Algal Biotechnology: Integrated Algal Engineering for Bioenergy, Bioremediation, and Biomedical Applications

Algae Biotechnology: Integrated Algal Engineering for Bioenergy, Bioremediation, and Biomedical Applications covers key applications of algae for bioenergy and how to integrate the production of biofuels with environmental, nutraceutical and biomedical processes and products. The book emphasizes cost-effective biofuels production through integrated biorefinery, combining continuous processes and various algae as feedstock to produce biofuel, bioenergy and various high value biochemicals. Novel algal culturing technologies and bioprocess engineering techniques are provided for the optimization of operational approaches for commercial-scale production, as well as to reduce the overall costs. New and existing molecular methods for genetic and metabolic engineering of algae are also presented.

Furthermore, methods for the optimization of existing biochemical pathways are explained, and new pathways are introduced, in order to maximize the potential for biofuels production and related nutraceutical and biomedical co-products. This book provides an ideal roadmap for bioenergy researchers and engineers who want to incorporate valuable nutraceutical and biomedical products and environmental practices into the production of biofuels.

• Addresses issues faced by the bioenergy sector and how to resolve them through the integration of algal biotechnology and engineering
• Provides a guide to the efficient and cost-effective production of bioenergy, while simultaneously mitigating pollution and producing valuable nutraceutical and biomedical biproducts
• Covers new and emerging approaches in integrated algal biotechnology
• Offers a roadmap to their application in the production of biofuels alongside nutraceutical, biomedical, and environmental processes and products

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 26, 2022
• ISBN: 9780323904360

Book preview

Part I

Environmental sector

Chapter 1: Algal engineering for bioremediation, bioenergy production, and biomedical applications

Ashfaq Ahmada,*; Fawzi Banata; Hanifa Tahera,b    a Department of Chemical Engineering, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates

b Research and Innovation Center on CO2 and H2 (RICH), Khalifa University, Abu Dhabi, United Arab Emirates

Abstract

Algae have received substantial consideration as a potential feedstock for extensive applications in the environmental sector, biofuel production, and biomedical engineering. Rapid climate changes, shrinking natural resources, and food crises are on the rise globally. The release of untreated industrial wastewaters containing vast amounts of carbon, nitrogen (N), and phosphorus (P) causes severe pollution and environmental damage. Hence, the recovery of such nutrients is required through a suitable sustainable process. Microalgae-based technologies have gained significant attention compared to other techniques due to their sustainable and cost-effective treatment strategies for removing wastewater nutrients. Algal biomass could potentially be used for bioenergy production and high-value bioproducts. High operational costs and low yield are the main limitations for developing microalgae-based biorefineries. Therefore, most researchers focus on an integrated approach for the cultivation of a suitable algal strain and its downstream processing. Algal biomass production for use as a biofuel is not feasible from a techno-economic perspective. A biorefinery concept could be a more suitable approach for bio-oil extraction, and the remaining biomass could be used for several biomedical applications. This chapter offers an overview of algae and their use in bioremediation, bioenergy production, and biomedical applications. Recent challenges and prospects in the algae-based sector, which has become quite promising at present, have also been discussed.

Keywords

Algae; Cultivation; Wastewater; Bioremediation; Bioenergy; Biomedical applications

Outline

1Introduction

2Industrial wastewater treatment

2.1Removal of total nitrogen (TN) and total phosphorus (TP)

2.2Heavy metal (HM) removal by algae

3Algae for CO2sequestration

4Bioenergy from algae

4.1Biodiesel production

4.2Bioethanol production

4.3Biogas production

5Biomedical applications

5.1Antioxidant activity

5.2Anticancer, antiangiogenic, and cytotoxic activities

5.3Antiobesity activity

5.4Antimicrobial activities

6Conclusion and future outlook

References

1: Introduction

Algae are photosynthetic aquatic plants that grow in ponds, streams, oceans, and even wastewater. Algae have a high tolerance for high temperatures, salinities, pH, and different light intensities and can grow alone or with other organisms because of their symbiotic relationship [1,2]. They are generally categorized as Rhodophyta (red algae), Phaeophyta (brown algae), and Chlorophyta (green algae). They can be grouped by sizes, e.g., macroalgae (seaweeds) are multicellular, large, and can be seen with the naked eye. In contrast, microalgae are unicellular, smaller in size, and can only be seen microscopically. Like conventional food crops, algae require water, sunlight, carbon dioxide (CO2), and nutrients to grow. However, they have a higher growth rate than other plants and provide ecological benefits [3,4]. Microalgae can be prokaryotic such as cyanobacteria (Chloroxybacteria), or eukaryotic such as green algae (Chlorophyta). Fig. 1.1 shows the green marine microalgae Nannochloropsis oculata and the freshwater/terrestrial algae species Eustigmatos splendida and Eustigmatos magnus [5].

