The high emission of greenhouse gases has been one of the foremost global threats with far-reaching consequences. Algae are photosynthetic eukaryotic organisms that are categorized as microalgae (unicellular) and macroalgae (multicellular) based on their size. Scientists have indicated that algae are extremely important to reach the global aim of zero carbon emissions by 2050.
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Since the industrial revolution, global carbon dioxide (CO2) emissions have escalated from 9.34 billion metric tons in 1960 to 36.44 billion metric tons in 2019. The US Environmental Protection Agency (EPA) has pointed out that some of the major contributors to atmospheric carbon dioxide are electricity and heat production, agriculture, forestry, industry, and transportation.
The United Nations Environment Programme stated that to prevent global warming to reach more than 1.5 ⁰C, the world would need to lower carbon emissions by roughly 7.6% annually for the upcoming decades.
Some of the recent strategies used to reduce the level of CO2 from the atmosphere involve bio capturing of CO2 and bioconversion to valuable products and renewable energy.
Capturing carbon is also known as sequestration, which involves taking carbon dioxide from the atmosphere, decarbonizing industries, and promoting clean fossil energy production. These strategies could effectively mitigate rapid climate change and global warming.
Strategies to Reduce Carbon Dioxide Emission
Algae are fast-growing aquatic organisms that are commonly found as seaweed, pond scum, and giant kelp. Scientists stated that one of the most sustainable strategies to capture and store CO2 from the atmosphere is photosynthesis.
Several studies have indicated that microalgae have exhibited maximum carbon-fixing capabilities. These studies have shown that microalgae can capture around 100 Gt of CO2 into biomass annually.
Carbon capture and storage (CCS) technologies have been indispensable in both fossil fuels- and biofuels-based power plants. This is because these technologies help to lower CO2 emissions from power generation by capturing carbon during pre-combustion, post-combustion, and oxy-fuel combustion. Although extensive research is being conducted to develop advanced conventional CCS technologies, public acceptance of this technology is still low.
As transportation and logistics sectors have been key contributors to CO2 emission, scientists believe that this emission can be significantly reduced via capturing and storing of CO2 onboard. Importantly, this system of recycling captured CO2 is similar to conventional fuel production from renewable energy sources. In addition, it can capture 90% of the emitted CO2 without any energy penalty.
Some of the drawbacks of the conventional CCS technologies involve high energy consumption, difficulties associated with transportation, immediate utilization of captured CO2, and a high possibility of leakage into the atmosphere during storage. The key advantages of microalgae-based CCS technologies are that they eliminate the above-mentioned limitation of conventional CCS technologies. Another advantage of this system is that compared to conventional CCS technologies, its operational cost is significantly low and is an eco-friendly strategy.
Microalgae and Lowering of Carbon Dioxide Emission
Blue-green algae (cyanobacteria), green algae, red algae, and diatoms are commonly referred to as microalgae. These organisms have been used for capturing carbon dioxide and sequestration.
At present, scientists have been involved in bio-capturing carbon dioxide using algae, and conversion of microalgae biomass containing CO2 into energy (biofuel) or other value-added products. This field of science, involving carbon dioxide mitigation, bioconversion, and circular carbon economy, is fast evolving.
Microalgae-based carbon capture technology is dependent on the species of microalgae, cultivation systems, and growth conditions (e.g., temperature, culture medium, pH, salinity, turbidity, light intensity, etc.). Optimization of algal growth also impacts the concentration of CO2 captured, for example, an increase of pH from 7.9 to 9.5 has led to a reduction of CO2 by two orders of magnitude.
Scientists have reported that Chlorella exhibited promising CO2 capturing capacity. This algal species can grow in an atmosphere containing 40% (v/v) CO2 with a fixation rate of 0.77 to 2.22 g/L/day.
Oscillatoria, a freshwater alga, also possesses the capacity to reduce carbon dioxide. Some of the advantages of using this alga in integrated biorefineries include its tolerance to high temperatures, toxic gases, and the possibility to produce high-value products.
Researchers reported that at an optimal temperature of 25-30°C and pH between 7 and 9, Oscillatoria exhibited significantly higher CO2 capture capacity. Several studies have shown Scenedesmus sp and Chlorococcum sp can be used to capture CO2 present in effluents and those emitted from industrial activities.
Conversion of Captured CO2 by the Microalgae
The CO2 captured from the atmosphere by microalgae via photosynthesis is stored in its biomass. It is converted to value-added products, such as bioenergy (e.g., biogas), bio-oil, biochar, and syngas.
Although the potential of microalgae to capture carbon is significantly high, other low-value bulk products, such as proteins for food application and fatty acids for nutraceuticals, are not feasible. However, recently, researchers have developed a concept of integrated bio-refinery using a microalgae-based biological carbon-capture approach that enables enhancement in the availability of biomass feedstock as well as extraction of high value-added products.
Various high-value and low-value marketable products are obtained from integrated biorefinery systems that include biofuel, i.e., biodiesel, biogas, bioethanol, and butanol. Additionally, algal pigments, such as chlorophyll a and b, phycobilins, lutein, astaxanthin, β-carotene, and C- phycocyanin, are widely applied in dyes, cosmetics, food and feed additives, pharmaceutical and nutraceuticals. Microalgae are also considered to be a rich source of amino acids, vitamins, fatty acids, and carbohydrates.
Challenges Associated with Capturing Carbon by Microalgae
Microalgae need to be cultivated outdoor and, thereby, the varying environmental conditions influence their growth.
Most of the research and development associated with carbon capture using microalgae is still in the laboratory phase, but researchers are optimistic about developing large-scale outdoor devices for optimal algal cultivation.
More research is required to understand the CO2 absorption process around the cell membrane of microalgae. During algal cultivation, there is a risk of contamination by certain bacterial and fungal species.
References and Future Reading
Koka, J. (2022) How can we further reduce CO2 emissions? New study reveals algae can help. [Online] Available at: https://www.anl.gov/article/how-can-we-further-reduce-co2-emissions-new-study-reveals-algae-can-help
Onyeaka, H. et al. (2021) Minimizing carbon footprint via microalgae as a biological capture. Carbon Capture Science and Technology. 1. https://doi.org/10.1016/j.ccst.2021.100007
Ou, L. et al. (2021) Utilizing high-purity carbon dioxide sources for algae cultivation and biofuel production in the United States: Opportunities and challenges. Journal of Cleaner Production. 321. https://doi.org/10.1016/j.jclepro.2021.128779
Jaiswal, A. et al. (2020) Biochemical Pathways Regulated by Algae to Mitigate Global Carbon Emissions: A Review. Journal of Environmental Pathology, Toxicology and Oncology: official organ of the International Society for Environmental Toxicology and Cancer, 39(4). pp. 317–334. https://doi.org/10.1615/JEnvironPatholToxicolOncol.2020034921
Singh, J. and Dhar, W.D. (2019) Overview of Carbon Capture Technology: Microalgal Biorefinery Concept and State-of-the-Art. Frontiers in Marine Science. 6. https://doi.org/10.3389/fmars.2019.00029