By Robert Polon, Biological Sciences Major, ’21

Author’s Note: In my UWP 102B class, we were assigned the task of constructing a literary review on any biology-related topic of our choice. A year ago, in my EVE 101 class, my professor briefly mentioned the idea that algae could be used to sequester atmospheric carbon dioxide in an attempt to slow the rate of climate change. I found this theory very interesting, and it resonated with me longer than most of the other subject matter in that class. I really enjoyed doing the research for this paper, and I hope it gives people some hope for the future. I’d like to thank my UWP professor, Kathie Gossett, for pointing me in the right direction throughout the process of writing this paper.

Abstract

With climate change growing ever more relevant in our daily lives, scientists are working hard to find solutions to slow and reverse the damage that humans are doing to the planet. Algae-based carbon sequestration methods are a viable solution to this problem. Photosynthesis allows algae to remove carbon dioxide from the atmosphere and turn it into biomass and oxygen. It has even been proposed that raw algal biomass can be harvested and used as a biofuel, which can provide a greener alternative to fossil fuel usage. Though technology is not yet developed enough to make this change in our primary fuel source, incremental progress can be taken to slowly integrate algal biofuel into daily life. Further research and innovation on the subject could allow full-scale replacement of fossil fuels with algal biofuel to be a feasible option. Methods of algal cultivation include open-ocean algal blooms, photo bioreactors, algal turf scrubbing, and BICCAPS (bicarbonate-based integrated carbon capture and algae production system). There are many pros and cons to each method, but open-ocean algal blooms tend to be the most popular because they are the most economical and produce the most algae, even though they are the most harmful to the environment.

Keywords

Algae | Biofuel | Climate Change | Carbon Sequestration

Introduction

As we get further into the 21st century, climate change becomes less of a theory and more of a reality. Astronomically high post-Industrial Revolution rates of greenhouse gas emissions have started to catch up with humans, as the initial consequences of these actions are now coming to light with fear that worse is on the way. Many solutions have been proposed to decrease greenhouse gas emissions, but very few involve fixing the damage that has already been done. It has been proposed that growing algae in large quantities could help solve this climate crisis.

According to the Environmental Protection Agency, 76% of greenhouse gas emissions come in the form of carbon dioxide. As algae grows, it removes carbon dioxide from the atmosphere by converting it to biomass and oxygen via photosynthesis. Algae convert carbon dioxide to biomass at relatively fast rates. On average, one kilogram of algae utilizes 1.87 kilograms of CO2 daily, which means that one acre of algae utilizes approximately 2.7 tons of CO2 per day [1]. For comparison, one acre of a 25-year-old maple beech-birch forest only utilizes 2.18 kilograms of CO2 per day [2]. This amount of carbon dioxide sequestration can be done by only 1.17 kilograms of algae. After its photosynthetic purpose has come to an end, the raw algal biomass can be harvested and used as an environmentally-friendly biofuel. This literary review will serve as a comprehensive overview of the literature on this proposal to use algae as a primary combatant against global warming.

Carbon Dioxide

For centuries, heavy usage of fossil fuels has tarnished Earth’s atmosphere with the addition of greenhouse gases [3]. These gases trap heat by absorbing infrared radiation that would otherwise leave Earth’s atmosphere. This increases the overall temperature of the earth, which leads to the melting of polar ice caps, rising sea levels, and strengthening of tropical storm systems, among many other devastating environmental effects [4]. The most commonly emitted greenhouse gas, carbon dioxide, tends to be the primary focus of global warming treatments. 

These algal treatment methods are no different. Any algal treatment option is dependent upon the fact that algae sequester atmospheric carbon dioxide through photosynthesis. It converts carbon dioxide into biomass and releases oxygen into the atmosphere as a product of the photosynthetic process [5].

Algal Cultivation

There are four proposed methods of algal cultivation: open-ocean algal blooms, photobioreactors, algal turf scrubbing, and BICCAPS. These techniques all differ greatly, with various benefits and drawbacks to each.

Open-Ocean Algal Blooms

Algae is most abundant on the surface of the open ocean. With the addition of its limiting nutrient, iron, in the form of iron(ii) sulfate (FeSO4), massive algal blooms can be easily sparked anywhere in the ocean [3]. This seems to be the way that most scientists envision sequestration because, of all proposed cultivation techniques, this one produces the most algae in the least amount of time. Intuitively, this method removes the most carbon dioxide from the atmosphere, as the amount of CO2 removed is directly proportional to the quantity of algae undergoing photosynthesis.

