano, Mathieu Pernice, Donna L. Sutherland, Audrey S. Commault, Christopher C. Hall, Leen Labeeuw et al. “Emerging technologies in algal biotechnology: toward the establishment of a sustainable, algae-based bioeconomy.” Frontiers in plant science 11 (2020): 279.
16. Uma, V. S., Zeba Usmani, Minaxi Sharma, Deepti Diwan, Monika Sharma, Miao Guo, Maria G. Tuohy et al. “Valorisation of algal biomass to value-added metabolites: emerging trends and opportunities.” Phytochemistry Reviews 22, no. 4 (2023): 1015-1040.
17. Narayanan, M. (2024). Biorefinery products from algal biomass by advanced biotechnological and hydrothermal liquefaction approaches. Discover Applied Sciences, 6(4), 146.
18. Sivaramakrishnan, R., Suresh, S., Kanwal, S., Ramadoss, G., Ramprakash, B., &Incharoensakdi, A. (2022). Microalgal biorefinery concepts’ developments for biofuel and bioproducts: current perspective and bottlenecks. International Journal of Molecular Sciences, 23(5), 2623.