A Solution in the Solution: Shrimp Shells as Means to Mitigate Harmful Algal Blooms
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The severity of algal bloom highlights a major shortcoming of the human race: our inability to understand the exponential function and foresee its catastrophic disasters. Consider an elementary question regarding exponentials: if it takes one day for algae to double in population size, and it takes one hundred days for algae to completely cover the lake, then at what day is the lake half-covered? The common, misinformed answer is fifty; but the correct, yet seemingly counterintuitive answer is the ninety-ninth day, because the half-covered pond doubles in size only on the final day. It is this inability of our intuition to understand exponentials which makes environmental crises feel like they emerge from nowhere. By the time the problem is large enough to be noticed, for example, on the ninety-eighth day when the lake is 25% covered with algae, it only takes two more days for the lake to be completely overwhelmed with algae.
The Dangers of Algae: Eutrophication
Algae are autotrophic protists which are generally eukaryotic. As such, all algal cells are capable of photosynthesis. A simplified reaction of algal photosynthesis is shown below:
CO₂ + PO₄³⁻ + NH₃ + H₂O → New cells + O₂ + H₂O
When high concentrations of nutrients such as nitrogen and phosphorus are available, typically by leaching of NPK (nitrogen, phosphorus and potassium) fertilizers, algae grow exponentially. This rapid, uncontrollable expansion is known as an algal bloom, where populations can increase from thousands to millions of cells per litre within days. The algal blooms form a green-coloured mat on the surface of the lake, blocking the penetration of sunlight, which adversely affect other aquatic plants which require sunlight for photosynthesis. As such, less aquatic plants produce oxygen. The depletion of oxygen is further exacerbated at night during respiration, when algae uses up oxygen from the water and produces carbon dioxide according to the following chemical equation:
Algal cells + O₂ → CO₂ + H₂O
This causes significant depletion of dissolved oxygen in the lake and affects the population of fish. Thus eutrophic lakes are characterised by unsightly green polluted waters, loss of species diversity and very low dissolved oxygen. Due to the thriving agricultural industry in Malaysia which utilizes large amounts of NPK fertilizers, eutrophication has become a severe problem, with the overabundance of dissolved nutrients in water fuelling its rampant growth. Studies show that of 90 lakes studied, 34 lakes (39%) are mesotrophic whilst the other 56 lakes (62%) are eutrophic (Sharip & Yusop, 2008). Though government policies regarding wastewater management are crucial for improvement, implementing government policies are often time-consuming, therefore scientists are looking for innovations in wastewater treatment to alleviate this issue.
The Challenges of Utilizing Microalgae
Despite the bleak picture that microalgae may paint due to its involvement in eutrophication, microalgae is a highly efficient and sustainable biological resource with a vast potential to address environmental and industrial challenges in the status quo. For example, microalgae biomass has long been considered as a promising source of third generation biofuels, such as biodiesel, bioethanol and bio-oil (Behera et al., 2015). Microalgae exhibit a fast growth rate, a high lipid content and a high photosynthetic efficiency (Wang et al., 2024), offering a renewable alternative to fossil fuels that do not require arable land, which is why it is considered to be a superior feedstock for biofuel production.
It should be clear that there is more to microalgae than meets the eye. However, microalgae harvesting, which is the process of separating and concentrating microscopic algal cells from their aquatic growth medium, is a significant challenge to the industrial production of biofuels from microalgae. One of these challenges arises from the negative surface charge of microalgal cells and its associated zeta potential. Most microalgae cells naturally possess a negatively-charged surface due to the presence of functional groups such as carboxylate (-COO-), phosphate (PO43-) and sulphate (SO42-). This negative charge creates strong electrostatic repulsion between individual cells, preventing them from easily aggregating or settling out of suspension. As a result, microalgae remain highly stable and dispersed in the growth medium, which is indicated by its high negative zeta potential (a key indicator of dispersion stability), preventing aggregation via electrostatic repulsion, and making conventional separation methods such as sedimentation inefficient.
Additionally, due to the dilute nature of the microalgae culture with a low biomass concentration of only 0.5 – 5 g/L, this means that over 99% of the culture volume is water that must be removed. Separating such a low concentration of solids from such a large volume of water is energy-intensive and energy-inefficient. In fact, the cost of microalgae harvesting constitutes up to 20 – 30% of the total production cost (de Morais et al., 2023), which makes it economically unviable. Consequently, the harvesting strategy has to be based on a low-energy method in order to overcome the problems and make algal biomass production economically feasible, which in turn would lead to a practical system that can compete with preexisting means of biofuel production.
Traditionally, microalgae harvesting has relied on conventional, high-energy methods such as centrifugation, but these methods either involve high capital investment or are energy-intensive (Qin et al, 2023). Therefore, flocculation is preferred as means to facilitate the removal of microalgae. By aggregating individual algal cells into larger, heavier flocs, the weight of the heavier flocs overcome buoyant forces, leading to quicker sedimentation of microalgae.
A Solution in the Solution
There are a variety of flocculants that can be used to harvest microalgae from wastewater. There are several important metrics used to determine the most suitable flocculant for this process. Firstly, the flocculant efficiency should be high. A good flocculant should achieve high biomass recovery with a low dosage to minimize operational costs. Secondly, the environmental safety and toxicity of the flocculant must be considered. Non-toxic and biodegradable flocculants are preferred over chemical-based flocculants that may leave harmful residues or contaminate biomass. Thirdly, cost-effectiveness and availability are also a crucial consideration, as flocculants must be readily accessible to support long-term large-scale industrial operations.
