Chapter 15
Overview of Microalgae-Based Carbon Capture and Utilization

Ye Sol Shin, Jaoon Y. H. Kim and Sang Jun Sim

15.1 Introduction

Starting with the invention of the steam-powered machineries that marked the industrial revolution, technologies transformed the quality of the human life beyond comparable to that of the animal. Unfortunately, the power is mainly driven from the exploitation of fossil fuel, which accompanies the emissions of CO2. Under the current policies, the carbon dioxide emission will increase from 29 to 43 Gt/yr [1]. The biofuel produced from the photosynthetic microalgae is an attractive alternative source of energy with sustainability. Microalgal cells can convert CO2 into raw materials that can later be used to produce biofuel and other high value bioactive compounds [2–5]. In addition, the ability of microalgae to effectively sequester CO2 can be applied in reducing gas effluents from fossil fuel power plants [6]. The CO2 fixation from microalgae does not involve a secondary pollution nor a consumption of extra energy from carbonizing sources [7]. This chapter describes the photosynthetic process of microalgae as a driving force for algal biofuel production. The upstream and the downstream processes are also covered because they are crucial in economizing the whole process. The final section of this chapter discusses various bioproducts from microalgae.

15.2 Capturing of Inorganic Carbon Using Photosynthesis

Photosynthesis is a biological way of capturing CO2 in form of biomass and it is a light energy-driven redox reaction. This process can be divided into two pathways: light-dependent and light-independent reactions. During the light-dependent reaction, photons from sunlight excite electrons from the splitting of two water molecules to a higher energy state that starts off a series of reactions to convert ADP and NADP+ into energy carriers, ATP and NADPH2. Oxygen is produced as a by-product synthesized from the splitting of two water molecule at the photosystem II. These energy carriers are later used during the light-independent reaction to fix CO2 into organic forms such as sucrose, starch, and cellulose. The overall reaction of photosynthesis is written as follows [5]:

15.1 equation

It is important for us to study factors that influence the efficiency of photosynthesis because they affect the overall performance of carbon dioxide conversion via microalgae. The classical way of assessing photosynthetic activity can be described through the light-response (P/I) curve, where the rate of the photosynthesis/respiration is shown in proportion to the light intensity. The rate of the photosynthesis shows a linear relationship with the light intensity under low light and has a positive initial slope (α = Pmax/Ik; Pmax is the maximum rate of photosynthesis and Ik is the saturation irradiance). However, with a prolonged irradiance with a value that surpasses the optimal irradiance magnitude, the rate of photosynthesis declines [8]. This phenomenon is called photoinhibition and hypothesized to occur due the photooxidative damage caused at a functional component in D1, the 32-kDa reaction center protein of photosystem II [9]. This is one of the many elements that contribute in lowering the actual photosynthetic efficiency and areal productivities (per volume) than the theoretical potential by two- to threefolds [10, 11].

Photosynthesis is a sensitive process in microalgae where many factors such as the salt concentration, the light intensity, and the magnitude of temperature in culture environment greatly influence the photosynthetic efficiency. It has been reported that inhibition of the activity of PSII by salt stress is associated with state-2 transition in Dunaliella tertiolecta and Chlamydomonas reinhardtii [12–14]. Cruz et al. demonstrated the destructive effect of an abrupt hyperosmotic shock on C. reinhardtii by completely blocking the linear electron transport. The primary site of disruption was between plastocyanin (PC) or cytochrome C6 and P700. The electron micrographs of osmotically shocked cells showed a significant decrease in the thylakoid lumen volume and lumenal space, which suggests to have hindered the movement of PC [15].

CO2 is a rate-limiting substrate for photosynthesis and many related processes in carbon metabolism in photosynthetic organisms due to its relatively low abundance in the atmosphere and its comparatively slower rate of diffusion of CO2 into the cell when dissolved in water than in air [16–18]. In the aquatic environment, the dissolved inorganic carbons exist in the forms of CO2, HCO3–, CO32–, and H2CO3 at the dynamic ionization equilibrium. Microalgae use bicarbonate transporters to pump bicarbonate into algal cells, which is the dominant (>50%) chemical species of CO2 in aquaculture [16, 19]. In the stroma of a chloroplast, the CO2 is fixed via the action of enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCo). Therefore, the efficiency of RuBisCo at capturing CO2 will greatly influence the overall efficiency of the photosynthesis.

