The Promise of Microalgae in a Changing World
Our reliance on agriculture for sustenance, healthcare, and resources has been essential since the dawn of civilization. However, traditional agricultural practices are no longer adequate to meet the demands of a burgeoning population amidst climate-driven agricultural challenges. Microalgae emerge as a beacon of hope, offering a sustainable and renewable source of food, animal feed, and energy. Their rapid growth rates, adaptability to non-arable land and non-potable water, and diverse bioproduct range, encompassing biofuels and nutraceuticals, position them as a cornerstone of future resource management. Furthermore, microalgae’s ability to capture carbon aligns with environmental conservation goals.
While microalgae offers significant benefits, obstacles in cost-effective biomass production persist, which curtails broader application. This review examines microalgae compared to other host platforms, highlighting current innovative approaches aimed at overcoming existing barriers. These approaches include a range of techniques, from gene editing, synthetic promoters, and mutagenesis to selective breeding and metabolic engineering through transcription factors.
Microalgae: A Sustainable Biotechnology Platform
Humans have cultivated plants as a sustainable source of food, medicine, and materials for millennia. Since the first Agricultural Revolution (10,000 BC), we have optimized our agricultural practices to meet the increasing demands of our civilization (Harlander, 2002). Today, with growing populations and food production shortcomings brought about by climate change, we can no longer count on the traditional crop optimization cycles to keep the world fed. According to the United Nations, the world population is expected to increase to about 10 billion by 2059 (World Population Prospects, 2022). Over-exploitation of arable land, rising global temperatures, changing climate, and extreme weather make land crops an increasingly strained source of food, feed, and energy (Kurukulasuriya and Rosenthal, 2013). Hence, new technology and resources are essential to meet the needs of future generations.
Microalgae hold significant promise as a sustainable and renewable source of food, feed, and energy. Microalgae are microscopic photosynthetic organisms that have high growth rates, can be cultivated using non-arable land and non-potable water, and have the ability to produce a variety of bioproducts, such as food supplements, biofuels, biopolymers, nutraceuticals, animal feeds, and medical therapeutics (Khan et al., 2018; Dolganyuk et al., 2020; Torres-Tiji et al., 2020; Diaz et al., 2023). Additionally, microalgae capture and utilize carbon dioxide (CO2) from the atmosphere to make these products, helping to mitigate greenhouse gas emissions (Onyeaka et al., 2021).
With their versatile bioproducts production capabilities, ability for carbon sequestration, and capacity to do this using non-arable land and non-potable water, microalgae offer a promising avenue for meeting society’s future demands, while reducing environmental impacts associated with this increased production. However, despite their immense potential, the lack of concerted domestication efforts has resulted in relatively expensive biomass production. This “chicken or egg” problem, where large-scale cultivation is needed to achieve low-cost production, and low-cost production is needed for large-scale utilization, has slowed the rate at which algae will attain widespread utilization (Chen and Wang, 2022). Overcoming the initial cost barriers will be crucial to fully exploit the advantages microalgae offer and establish them as low-cost, sustainable, and scalable solutions for the future (Lane, 2022).
Advancing Microalgae through Innovative Approaches
Past research endeavors have demonstrated continuous improvements in a number of the properties of algae cultivation, aiming to either boost biomass production or optimize the downstream process (Chu, 2017; Maity, 2019; Kang et al., 2022; Chettri et al., 2023). Several methods have been utilized to enhance biomass production, including improved pond design, improved crop protection, better growth media, and water chemistry, improving photosynthetic efficiency, working with extremophile strains, and optimizing strain development through molecular engineering, breeding, selection, and in-vitro evolution.
For enhanced metabolic engineering, multiple techniques are available, one of which involves either overexpressing or repressing functional genes (Mochdia and Tamaki, 2021; Chettri et al., 2023). Earlier literature has surveyed the landscape of engineering tools for algae (Mochdia and Tamaki, 2021; Sproles et al., 2021; Dhokane et al., 2023; Khoo et al., 2023; Patel et al., 2023). In our current review, we update and expand upon these evaluations, providing fresh insights into the field.
Our discussion starts with a comparison of microalgae against alternative production platforms, emphasizing new methods intended to improve the quality of microalgae-derived biomass. We delve into various methodologies such as gene editing, the introduction of synthetic promoters, mutagenesis, selective breeding, adaptive laboratory evolution, and metabolic engineering driven by transcription factors. We also present a thorough survey of studies focused on transcription factor-mediated metabolic engineering in microalgae. Additionally, we confront the existing hurdles and forecast potential developments, stressing the crucial integration of these innovative tools into commercially valuable algae strains.