Fig. 1.1

Fig. 1.1 Microalgae convert CO 2 into carbohydrates, lipids, and other valued bioproducts by using sunlight. From M.I. Khan, J.H. Shin, J.D. Kim, The promising future of microalgae: current status, challenges, and optimization of a sustainable and renewable industry for biofuels, feed, and other products, Microb. Cell Fact. 17 (1) (2018) 36.

Algae can potentially be used to produce biofuel, bioproducts, medicines, and cosmetics as they are a rich source of carbon compounds [6]. Bioproducts produced by algae are polysaccharides, lipids, pigments, proteins, vitamins, bioactive compounds, and antioxidants that can be used for various purposes. Algae have extensive applications in industrial wastewater treatment and CO2 sequestration [7]. Algae feedstock is deemed renewable and sustainable for biofuels, which has encouraged setting up biorefineries. Integrated algal engineering approaches for improving their growth rate and genetic modification can enhance their future applications for producing renewable bioproducts. Rapid climate change is being caused due to the burning of fossil fuels, the release of anthropogenic CO2, and the increasing population worldwide. Microalgae and cyanobacteria can be promising biological tools to tackle these persistent problems [8,9]. Algal biotechnology aims to produce sustainable biofuels with zero CO2 emissions. An algal strain can be modified through genetic engineering to enhance biofuel production by targeting either a single gene or multiple genes [10]. Fig. 1.1 presents the conversion of CO2 into carbohydrates, lipids, and other valued bioproducts by using sunlight.

Commercial algae cultivation for generating biofuels and bioproducts has significantly increased recently [4]. An enormous amount of algae is being produced and sold for different purposes, such as the production of food and nutrient supplements. Algal extracts and by-products can be used in the pharmaceutical and cosmetic industries [5–7]. Algae feedstocks are proficient and desirable for biofuel production. They do not require vast lands for cultivation and can quickly grow in industrial wastewater. Algae do not contest human and animal food chains and mitigate atmospheric CO2 [11–13]. Microalgae do not have lignocellulosic materials in the cell wall. This facilitates the pretreatment method and reduces production costs. Algae can grow in industrial wastewater and require less energy for their cultivation than the energy they can produce [14–16]. Production of second-generation biofuels from terrestrial plants is an immensely debated issue because biofuels’ production from such food crops is expensive and competes with food and feed requirements. Moreover, crop foods need arable land and an enormous quantity of water, making biofuel production unsustainable. Therefore, liquid fuels from algae are an incomputable alternative [17,18]. Biofuel generation from microalgae is still in the developing phase, and a significant improvement is essential for its commercial application and attracting investors and consumers.

2: Industrial wastewater treatment

Several physical, chemical, and biological treatment techniques have been used for industrial wastewater treatment. Conventional methods for wastewater treatment involve intensive aeration for the oxidation of organic carbon and removal of other contaminants using microorganisms. An enormous amount of energy is required for the aeration of wastewater treatment plants, accounting for 50% or more of the total energy costs [19–21]. Numerous studies have suggested that algae can use various types of wastewater such as industrial, domestic, municipal, or agricultural wastewater. Combining sewage with the flue gas (atmospheric CO2) enhances microalgae biomass productivity [22,23]. Organic carbon oxidation directly emits CO2 into the atmosphere, whereas the energy used for the aeration of treatment plants can indirectly emit CO2 [24,25].