After a series of reactions, the glyceraldehyde-3-phosphate is produced as the end product from photosynthesis. This is an intermediate product that is converted into the final products such as carbohydrate, amino acids, and lipids [4, 5]. However, oxygen at the active site of RuBisCo competes with CO2 for fixation by RuBisCo, resulting in photorespiration. The products of the oxygenase reaction are 3-phosphoglycerate and 2-phosphoglycolate. Further metabolic reaction with phosphoglycolate will give rise to glycine, leading to condensation of two glycines to produce serine. This is referred as the carbon loss (one carbon per two molecule of glycine) that lowers the efficiency of carbon fixation that produces bioproducts of high value [19, 20].

Photorespiration reduces the efficiency of photosynthetic carbon fixation by 20–30% [21]. Hence, microalgae have developed a mechanism that increases the level of CO2 at the active site of RuBisCo by transporting the inorganic carbon or carbon dioxide into the cell. This is so called the carbon concentrating mechanism (CCM) [22]. This mechanism compensates for not only the limited availability of CO2 due to low solubility and concentration but also the slow catalytic turnover of CO2 assimilating enzyme, RuBisCo, by enabling the usage of both CO2, and HCO3– as a carbon source for photosynthesis [16]. This allows aquatic photosynthetic organisms to have the flexibility to acclimate to low concentration of inorganic carbon sources. In addition, CCM overcomes the problem of the rapid diffusion of CO2 through the membranes by allowing microalgae to accumulate HCO3–, a charged species with much slower rate of diffusion than CO2, which will assure an optimal concentration of inorganic carbon source inside the cell [23]. Later, HCO3– is catalyzed as CO2 and OH– by periplasmic carbonic anhydrase (pCA) so the converted CO2 can react with RuBisCo to begin the carbon fixation [22, 24].

15.3 Microalgal Biofuel Production

The system of converting CO2 using microalgae can be partitioned into two bioprocess stages: upstream and downstream, consisting of sub-steps of strain selection, cultivation, biomass harvesting, drying, lipid extraction, and transesterification [25]. Optimizing of any of these stages will improve the production yield and the quality of the bio-oil, expediting the actualization of the system of microalgae (Figure 15.1).

Scheme for synthesis of bio-oil.

Figure 15.1 A general scheme of the biological CO2 converting system of microalgae, separated by upper and downstream processes.

15.3.1 Upstream Process: Strain Selection and Cultivation of the Selected Strain

15.3.1.1 Strain Selection

Microalgae are simple aquatic photosynthetic organisms with a great diversity of approximately 200 000–800 000 algal species [25, 26]. As a result, the biomass productivities and lipid contents varies among strains. For instance, Chlamydomonas sp. when compared with Chlamydocapsa bacillus showed a lower growth rate but indicated a higher lipid content [27]. The selection of the algal strain is the first and a crucial step for biofuel production. In the case of biomass that are subjected to hydrothermal liquefaction (HTL) process, a chemical process of converting whole biomass to bio-oil, the quality of resulting end products varies from type 1 kerogen to a higher grade bio-oil depending on the initial biomass composition. Hence, selecting species with the desired biomass composition that is already optimized for HTL process will increase the production yield of high-quality biocrude [28–30]. The lipid productivity of strains is another selective criterion for selecting strain. A strain with high lipid content is most suitable for the transesterification of lipid [31].