The Bioeconomy and the Role of Microalgae
Today, we utilize renewable biological resources for food, materials, and energy, which offers an alternative to fossil resource-based economies (Bugge et al., 2016). A sustainable bioeconomy is one that includes a stronger focus on sustainability, including energy utilization, reduced greenhouse gas emissions, more efficient water utilization, and overall a transformative change in resource production and consumption to a more environmentally focused alternative to the current fossil-based economy (Antar et al., 2021).
The bioeconomy presently utilizes a diverse range of production organisms, encompassing bacteria, yeast, fungi, plants, and mammalian cells (Table 1; Antranikian and Streit, 2022; Hankamer et al., 2023; Jo et al., 2023; Kamaludin and Feisal, 2023; Navarrete and Martínez, 2020; Nguyen and Lee, 2021; Sarwar and Lee, 2023; Soong et al., 2023; Zhang et al., 2017).
Table 1. Comparative overview of host organisms in biotechnology: Advantages and Disadvantages.
| Host Organism | Advantages | Disadvantages |
|---|---|---|
| Bacteria (e.g., Escherichia coli) | – Rapid growth – Well-characterized genetics – Easy genetic manipulation – Cost-effective for protein, chemical, and biofuel production |
– Less suitable for complex proteins requiring specific post-translational modifications – Challenges in processing cheap raw materials containing inhibitory compounds |
| Fungi (e.g., Aspergillus, Trichoderma reesei, Saccharomyces cerevisiae) | – Excellent at producing enzymes, acids, and pharmaceuticals – Generate sustainable biomaterials and food ingredients – Aid in fermentation processes |
– Limited knowledge of fungal genetics, metabolism, and physiology – Challenges in large-scale cultivation and product recovery |
| Mammalian Cells (e.g., CHO, BHK21, NS0, Sp2/0) | – Preferred for biopharmaceuticals requiring complex post-translational modifications – Secrete proteins directly |
– Maintenance and scalability challenges – Susceptibility to viral contamination – Decreased viability with successive passages due to genetic changes – Labor-intensive and expensive |
| Plants | – Sole sustainable source for food, feed, fiber, renewable fuels, and pharmaceuticals – Key for fermentation due to starch and sugar content |
– Risks to forest areas and biodiversity from biofuel production – Potential escalation of food prices – Strain on water resources – Environmental stressors reducing yields and causing economic losses |
| Microalgae | – Photosynthetic capabilities to transform light and CO2 into organic products – Ability to produce diverse bioproducts (food supplements, biofuels, nutraceuticals, etc.) – Rapid growth, lack of competition for resources, higher yields, metabolic diversity, utilization of non-arable land, and efficient carbon capture |
– Achieving economic viability for microalgae-based bioproducts remains a challenge – Optimization of cultivation, harvesting, extraction, and downstream processing costs is required – Deficit in sequenced genomes and genetically transformed microalgae strains – High risk of biological contamination in open pond cultivation – High downstream processing costs |
Bacterial hosts are employed as cell factories in the production of fuel, recombinant proteins, vitamins, chemicals, and plasmid DNA (Gonçalves et al., 2012; Ferrer-Miralles and Villaverde, 2013; Acevedo-Rocha et al., 2019; Cho et al., 2022). Notably, Escherichia coli is a popular choice in biomanufacturing due to its rapid growth, well-characterized genetic makeup, and the ease with which it can be genetically manipulated. It is commonly used for producing proteins, antibiotics, and small molecules, due to its cost-effectiveness, versatility, and rapid growth on various nutrients (Blount, 2015). It is a key model organism in molecular biology, contributing to understanding genetic code, replication, transcription, and translation. However, it is less suitable for complex proteins requiring specific post-translational modifications (PTMs) (Corchero et al., 2013). Escherichia coli produces about 30% of therapeutic proteins but lacks PTMs like glycosylation and phosphorylation, crucial for many protein therapies (Baeshen et al., 2015). Efforts are ongoing to engineer E. coli strains to overcome this limitation, but currently, complex proteins are mainly produced using mammalian cell culture. Escherichia coli is also utilized in biofuel and bio-alcohol production, benefiting from its ability to grow in different conditions and its high growth and metabolism rates (Chen et al., 2013; Koppolu and Vasigala, 2016; Wang et al., 2017; Liang et al., 2020). Challenges include processing cheap raw materials like cellulosic and hemicellulosic hydrolysates, which can contain growth-inhibiting toxic compounds and cause osmotic stress when using concentrated sugars (Koppolu and Vasigala, 2016).