Additionally, substantial quantities of potent greenhouse gases, such as nitrous oxide (N2O), are also discharged in the latter case. One of the main constraints for traditional wastewater treatment is the recovery of N and P after the treatment [24,25]. Therefore, the algal wastewater treatment approach can be economical and ecologically friendly, mainly for removing and recovering N and P. Algae can produce oxygen (O2) through photosynthesis and assimilate CO2 during the photosynthetic process. They have a symbiotic relationship with bacteria. During the oxidation of organic carbon, the bacteria utilize the O2 produced by photosynthetic algae, and the algae simultaneously assimilate the CO2 generated by bacterial respiration. Therefore, the integration of algae in wastewater treatment can decrease aeration requirements and CO2 emissions. Algae absorb N and P and photosynthetically fix carbon during their growth. This reduces the bacterial requirements for N and P removal and the associated aeration demands and N2O emissions [21]. Another study has reported that algae could be cultured via an integrated cultivation system using wastewater from the food industry and CO2 from the atmosphere. Further, biomass can be used to produce bioenergy and bioactive compounds [22].

Moreover, algae biomass produced in the wastewater treatment process could be recycled for diverse applications, as shown in Fig. 1.2. Algal biomass contains lipids, carbohydrates, and proteins with high nutritious and calorific value. After its harvesting, it can be used as animal feed [26], slow-release fertilizer [27], or biofuel [28,29], thus turning waste into valuable resources. Biogas, such as biomethane and biohydrogen, can be produced through anaerobic digestion from wastewater. The biomass can also be utilized as supplementary feed for aquaculture and animals and as fertilizer for crops.

Fig. 1.2

Fig. 1.2 An integrated approach of microalgae cultivation in different wastewaters for bioproducts application. From R.K. Goswami, et al., Microalgae-based biorefineries for sustainable resource recovery from wastewater, J. Water Process. Eng. (2020) 101747.

2.1: Removal of total nitrogen (TN) and total phosphorus (TP)

Fig. 1.3 explains the standard wastewater treatment process, including primary, secondary, and final processing stages. Raw wastewater usually comprises organic N that can quickly degrade into ammonium. Organic N and inorganic N, including urea, and such wastewater could be used to grow filamentous algae. Putatively, it is favorable to cultivate freely suspended cells of filamentous algae in raw wastewater. However, the existence of a suspended substance in wastewater makes it too turbid and blocks light penetration, inhibiting algae's photosynthetic activity. The solid wastes present in the wastewater can be typically removed through the primary process of sedimentation or dissolved air flotation to get a relatively perfect effluent that has soluble organic carbon and ammonium [21].

Fig. 1.3

Fig. 1.3 Wastewater treatment process (orange boxes, gray in print version) and the different wastewater streams (blue boxes, dark gray in print version) in which algae could be cultivated. From J. Liu, et al., Wastewater treatment using filamentous algae—a review, Bioresour. Technol. 298 (2020) 122556.