In light of all this, chitosan stands out as a highly promising flocculant for microalgae harvesting. Chitosan is a natural, biodegradable linear polysaccharide composed of randomly distributed β--linked D-glucosamine and N-acetyl-D-glucosamine. Chitosan is produced commercially by the deacetylation (removal of the acetyl group, -COCH3) of chitin, by using sodium hydroxide as a reagent. By removing the acetyl group of the chitin polysaccharide and leaving behind a complete amino group (-NH2), chitosan is formed. In acidic environments below pH 6.5, the free amino groups on chitosan act as weak bases and undergo protonation, forming the positively-charged R-NH3+, leading to its strong cationic nature, which allows it to effectively neutralize the negative surface charge on microalgal cells and reduce their zeta potential, promoting cell aggregation and rapid floc formation. Therefore, it is considered a high-efficiency flocculant (Elcik et al, 2023).
Chitosan is also renowned for its biodegradability, biocompatibility and non-toxicity. Chitosan is derived from chitin, a natural biopolymer found in the shells of crustaceans such as shrimps, crabs and lobsters. Because chitosan is organic in nature, it can be broken down by naturally occurring enzymes such as lysozymes and chitinases (Aranaz et al., 2021). These enzymes cleave glycosidic bonds, converting it into harmless oligosaccharides that can be safely eliminated. Furthermore, its polysaccharide structure is similar to components found in the extracellular matrix, so living cells do not recognize it as foreign substances which may trigger an inflammation reaction. This makes it safe for aquatic systems as it does not harm aquatic wildlife. Therefore, by virtue of its chemical properties, chitosan is considered a safe substance and can be used extensively in algal harvesting.
Sustainability for the Ages
The production and use of commercial chitosan inherently encourage waste revalorization. Chitin, being found in the shells of crustaceans such as shrimps, crabs and lobsters, is naturally abundant but often discarded as seafood waste. However, by extracting chitin from these byproducts and converting it into chitosan, we transform a burdensome waste into a high-value functional material that not only minimizes the environmental footprint of the seafood industry, but also provides an eco-friendly solution for wastewater treatment, which aligns with the core principles of Sustainable Development Goals no. 11 (Sustainable Cities and Communities) by reducing domestic waste produced.
It is only endlessly interesting that the answer to one of the more pressing problems in microalgae harvesting, and wastewater treatment as a whole, is a naturally-occurring biopolymer that can be derived from something as common as shrimp shells, as if the solution had been lying under our noses all this time. This leads us to question: What other substances can we use to tick off the list of ever-pressing problems on the human agenda? Are there any other substances or compounds out there, which are hiding in plain sight, that we have yet to fully harness? It is only through this repeated questioning and analysis, coupled with humanity’s undying spirit of innovation, that we are able to herald a new era of sustainable and clean technology, one which aligns itself with the Sustainable Development Goals of the world.
References:
1. Zati Sharip and Zulkifli Yusop (2008). National overview - The Status of Eutrophication of Lakes in Malaysia. Zenodo (CERN European Organization for Nuclear Research). doi:https://doi.org/10.5281/zenodo.6600980.
2. Behera, S., Singh, R., Arora, R., Sharma, N.K., Shukla, M. and Kumar, S. (2015). Scope of Algae as Third Generation Biofuels. Frontiers in Bioengineering and Biotechnology, [online] 2. doi:https://doi.org/10.3389/fbioe.2014.00090.
3. Wang, M., Ye, X., Bi, H. and Shen, Z. (2024). Microalgae biofuels: illuminating the path to a sustainable future amidst challenges and opportunities. Biotechnology for Biofuels and Bioproducts, [online] 17(1). doi:https://doi.org/10.1186/s13068-024-02461-0.
4. de Morais, E.G., Sampaio, I.C.F., Gonzalez-Flo, E., Ferrer, I., Uggetti, E. and García, J. (2023). Microalgae harvesting for wastewater treatment and resources recovery: A review. New Biotechnology, [online] 78, pp.84–94. doi:https://doi.org/10.1016/j.nbt.2023.10.002.
5. Qin S, Wang K, Gao F, Ge B, Cui H, Li W. Biotechnologies for bulk production of microalgal biomass: from mass cultivation to dried biomass acquisition. Biotechnol Biofuels Bioprod. 2023 Aug 29;16(1):131. doi: 10.1186/s13068-023-02382-4. PMID: 37644516; PMCID: PMC10466707.
6. Harun Elcik, Dogan Karadag, Ayse Irem Kara and Mehmet Cakmakci (2023). Microalgae Biomass Harvesting Using Chitosan Flocculant: Optimization of Operating Parameters by Response Surface Methodology. Separations, 10(9), pp.507–507. doi:https://doi.org/10.3390/separations10090507.
7. Aranaz, I., Alcántara, A.R., Civera, M.C., Arias, C., Elorza, B., Heras Caballero, A. and Acosta, N. (2021). Chitosan: An Overview of Its Properties and Applications. Polymers, [online] 13(19), p.3256. doi:https://doi.org/10.3390/polym13193256.
This article was prepared by Foo Yu Shen (Sunway College KL).
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