Climate change is the long-term shift of weather patterns triggered by changes in atmospheric temperature. Human interference over the last two centuries had accelerated this slow natural process. Which led to increasing atmospheric temperature termed ‘Global Warming‘. To this, the major contributors are increased anthropogenic Carbon dioxide (CO₂) and other Greenhouse Gases (GHGs) in the atmosphere. Greenhouse gases are emitted by the combustion of fossil fuels during industrial development and transportation. To stop the climate change scenario, reducing air pollution, controlling CO₂ emissions, and environmental Carbon capture are the only solutions.

Naturally, photosynthetic species of microorganisms and plants are major CO₂ fixers on the Earth. But, only a natural process won’t be enough and requires positive human intervention. Novel approaches for CO2 scrubbing include chemical and physical techniques of CO₂ absorption along with novel membrane-based adsorption technologies. Nevertheless, Ecological solutions also have a potential way out and algae-based carbon capture could be a significant alternative approach. 

What are Greenhouse Gases (GHGs)?

The gases that trap heat energy and increase the atmospheric temperature are called Greenhouse Gases (GHGs). Moreover, different GHGs have a varying capacity for heat entrapment, which is generally referred to as Global Warming Potential (GWP). GWP measures relative heat absorption by 1-ton emission of any GHGs in comparison with 1-ton emission of CO₂. CO₂, Methane (CH₄), and Nitrous oxide (N₂O) are major GHG but Fluorinated-gases, especially Hydrofluorocarbons (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons) are high-GWP gases even in the least concentrations.

Major CO₂ contributors

Industrialization and population growth demanded a forever-increasing need for energy, natural resources, and transportation. This has led to the miss managed exploitation of fossil fuels and natural resources. No doubt, the major industrial sector that contributed to a large amount of CO₂ release were energy and transportation. Moreover, manufacturing & construction, agriculture, urbanization and its waste, aviation & shipping, etc. have also contributed to this. Within the last 30 years, CO₂ emissions have doubled in the sector of energy to generate electricity and heat. Country-wise, the major CO2-emitting countries are either manufacturers, producers, or consumers of the world’s resources. The countries like China (35.4%), the United States (19%), India (8.9%), Russia (6.3%), and Japan (3.8%) contribute to almost 3/4^th of CO₂ emissions in comparison with the rest of the world (26.6%).

The trend of World CO2 rise in the last 200 Years

In the pre-industrialization era, the atmospheric CO₂ was 278ppm which has increased in the last 200 years to 417ppm. Which is almost a 50% increase in the CO₂ level from the original. Additionally, in the last 70 years, it has rapidly risen from 5,000 million metric tons to more than 30,000 million metric tons. This significant rise in atmospheric CO₂ level has disturbed the Earth’s global temperature balance and led to an increase in the atmospheric temperature almost by 1 degree Celsius (1.8 degrees Fahrenheit). And it is increasing at a rate of more than 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade.

Global Warming

At a rate of 0.2 degree Celsius per decade, the world’s temperature would attain one degree Celsius more raise in the coming 50 years. This will make a total of 2-degree Celsius increase in the preindustrial era. A sudden increase in temperature will significantly impact Earth’s atmosphere affecting the ocean’s cyclical pattern to volcanic activities. All these changes will lead to devastation on Earth that never happened in human history. Reducing CO₂ emission, and capturing to sequester the environmental CO₂ are the only viable solutions to reduce the global warming impact.

Ways of CO₂ Sequestration and Associated Challenges

With current ongoing applications of fossil fuels and the lack of prominent alternative renewable energy, the release of CO₂ will be unavoidable. Keeping CO₂ below the level of the specified limit of GHGs to avoid global warming is known as a carbon budget and only that much CO₂ release could be permissible. Major fundamental optimizations in industrial operations are required to attain net-zero environmental CO₂ release. For CO₂ sequestration, Carbon Capture and Utilization (CCU), and Carbon Capture and Storage (CCS) are the two considerable options.

Along with CO₂ reduction, CCU offers consumption of CO₂ as a raw material for various industrial, research, and commercial prospect and reduce the need of generating new CO2. Under CCS, captured CO2 can be stored under earth-crest-depleted oil and gas reservoirs, and under the oceanic bed, making sure that it will never be released back into the environment. But the risk associated with CCS needs critical evaluation before implementation.