Wastewater contains organic, and inorganic P. Algae utilize orthophosphate, polyphosphate, pyrophosphate, and metaphosphate for their growth [30]. N and P are important nutrients required for algal growth and are assimilated instantaneously. Table 1.1 presents the wastewater contamination removal rate for different species of microalgae. An N to P ratio (N:P ratio) is usually defined to identify whether they are the limiting nutrients for algal growth in certain wastewater. Microalgae Pantanalinema has been reported for N and P removal from wastewater under the dark-light condition. Around 86% of P was removed within a dark-light cycle of 6 h. Cellular and polyphosphate mechanisms of microalgal-bacterial granules were responsible for accumulating P. Approximately 70% of soluble P removal was reported due to polyphosphate development in Pantanalinema algal cells [31]. The algal-bacterial symbiosis (ABS) system has been reported to improve the removal of nutrients using a sequencing batch biofilm reactor (SBBR). The total N's elimination efficiencies increased from 38.5% to 65.8% and P from 31.9% to 89.3% using the algae-assisted SBBR. Moreover, chlorophyll-a (3.59 mg/g) production increased at a stable stage and was 4.07-fold higher than that of freely suspended cells. An analysis of the mechanisms proposed that the high removal of N and P is mostly due to enhancing both algal biomass and total biomass in the biofilm [32]. Chlorella vulgaris and Neochloris oleoabundans have been reported to remove the Chemical Oxygen Demand (COD), inorganic N, and total dissolved P at 36°C from primary and secondary effluents and centrate (CEN). The efficiency of COD's removal achieved with C. vulgaris was 51% from primary effluent, 55% from secondary effluent, and 80% from CEN. In contrast, the efficiency of COD's removal achieved with N. oleoabundans was 63% from primary effluent, 47% from secondary effluent, and 72% from CEN. Simultaneously, ammonia removal efficiencies (70%–84%) were obtained with both species in different wastewaters. High P concentrations that were removed from primary effluent were > 84%. These were moderate in CEN (> 22%) and less in secondary effluent (< 15%). These studies confirmed that algae could grow in wastewaters at a hot temperature of 36°C and remove contaminants such as organic carbon, N, and P [33]. Algae-based membrane bioreactor (A-MBR) has been reported to cultivate algae with high cell density to remove P. The concentration of algae cells was increased from 385 to 4840 mg/L, and the average solids yield production rate of 32.5 g− 3/day was attained. Total P removal of 66% was achieved from wastewater in A-MBR. This study suggested that algae-induced phosphate precipitation is the key to removing P. The high-cell density of algal cultivation can produce P-rich biomass with brilliant harvesting properties [34]. The discharge of excessive P causes extensive eutrophication and water pollution that threatens both ecological and human health. However, P is an important component for all living microorganisms, but it is nonrenewable. Further, its natural reserves are depleting rapidly. Algae can sustainably reuse P from wastewater for their growth. Ultra-membrane-treated landfill leachate can be utilized as a nutrient medium for culturing indigenous algal species with immediate elimination of P and N. Maximum N removal of 69% and P removal of 100% were achieved from 100 mg/L P- si1_e -supplemented medium. Algae can be grown and used to sustain P and N from landfill leachate [35]. A study has recommended natural algae granulation in open sequencing batch reactors (SBRs) for treating synthetic wastewater to overcome the high separation cost of algae. High removal of P content (33 mg-P/g-TSS) with higher P bioavailability (92%) was achieved with algae granules as compared to seed algae (20 mg-P/g-TSS). The algae granules have a rich perspective for P rescue and reuse [36]. The anaerobic-aerobic-anoxic sequencing batch reactor (AOA-SBR) system has been suggested for instantaneous carbon, N, and P removal. High removal proficiencies of COD (97%), TN (96%), and TP (94%) were achieved with the AOA-SBR system in 6-h cycles [21]. Algae immobilization has great potential to eliminate nutrients from wastewater. However, its commercial application is challenged by the high cost and the maintenance of many viable and active microalgal cells. Agar-immobilized microalgal cells efficiently removed si2_e -N (96%) and si3_e -P (99%) in both batch and continuous modes. The immobilized algal cells were still active and could eliminate 94% of si2_e -N and 66% of si3_e -P after being recycled for 8 cycles. Further, their nutrient removal efficiency was still more than 60% even after their preservation for 120 days under normal conditions. This study suggested that algae immobilization is simple, less costly, and practicable in preserving the microalgae at room temperature for a long time. Applying algae to remove nutrients in wastewater treatment is more suitable and advantageous [37]. Another study reported removing more than 94% ammonium-nitrogen and phosphate-phosphorus with the use of Chlorella sorokiniana cultivated in 10-L flat-panel bioreactors. This study indicated that controlled pH and high hydraulic retention time (HRT) could maximize the algae yield and improve the uptake of nutrients without magnesium (Mg) enrichment through continuous cultivation [38]. Outdoor cultivation of C. vulgaris in a thin-film flat plate photobioreactor (PBR) using digested piggery wastewater as the culture medium has also been reported. High levels of TN (72.48%), TP (86.93%), and COD (85.94%) removal were achieved with C. vulgaris. This study suggested that algal cells can adapt quickly to wastewater in outdoor conditions [39].

Table 1.1

From N.S.M. Aron, K.S. Khoo, K.W. Chew, A. Veeramuthu, J.S. Chang, P.L. Show, Microalgae cultivation in wastewater and potential processing strategies using solvent and membrane separation technologies, J. Water Process Eng. 39 (2021) 101701.