15.3.1.2 Cultivation Condition

There are four different types of cultivation conditions: photoautotrophic, heterotrophic, mixotrophic, and photoheterotrophic [32]. The photoautotrophic cultivation is the most common way of cultivating microalgae, and it was reported to be economically feasible to grow microalgae in commercial scale using open culture system [33, 34]. Under photoautotrophic condition, microalgae use light energy as their energy source for carbon fixation and to accumulate biomass. The carbon source is fundamental for the growth of microalgae, but excessive amount of carbon source can make their cultivation system acidic, inhibiting the cell growth [25, 34]. However, photoautotrophic condition has limitations for countries where sunlight availability fluctuates throughout the year. This can be solved through the usage of heterotrophic cultivation [35]. Heterotrophic cultivation allows microalgae to grow under dark environment utilizing organic compound as both energy and carbon source, resulting in relatively high lipid and biomass productivity. Heterotrophically cultivated Chlorella protothecoides accumulated 55% of its dry weight as oil, whereas autothropically cultured cells produced 14% of its dry weight as oil [36, 37]. Therefore, choosing a right cultivation condition for the selected strain is important.

15.3.2 Downstream Process: Harvesting, Dewatering, Disruption, Extraction, and Transesterification

Downstream processes of microalgae are harvesting, dewatering of the wet biomass, disruption of the cell, extraction of the lipids, and conversion of biomass to biodiesel [38]. The downstream process takes up as high as 60% of the total production cost of biofuel from microalgae. Therefore, innovative techniques should be invented to improve the efficiency of the process and reduce the cost of the overall production. There are two main techniques for bio-oil production from biomass: pyrolysis and HTL [39]. Pyrolysis requires harvesting of biomass, drying of wet biomass, then extraction of lipid, and transesterification. Harvesting methods includes centrifugation, flocculation, filtration, flotation, magnetic separation, electrolysis, ultrasound, and immobilization. Usually the harvesting process (harvesting and dewatering processes) takes up to 20–30% of the total biomass production cost [40–42]. A major limitation from harvesting and dewatering process is caused by the dilute microalgal biomass in culture of 0.3–0.5 g dry mass per liter and a density similar to water [38, 43]. HTL does not require the drying of biomass (energy intensive process) since it's conducted under an aquatic environment. Therefore, it is much better in terms of energy balance than pyrolysis. Instead, pretreatment of the cell disruption (physical and chemical methods) is necessary before the extraction process [38]. HTL has comparatively low extraction yield than pyrolysis due to the immiscibility of water and nonpolar organic solvent in wet biomass [44].

15.4 Application of Microalgal By-Products

15.4.1 Bioproducts

Microalgae are rich source of essential organic and inorganic nutrients such as carbohydrates, proteins, different types of vitamins, fibers, and minerals. Therefore, microalgae have been consumed as a source of food or as dietary supplement [45]. Some of the already widely commercialized and commonly used strains of microalgae are Chlorella vulgaris, Haematococcus pluvialis, Dunaliella salina, and Cyanobacteria Spirulina maxima. Recently, a blue-green alga, Spirulina platensis have increasingly become popular due to its high protein content.

Microalgae are also widely used in cosmetic industry as pigments, thickening agents, water-binding agents, and antioxidants. Arthorospira and Chlorella are already well-established strains of microalgae for skin care section (e.g., antiaging, regenerative cream, and emollient) [46]. Some of microalgae that are used in sun protection and hair care products are Chondrus crispus, Mastocarpus stellatus, Ascophyllum nodosum, Nannochloropsis oculata, C. vulgaris, and more. These strains are capable of synthesizing organic metabolites such as sporopollenin, scytonemin, and mycosporine-like amino acid that are able to intercept UV radiation and dissipate its energy as heat, providing the potential to be used as a source of UV protection [47].

15.5 Conclusion

We are still challenged with several limitations in microalgal industry that holds us back from fully exploiting its biomass. In a large-scale cultivation system, increasing the productivity is the most crucial factor. This can be improved by adjusting factors in upstream and downstream of the production process such as strain development, adjustment of CO2 and O2 level or light intensity of the harvesting system, and more. Therefore, in order for the microalgal industry to be realized, there need to be a constructive plan that considers economical, technical, and political perspective of this matter.

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