The fungal kingdom, encompassing yeasts to filamentous fungi, excels in producing bio-based products like enzymes, acids, and pharmaceuticals (Meyer et al., 2016; Sanchez and Demain, 2017; Corbu et al., 2023). Species like Aspergillus, Trichoderma reesei, and Saccharomyces cerevisiae are key in recombinant protein and industrial enzyme production (El-Gendi et al., 2021; Lübeck and Lübeck, 2022). Fungi also generate sustainable biomaterials, food ingredients with prebiotic benefits, and aid in fermentation in food and beverage industries (Patel et al., 2016; Singdevsachan et al., 2016; Wösten, 2019). Agriculturally, they enhance crop growth through nutrient uptake symbiosis (Wu et al., 2022). However, some fungi pose health threats (Fisher et al., 2020). Despite their potential, our knowledge of fungal genetics, metabolism, and physiology is limited, necessitating advanced tools for better utilization (Naranjo-Ortiz and Gabaldón, 2020; El Enshasy, 2022). Challenges in large-scale fungal cultivation and product recovery remain, requiring further research for improved productivity and cost-efficiency in fermentation.
Mammalian cell lines like Chinese hamster ovary (CHO) cells, baby hamster kidney (BHK21) cells, and murine myeloma cells (NS0 and Sp2/0) are preferred for biopharmaceuticals, particularly for complex proteins with specific PTMs (Dumont et al., 2016; Arnau et al., 2019). These cells secrete proteins directly, avoiding the need for cell lysis and protein refolding required in bacterial production. However, non-human mammalian cells might introduce non-human PTMs, potentially causing antibody responses in humans (Ghaderi et al., 2010). While crucial for therapeutic proteins and vaccines, mammalian cell cultures face challenges like maintenance, scalability, susceptibility to viral contamination, and decreased viability with successive passages due to genetic changes (Li et al., 2010; Seth, 2012; Barone et al., 2020). Mammalian cell-based production, though effective, can be labor-intensive and expensive.
Plants are crucial for a sustainable bioeconomy, being the sole sustainable source for food, feed, fiber, renewable fuels, pharmaceuticals, and carbon sequestration (Vanholme et al., 2013; Munir et al., 2022). They support all life as primary producers and are key for fermentation due to their starch and sugar content. However, utilizing plants for the production of biofuels may pose risks to forest areas and biodiversity, potentially escalate food prices, and create a strain on water resources (Koh and Ghazoul, 2008; Furtado et al., 2014; Ramos et al., 2016). Moreover, plants face challenges from environmental stressors like climate change, reducing yields, and causing economic losses (Dhankher and Foyer, 2018; Zaidi et al., 2020). Genetic engineering and traditional breeding improve crop resilience and nutritional content but raise concerns like cross-pollination and resistance to pests (Van Acker et al., 2007; Bawa and Anilakumar, 2013). Intense cultivation also leads to environmental issues like eutrophication and biodiversity loss (Tilman, 1999; Schütte et al., 2017; Li et al., 2020). Balancing the benefits and impacts of plant-based bioeconomy is a significant ongoing challenge.
Microalgae have emerged as a promising platform for the sustainable bioeconomy, due to their unique characteristics and versatile applications, and the fact that they do not compete with traditional crop cultivation (Khan et al., 2018). Compared to bacteria, fungi, and mammalian cells, microalgae offer several advantages in the context of sustainable bioproduct production (Table 1). Microalgae exhibit photosynthetic capabilities, enabling them to transform light and CO2 into organic carbon products, including proteins, lipids, and carbohydrates (Rasala and Mayfield, 2015). Microalgae possess a remarkable capacity to generate a diverse range of bioproducts, encompassing food supplements, biofuels, biopolymers, nutraceuticals, animal feeds, and medical therapeutics (Khan et al., 2018; Nur and Buma, 2019; Dolganyuk et al., 2020; Torres-Tiji et al., 2020; Diaz et al., 2023). Additionally, in comparison to plants, their biomass production offers several advantages, including rapid growth, lack of competition for resources used by crops, higher yields, metabolic diversity, utilization of non-arable land, nutrient recovery from wastewater, efficient carbon capture, and accelerated development of new production strains (Rasala and Mayfield, 2015; Fu et al., 2016; Benedetti et al., 2018).
However, achieving economic viability for microalgae-based bioproducts remains a challenge. The optimization of cultivation, harvesting, extraction, and downstream processing costs will all be required to ensure the competitiveness of these products against traditional sources. Although microalgae offer great genetic diversity, there is still a significant deficit in the number of sequenced genomes and the number of microalgae that have been genetically transformed in the lab (Lin et al., 2019; Maréchal, 2021). The full potential of microalgae is still largely unrealized due to our limited understanding of their metabolic pathways, regulatory networks, and genetic makeup (Kumar et al., 2020). At the commercial scale, growing microalgae in open ponds is a challenging task due to the high risk of biological contamination (Lam et al., 2018). Additionally, the high cost of downstream processing must be reduced to make microalgae a platform capable of producing products with commodity pricing (Khoo et al., 2020). All of these bottlenecks can be surmounted, albeit with substantial investments of time and resources. However, such investments are imperative for preserving our