2.2: Heavy metal (HM) removal by algae

HM contaminants are increasing in water bodies due to urbanization, industrialization, and natural earth processes. HMs accumulate in the human body by consuming polluted water and food. The conventional methods reported for their removal are electrolysis, ion exchange, precipitation, chemical extraction, hydrolysis, polymer microencapsulation, and leaching. Several algal species are said to be useful biosorbent materials for removing HMs [50–55]. However, these approaches are not economical, especially on a large scale, due to continuous monitoring and control required and because of their lower HM-removal efficiency. Filamentous algae can adsorb HMs from wastewater. Algae can remove HMs by biosorption and bioaccumulation, as well as through physical and chemical mechanisms. HM removal by dried algal cells is called biosorption, and the use of accumulating abilities of alive algal cells is called bioaccumulation. A majority of the studies on this subject have focused on the biosorption of HMs from wastewater by utilizing the dry mass of filamentous algae as bio-adsorbent material. HM removal by inactive and dead biomass depends on the metal ions and the biomass's high affinities. Fig. 1.4 illustrates the complex mechanisms of metal ion binding. More precisely, the properties of algal cell wall constituents, such as alginate and fucoidan, are specifically responsible for metal ions’ sequestration. The main functional groups existing in the brown and green algae, such as carboxyl, hydroxyl, sulfate, phosphate, and amine groups, play an essential part in metal binding [50,56].

Fig. 1.4

Fig. 1.4 The complex mechanisms of metal ion binding. From J. He, J.P. Chen, A comprehensive review on biosorption of heavy metals by algal biomass: materials, performances, chemistry, and modeling simulation tools, Bioresour. Technol. 160 (2014) 67–78.

The alginates in the cell wall and the intercellular substance of brown algae have a greater uptake for divalent cations such as Pb² +, Cu² +, Cd² +, and Zn² + [50]. In this case, the use of dried biomass of Oedogonium, Spirogyra, and Cladophora for the biosorption of lead (Pb), cadmium (Cd), nickel (Ni), and mercury (Hg) was reported [51–54]. Filamentous algae biomass performs the dual function of removing N from wastewater and subsequently being useful as a biosorbent for removing HMs from other contaminated water. Algae can offer an unconventional, sustainable, and environment-friendly approach for the bioremediation of HMs [57,58]. Numerous studies have reported the use of diverse filamentous algae species, including Oedogonium, Rhizoclonium, and Hydrodictyon, for maximum removal of HMs. The cultivation of these algal species in ash dam water confirmed more than 60 mg/g of HMs’ accumulation from wastewater [59]. It was observed that the cultivation of Cladophora in synthetic wastewater removed more than 80% of Cd in batch and semibatch systems [60].

The alga Distigma proteus, isolated from industrial wastewater, has been reported to have a high tolerance against HMs such as Cd² +, chromium (Cr⁶ +), Pb² +, and copper (Cu² +). The growth rate of the algae was found to slow down by day 8 against the metal stress of Cu² + (90%), Cd² +(84%), Cr⁶ + (71%), and Pb² + (63%). The highest Cd² + removal rate of 90% was achieved from the medium after 8 days with the use of the Distigma algae. The metal removal capability of Distigma can be explored for metal detoxification and environmental clean-up [61]. The use of freely suspended and immobilized cells of Anabaena doliolum and C. vulgaris has been reported for Cu and iron (Fe) removal. The immobilized algal cells showed a high removal rate for both Cu and Fe. This means that immobilization can protect the algal cells from toxic metals compared to the freely suspended cells. The immobilization technology can also ease the harvesting process and can potentially be used during repeated cycles [62]. Immobilized Microcystis has been used in the packed column for Cu² + removal at different flow rates and metal ion concentrations. The highest reduction of Cu² + (54%) was reached at a flow rate (0.75 mL/min) with an initial metal ion concentration of 30 μg/mL and biomass dosage of 0.016 g. The elimination of Cu² + was found to be influenced by inlet metal ion concentration and biomass density. Increasing the biomass dosage from 0.016 to 0.128 g increased the removal percentage, and the Cu² + adsorbed per unit dry weight dropped [63]. The use of brown seaweeds, such as Hizikia fusiformis, Laminaria japonica, and Undaria pinnatifida, for the biosorption of heavy metal ions (Pb² +, Cd² +, Mn² +, Cu² +, and si6_e ) has been reported [64]. Dried biomass of brown marine algae Ecklonia radiata has been observed to uptake Cd from an aqueous solution. The maximum removal of Cd (1634 mg/g) dry biomass at an optimum pH of 4 and 50°C temperature has been reportedly achieved. Adsorption temperatures and pH levels can play an essential role in Cd uptake [65]. The biomass of the brown marine algae Sargassum fluitans has been reported to maximize Cr, Cu, and Al removal [66].