There are many benefits to open-ocean algal blooms. There is no shortage of space on the surface of the ocean, so, hypothetically, there is a seemingly infinite amount of algal mass that can be cultivated this way. This technique is also very cost-efficient, as all you need to employ it is some iron(ii) sulfate and nature will do the rest [3]. 

Once the algal bloom has grown to its maximum size, there is an overabundance of algal biomass on the surface of the ocean. Some researchers have proposed that this mass be collected and used as a biofuel [5,6,7]. Others have proposed that we let nature play its course and allow the dead algae to sink to the bottom of the ocean. This ensures that the carbon dioxide it has taken out of the atmosphere is stored safely at the bottom of the ocean [8]. Here, the algal biomass is easily accessible for consumption by shellfish, who store the carbon in their calcium carbonate shells [3].

This solution is not an easy one to deploy, however, because algal blooms bring many problems to the local ecosystems. Often referred to as harmful algal blooms (HABs), these rapidly growing algae clusters are devastating to the oceanic communities they touch. They increase acidity, lower temperature, and severely deplete oxygen levels in waters they grow in [9]. Most lifeforms aren’t prepared to handle environmental changes that push them out of their niches, so it’s easy to see why HABs kill significant portions of marine life.

HABs can affect humans as well. Many species of alga are toxic to us, and ingestion of contaminated fish or water from areas affected by these blooms can lead to extreme sickness and even death. Some examples of these diseases are ciguatera fish poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, amnesic shellfish poisoning, and diarrheic shellfish poisoning [10]. The effects of harmful algal blooms have only been studied in the short-term, but from what we have seen, they are definitely a barrier in using this form of algae cultivation [11].

Photobioreactors

Photobioreactors are another frequently-proposed tool for cultivating algae. These artificial growth chambers have controlled temperature, pH, and nutrient levels that make for optimal growth rates of algae [12]. They can also run off of wastewater that is not suitable for human consumption. Photobioreactors minimize evaporation and, with the addition of iron, magnesium, and vitamins, increase rates of carbon dioxide capture are increased [1]. Due to the high concentration of algae in a relatively small space, photobioreactors have the highest rates of photosynthesis (and subsequently carbon dioxide intake) out of all of the cultivation methods mentioned in this paper.

This innovative technology was driven primarily by the need to come up with an alternative to triggering open-ocean algal blooms. Photobioreactors eliminate pollution and water contamination risks that are prevalent in harmful algal blooms. Furthermore, they make raw algal biomass easily accessible for collection and use as a biofuel, which open-ocean algal blooms do not [12].

The main drawback to this method is that the cost of building and maintaining photobioreactors is simply too high to be economically feasible right now [12]. Technological developments need to be made to lower the short-term cost of operation and allow for mass production if we want to use them as a primary source of carbon sequestration. Their long-term economic feasibility still remains unknown, as most of the cost is endured during the production of the photobioreactors. Money is made back through the algae cultivated, but the technology hasn’t been around long enough to show concrete long-term cost-benefit analyses without speculation [14]. 

Algal Turf Scrubbing (ATS)

Proposed in 2018 by botanist Walter Adey, algal turf scrubbing (ATS) is a new technique created to efficiently cultivate algae for use in the agriculture and biofuel industries. The process involves using miniature wave generators to slightly disturb the flat surface of a floway and stimulate the growth of numerous algal species in the water. Biodiversity in these floways increases over time, and a typical ATS floway will eventually have over 100 different algae species [11].

Heavy metals and other toxic pollutants occasionally make their way into the floways; however, they are promptly removed, to ensure that the product is as nontoxic as possible. The algal biomass is harvested bi-weekly and has a variety of uses. Less toxic harvests can be used as fertilizers in the agricultural industry, which research claims is the most economically efficient use for the harvest. It can also go towards biofuel use, although the creators of the ATS system believe the majority of their product will go towards agricultural use because they will not be able to produce enough algae to keep up with the demand (if our society moves towards using it as a biofuel) [11].

The problems with ATS are not technological, but sociopolitical, as the research team behind it fears that they will not get the funding and resources needed to perform their cultivation at an effective level [11].

BICCAPS

The bicarbonate-based integrated carbon capture and algae production system (BICCAPS) was proposed to reduce the high costs of commercial algal biomass production by recycling bicarbonate that is created when algae capture carbon dioxide from the atmosphere and using it to culture alkalihalophilic microalgae (algae that thrive in a very basic pH above 8.5). Through this ability to culture more algae, the system should, in theory, cut costs of carbon capture and microalgal culture. It is also very sustainable, as it recycles nutrients and minimizes water usage. The algae cultivated can also be turned into biofuel to lower fossil fuel usage [13].

