Microalgae are a diverse group of single-celled plant-like organisms that are usually photosynthetic and can be found in marine and freshwater ecosystems. They are the main primary producers in these systems and play a major role in aquatic food webs. However, increasing attention is now being drawn to the economic potential of microalgae, thanks to the wide variety of molecules they produce, which have extensive applications in food/feed, pharma, and industry.
The molecules produced by microalgae can largely be divided into two categories: (i) low-value compounds, like proteins, lipids, and carbohydrates, which are produced in large amounts, and (ii) high-value compounds, such as vitamins, carotenoids, and MAAs, which are produced in small amounts. The limited production of high-value compounds makes them more expensive and restricts the economic viability of the production process. “Upscaling the production of high-value compounds within the organism could result in a more economically viable production system and provide a major boost towards a more sustainable bioeconomy,” says Elke Vereecke, predoctoral fellow in plant genome editing working on GeneBEcon at Flanders Research Institute for Agriculture, Fisheries and Food (ILVO), Belgium. “One way to reach this goal is through selection of the right species and gene editing.”
Picking the right species for the job
Selecting the most suitable species is an essential first step to an economically viable production process. While there are numerous species of microalgae, not all species are equally suited for large-scale production due to differences in their metabolism.
Most microalgae are autotrophic and use light energy to convert carbon dioxide into sugars while boosting the production of high-value compounds such as MAAs. However, some microalgae, such as Chlorella species, are mixotrophic, which can provide some major advantages for the production process. Mixotrophic microalgae can switch between an autotrophic metabolism in the presence of light and a heterotrophic metabolism in the absence of light, where they consume organic molecules for growth.
“Growing microalgae in the absence of light provides larger yields while simultaneously requiring less space and reducing costs,” says Vereecke. “Once enough microalgae are produced, their autotrophic metabolism can be triggered by providing light, thus kickstarting the production of the high-value components. By combining heterotrophic and autotrophic growth, large quantities of microalgae can be produced in a small amount of space and time while reducing costs and still creating the desired high-value compounds.”
Upscaling the production process with gene editing
After selecting the optimal species and cultivation mode, the production of high-value compounds can be further increased by introducing mutations. Historically, mutations were introduced in plants by exposure to high doses of UV light or radiation. However, this method led to uncontrolled mutations, which could create unwanted and unexpected byproducts. By using targeted gene editing techniques, such as CRISPR-Cas, the production of desirable high-value compounds can be increased while minimizing the risks of unforeseen byproducts.
CRISPR-Cas is a technique used to edit specific parts of an organism’s genome by either removing, adding, or changing small sections of the DNA sequence. Conceptually, it involves the targeted cutting of the DNA strand at a predefined location, allowing the insertion or removal of a small part of DNA. Subsequently, the DNA will repair itself, encapsulating the introduced changes.
To date, gene editing is widely used in larger terrestrial plants but has received limited attention in microalgae, largely because the genome of most microalgae is still unknown, making targeted DNA cuts impossible. The genomes of some model microalgae like Chlamydomonas species are known, but this genus is not universally mixotrophic, thus providing other difficulties in the production process. However, because of the substantial bioeconomical potential of microalgae, microalgal research and genomic resources are steadily catching up, gradually opening the door for gene editing in species more suited for the production process.
Towards a more sustainable bioeconomy
To date, the overall cost to grow microalgae is high, mainly because current infrastructure largely aims to extract single compounds while discarding other potentially useful but low-value products. For instance, waste streams include proteins that could be used in food or feed, and essential amino acids like omega 3 that could be used in supplements. By combining the selection of the right species with gene editing, we could open the door to dual-purpose production, wherein high-value compounds are extracted, and the residual ‘waste’ is redirected into other applications. Doing so would facilitate the transition to a more sustainable bioeconomy and reduce the relative cost of production for individual compounds.
While the biofuel bubble supported by microalgae burst about a decade ago, the potential of microalgae to help reduce the impact of the economy on the environment remains relevant today. Microalgae can be a source of numerous low- and high-value products, but to upscale the production of high-value compounds, we need to boost the production of these molecules in individual microalgae organisms, potentially through gene editing. If we can follow this chain of production and transition from a single-purpose production towards a multi-purpose production process, we can fully exploit the potential of microalgae and move towards a more sustainable tomorrow.