A native cyanobacterial species Nostoc muscorum has been reportedly used for Cu [67], Zn [67], Pb [67], and Cd [67] biosorption from an aqueous solution. The highest biosorption of Pb (96.3%) [67] and Zn (71.3%) [67] were obtained at 60 h of incubation with the algae. The biosorption of metals was attributed to passive biosorption and accumulation by the actively growing N. muscorum biomass. This study demonstrated that cyanobacteria could be used to remove metals from a multicomponent system [68]. Another study reported the use of dried biomass of the most common filamentous algae for the biosorption of HMs in the batch process. The removal of Pb² + by 97% and 89% were reported with Pithophora oedogonia and Spirogyra neglecta, respectively, in 30 min from an initial concentration of 5 mg/L metal ions. The removal efficiency of Pb² + decreased as the initial concentration of metal ions increased. Reduction of Pb² + 70% was obtained with S. neglecta at an initial concentration of 75 mg/L of Pb² +, while more than 75% of Pb² + and Cu² + reduction were achieved with S. neglecta and P. oedogonia from mixed-metal ions solutions. Some algal species, such as Hydrodictyon reticulatum, Cladophora callicoma, and Aulosira fertilissima, could not efficiently remove metal ions from mixed solutions [69].

Conversely, high concentrations of HMs can cause toxic effects on algal cells and hinder their growth and cellular metabolism. The absorption of HMs through algae could be determined by the latter's growth phase and other environmental settings. Numerous studies have reported that the use of dead algae shows more significant advantages than the use of living cells. Dead algae biomass comprises cellulose, glycoprotein, and pectins that act as biosorbents and adsorbents of HMs via the extracellular process [70].

3: Algae for CO2 sequestration

CO2 is continuously increasing in the atmosphere due to various anthropogenic activities. The burning of fossil fuels has contributed around 87% of CO2; deforestation contributes 9% of it, whereas 4% is generated by industrial, manufacturing, and human activities [71]. In general, the three sectors that contribute to CO2 emissions are transport, industrial production, and fuel combustion for some other activities [72,73]. Recent statistics have revealed that transportation and industrial production account for about 80% of fuel combustion, while the electricity sector accounts for the remaining 20% [74]. The rapid increase in atmospheric CO2 levels is a key contributory factor for global warming, which is one of the leading challenges threatening environmental sustainability [75,76]. Several conventional technologies have been developed to deal with this challenge that involves the sequestration of CO2, particularly along with energy retrieval, which has become an urgent need. The processes involving the mitigation of CO2 by chemical absorption, membrane separation, and physical adsorption and the cryogenic processes are all expensive and cause secondary pollution [75,77]. In the natural environment, CO2 fixation can be conceivably done via photosynthetic land plants and microorganisms.

Algae and cyanobacteria have been reported for CO2 fixation due to their fast growth rate and greater fixation efficiency than land plants [78]. Hence, the sequestration of CO2 via photosynthetic algae and cyanobacteria is an ideal approach. The microalgae Spirulina platensis has been reported as a favorite strain for the fixation of CO2 due to its fast growth rate, high resistance to high CO2 concentration and temperature, nutrient deficiency, and pH flocculation. Additionally, it can produce precious bioactive compounds to improve immunity [79] and prevent aging [80]. Biofixation of CO2 through algae is considered favorable as it can fix CO2 and instantaneously yield value-added chemicals too [81].

Algae-based CO2 sequestration offers an encouraging prospect to decrease CO2 and convert carbon into bioproducts. Further, this process has fewer safety requirements than that of the storage of CO2. Algae's high photosynthetic ability makes their CO2 utilization 10–50 times greater than any other terrestrial plants using sunlight [82]. The bubbling-type photosynthetic algae microbial fuel cell (B-PAMFC) has been reported for wastewater treatment, and it facilitates CO2 sequestration along with instantaneous power production. The highest fixation rate of CO2 was achieved with C. vulgaris at 2.8 g/L urea in the B-PAMFC. The absorption efficiency of CO2 and lipid productivity (105.9 mg/L/day) was enhanced due to urea's application. The highest net energy of 1.824 kWh m− 3 was produced. This study demonstrated that B-PAMFC with urea as the N source offered a beneficial method for instantaneous CO2 mitigation and bioenergy production [83]. Microalgae Phormidium valderianum BDU 20041 reported high tolerance at 15% CO2, thus proving to be a suitable contestant for carbon capture. An increased amount of lipid at an elevated CO2 level was achieved with actual flue gas conditions than that achieved in ambient air [84].