The main drawback to this closed-loop system is that it does not cultivate as much algae as the other systems, though work is currently being done to improve this. It has been proven that the efficiency of BICCAPS significantly improves with the addition of sodium to the water, which stimulates the growth of alkalihalophilic microalgae [13]. This means that, with a little bit of improvement to the efficiency of the system, BICCAPS could become a primary algal biomass production strategy because of its low cost and sustainability. 

Use of Algae as a Biofuel

While algae may not have the energetic efficiency of fossil fuels, it is not far behind. It can be burned as a biofuel to power transportation, which would allow us to lower our use of fossil fuels and, subsequently, our greenhouse gas emissions. When dry algal biomass is burned, it releases more oxygen and less carbon dioxide than our current fuel sources. The increase in oxygen released into the atmosphere not only helps to lower CO2 emissions but increases the overall atmospheric ratio of oxygen to carbon dioxide. More research still has to be done to find the best possible blend of algal species for fuel consumption [12]. Solely using algae as a biofuel would not meet the world’s energy demand, but the technology for photobioreactors continues to improve, giving hope to one day use algae more than fossil fuels [6].

A common counterargument to proposals for algal biofuel usage is that burning dry algae only provides half the caloric value of a similarly-sized piece of coal. While this is true, it should be taken into consideration that that coal has an extraordinarily high caloric value and that the caloric value of algae is still considered high relative to alternative options [3].

It is often suggested that bioethanol, which essentially uses crops as fuel, should be used over algal biofuel. The main problem with this proposal is that farmers would spend more time cultivating inedible crops because they make for better fuel. This would lead to food shortages on top of the current hunger problem in our world. Farming crops also take up land, while growing algae does not [7].

Drawbacks

The main problems associated with using algae as a biofuel are technological and economical. We simply do not have the technology in place right now to produce enough algae to completely replace fossil fuels with it. In order to do this, we would have to establish full-scale production plants, which is not as economically viable as simply continuing to use the fossil fuels that degrade our planet [12]. Receiving funding for the commercialization of algae is the biggest obstacle this plan faces. It’s difficult to get money allocated to environmental conservation efforts because, unfortunately, it doesn’t rank very highly in our government’s priorities. Algal carbon sequestration has also never been observed at a commercial scale, so there is hesitation to fully commit resources to something that seems like a gamble.

Alternative Uses

It has also been proposed that algal biomass grown to sequester carbon dioxide should be used in the agricultural industry. As previously mentioned, the creators of ATS have suggested using it as a fertilizer [11]. Others say that it can be used to feed livestock or humans, as some cultures actually consume algae already [12]. The seemingly infinite supply of microbes can also be harvested and used heavily in the medical industry in the form of antimicrobial, antiviral, anti-inflammatory, anti-cancer, and antioxidant treatments [7].

Conclusion

Algae can be used to fight climate change because it removes carbon dioxide from our atmosphere, stores it as biomass, and replaces it with oxygen. Arguments have been made in many directions over the best method of algal cultivation. Triggering open-ocean algal blooms is certainly the most cost-efficient of these methods, and it produces the most algal biomass. The problem with using this technique is that these algal blooms have devastating ecological effects on the biological communities they come in contact with. Photobioreactors are another popular method among those who favor this strategy because of their ability to efficiently produce large quantities of algae; however, the main inhibition to their usage is the extremely high cost of construction and operation. With more focus on developing lower cost photobioreactors, they can potentially become the primary source of algal growth. Algal turf scrubbing is another strategy of algae cultivation that struggles with the problem of acquiring adequate funding for the operation. BICCAPS is a relatively inexpensive and eco-friendly way to grow algae in a closed system, but it yields low quantities of algal biomass compared to the other systems.

The raw algal biomass from these growth methods can potentially be used as a biofuel. Dry alga has a high caloric value, which makes it great for burning to power equipment. It does not burn as well as fossil fuels, but it does release more oxygen and less carbon dioxide than fossil fuels when burned. Of course, funding will be needed for increased algae production to make this a possibility, but with more research and advances in the field, algal growth would be a great way to remove large amounts of carbon dioxide that is stuck in Earth’s atmosphere and become our primary fuel source down the line.

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