Algae show opportunities in both CCU and CCS. Algae sequester and utilize CO₂ as a carbon source and store it in the form of algal biomass.

Carbon sequestration by Algae

Oceans are major sinks for global anthropogenic carbon and algae plays a major role in it. Algae photoautotrophically utilizes CO₂ and Water in presence of Sunlight to produce Glucose and O₂. Photosynthesis reaction has light-dependent and light-independent Phases, both happening inside the chloroplast’s thylakoid and stroma respectively. In the light-dependent phase, light photons donate energy to produce chemical energy ATP and NADPH, using water and releasing O₂. This chemical energy (ATP) is utilized in the light-independent phase to produce glucose from CO₂.

Microalgae can fix > 45 % of the total CO₂ and contribute to 40% of total oceanic productivity. They carry massive amounts of organic carbon into the ocean contributing to carbon biological pumps (Reference: Marella et al. 2020, Tréguer et al. 2018). Microalgae production sequesters ~1.8Kg of CO₂ /Kg of dried algal biomass. And 2.7 tons/day of CO₂ /Acre. Carbon capture by microalgae is 10 to 50 times higher in amounts than by terrestrial plants. Algae-based CO₂ sequestration on an industrial scale has proven to be one of the promising ways to deal with climate change.

Carbon capture by algae in wastewater

Large-scale cultivation of microalgae either in freshwater or marine water with additional nutrients is depending upon their growth requirements and intended final use. Certainly, algae cultivation for CO₂ sequestration demands a lot of water. Domestic and industrial wastewater contains lot many contaminants and nutrients that support algal growth. The ratio of C: N: P calculated for wastewater is around 20:8:1 and algae require this ratio at 50:8:1. So, instead of releasing inefficiently treated wastewater into natural water bodies this water could be fortified with additional CO₂ and then utilized for algae cultivation along with CO₂ sequestration.

Various microalgal species are potential CO₂ scavengers and copious growing diatoms are one example. Diatoms basically grow in highly polluted water bodies to neutralize eutrophication. Diatoms fix 20% of the total anthropogenic CO₂, which makes them a potential candidate for wastewater bioremediation along with CO₂ sequestration.  

Apart from CO₂ sequestration potentials, microalgal biomass has many commercial applications including in biofuels and nutritional products.

Limitations of the Algae-based carbon sequestration technology

Light, water, and nutrients are the basic requirement of algae for their growth. Sunlight is available for half day period and using freshwater & pure nutrients for algae cultivation would lead to an unsustainability issue. Facilitating algae cultivation with specialized light sources for nighttime could resolve the issue at an increased cost for light energy, but this will help to keep the process continuous. For fresh water and nutrient sources as mentioned earlier, wastewater could be the potential source and other water resources like marine water can also play an important role.  

Conclusion and future prospects

Algae are the best know environmental agents in carbon capture. Along with their various industrial application, they have proven to create a pavement for the global carbon issue. Finding robust microalgae strains or consortiums for effective CO₂ sequestration is the key component in the reduction of GHGs and water pollutants.  Algae biotechnology promises the development of circular economy and biorefinery concepts. Profound research and an effective transition from laboratory studies to industrial scale will be critical steps in this process. Already established comprehensive scientific knowledge on algae-based CO₂ sequestration, wastewater treatment, biofuels, and various commercial applications of algae has started taking a shape for sustainable development. And the progress made in this field will definitely lead to carbon neutrality in near future.

Reference

1. Hannah Ritchie, Max Roser and Pablo Rosado (2020) – “CO₂ and Greenhouse Gas Emissions”. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions’ [Online Resource]
2. Marella, T. K., López-Pacheco, I. Y., Parra-Saldívar, R., Dixit, S., & Tiwari, A. (2020). Wealth from waste: Diatoms as tools for phycoremediation of wastewater and for obtaining value from the biomass. Science of the Total Environment, 724, 137960.
3. Tréguer, P., Bowler, C., Moriceau, B. et al. Influence of diatom diversity on the ocean biological carbon pump. Nature Geosci11, 27–37 (2018). https://doi.org/10.1038/s41561-017-0028-x