Mixed algal cultures in batch mode with an external supply of CO2 from wheat straw fermentation have been reported. High sequestration of CO2 (287 mg/L/day) has been observed with mixed cultures of algae. Removal of 87% ammonium, 78% phosphate, 68% COD, and 65% nitrate was also achieved. About 12.29% of lipids were produced with the help of enriched CO2 and wastewater for the supply of nutrients. The total amount of chlorophyll and protein content achieved was 14.3 and 12.3 μg/L and 0.13 and 0.15 mg/L, respectively. This study indicated that algae consortia could be potentially used for CO2 mitigation, wastewater treatment, and bioenergy production [85]. Freshwater algae C. vulgaris was cultivated in PBRs with low doses of sugars to enhance CO2 mitigation under the light-emitting diode's illumination. Glucose addition at low concentration improved the photoautotrophic growth and biomass generation and CO2 capture by 10%. A techno-economic analysis (TEA) suggested that LED-based PBRs are a feasible approach for transforming CO2 into value-added algal biomass [86]. Another study reported the use of Chlorococcum humicola, C. vulgaris, and Scenedesmus quadricauda for total CO2 fixation and chlorophyll, protein, and carbohydrate production. Sodium bicarbonate at a concentration of 0.2%–1% was used in the bold basal medium for the uptake of carbon, and it acted as a substitute source of atmospheric CO2. A high amount of lipid content and CO2 fixation rate of 0.4% were achieved, whereas the maximum amount of chlorophyll content was obtained at 0.6% of sodium bicarbonate concentration. This study showed that S. quadricauda could be suitable for CO2 mitigation and the high production of polyunsaturated fatty acid [87].

Microalgae cultivation has been reported in an outdoor raceway pond to produce biomass and for CO2 mitigation from the flue gas under diverse conditions. Algal growth is affected by numerous physiochemical parameters, predominantly temperature, solar irradiance, CO2, and cross-contamination. Lower algal biomass productivity in batch cultivation was achieved without the addition of external carbon supplementation. In contrast, feeding of flue gas at a concentration of 10% CO2 v/v improved the dissolved medium carbon concentration, which enhanced the rate of CO2 fixation. The semicontinuous method has been adopted to strengthen the performance of the system further. The highest biomass density of 0.42 g/L and a 3.5-fold improvement in areal productivity of 11.488 g/m² were achieved [88]. The effect of N:P ratios on the growth and CO2 fixation of Chlorella sp. was evaluated in the bubble column PBR. The maximum biomass yield (3568 mg/L) was achieved at the N:P ratio of 15:1. The highest algae cell density of 105 × 10⁶ cells/mL and sequestration of CO2 at 28% was obtained after 92 h. This study demonstrated that organic and inorganic carbon could influence algal cultivation [89]. The marine algae Nannochloropsis salina has been reportedly used for CO2 fixation and high lipid production. The optimum growth rate of N. salina was reported at the concentration of 6% CO2, while some cells were found to grow well under the CO2 concentration of 20%. Increasing the level of CO2 caused acidification in the medium, which reduced the pigment and inhibited the cells’ growth [90]. Conversely, CO2 fixation and the production of specific lipids increased with the removal of O2 from the inlet gas. Increasing the concentration of CO2 from 25% to 100% caused inhibited cell growth entirely. These findings suggest that, in the future, a more efficient approach to algal biotechnology can be developed and applied for both CO2 mitigation and biofuel production [90]. Chlorella minutissima cultivation in an indoor PBR for CO2 absorption from the anaerobic digestion system has been reported. The intake of CO2 by C. minutissima was found to be in the range of 75%–85% at a light intensity of 1296 μmol m²/s and gas flows at 0.33 vvm [91].

The regeneration of different solvents used in the absorption of CO2 is a significant challenge. A hybrid system of ammonia and microalgae for capturing CO2 has been reported. The fixation of carbon over 85% was obtained with Chlorella sp. L166, L38, and UTEX1602 suggested that algae could be used in a chemical-biological hybrid system to capture CO2. This study demonstrated that the new absorption-algae hybrid process could replace the conventional method for CO2 absorption [92]. However, it was also found that slow diffusion of CO2 in water for a short resident time could limit microalgae growth. Thus, polyethylene glycol (PEG) 200 enhanced the transfer of CO2 from the gaseous phase to the liquid phase. The sequestration of CO2 increased the growth of Nannochloropsis oceanica with the CO2 bubbling of 15% vol. The maximum specific growth rate of N. oceanica (1.41/day) was achieved with 1 mmol of PEG 200 in the culture medium. The algae biomass increased by about 79% with increased TIC because of more CO2 dissolution in the culture medium. Thus, as a CO2 absorbent, PEG 200 can efficiently capture CO2 from flue gas to grow algae [93]. A substantial amount of natural gases are generated in the process of oil extraction. The composition of natural gases generated during the process varied, but the most dominant ones were methane (CH4) (80%–95%) and CO2 [92].

Conversely, the release of both these gases is considered to be the primary cause of global warming and climate change. Therefore, it is necessary to convert natural gas into other valuable products by the process of mitigation. A microbial consortium of algae and bacteria has been suggested for quick metabolization of the high levels of CO2 and CH4 and their transformation into value-added bioproducts. The consortium (algae and bacteria) isolated from mangroves can survive in 70% of CH4 and 30% of CO2 [94]. A novel airlift photosynthetic microbial fuel cell (AL-PMFC) with C. vulgaris has been reported for the biofixation of CO2 and bioenergy. The maximum CO2 fixation rate of 835.7 mg/L/day was achieved with AL-PMFC. The highest CO2 fixation rate of 1292.8 mg/L/day, lipid productivity of 234.3 mg/L/day, and power density of 5.94 Wm³ have been attained at the optimized C. vulgaris inoculum size, level of CO2, and aeration rate. Thus, AL-PMFC can provide an attractive approach for CO2 fixation and bioenergy production [95]. Cultivation of N. oculata in semibatch PBRs for the bioconversion of CO2, wastewater treatment, and biomass production has been reported. The maximum growth rate of N. oculata with the productivity of 0.088 g/L/day was obtained at 18% of CO2 and the optimal pH range of 5.5–6.5 [96].

4: Bioenergy from algae

With a growing population worldwide, bioenergy demands are increasing globally. Fossil fuels are reducing worldwide and are close to their exhaustion point due to high and unsustainable consumption and due to their nonrenewable nature. Therefore, biofuels are gaining much attention globally as an alternative to fossil fuels. Biofuels, including biodiesel and bioethanol, are now being produced commercially in several developed countries. Alternative biofuels can be produced from numerous renewable substrates such as food crops, crop or fruit wastes, woody parts of plants, garbage, and algae [97,98]. The key benefit of producing and using biofuels is that they are renewable and sustainable and can considerably reduce environmental pollution and global warming. A leading source of global warming is the emission of greenhouse gases, such as CO2, generated due to the burning of fossil fuels. Approximately 29 gigatons of CO2 are generated annually, and a total amount of 35.3 billion tons of it has been produced until now due only to the burning of fossil fuels [99]. Algal biofuels could be an alternative to fossil fuels as they have 10%–45% of O2 and fewer sulfur emissions [100].

In contrast, petroleum-based fuels emit a high level of sulfur and do not have O2. Biofuel does not create environmental pollution. Moreover, it is a readily available, sustainable, and reliable fuel produced from bioresources. Biofuel from algae is environment-friendly and nontoxic, and it is considered to be a strong product for fixing CO2 worldwide. It is said that 1.83 kg of CO2 can be fixed per kg of algae. Moreover, several algal species consume flue gases, such as SOx and NOx, together with CO2 as nutrients [101]. CO2 constitutes 50% of the dry weight of algae biomass. The assortment of algal biomass is vital for regulating biofuel production costs and optimizing the energy structure. The variety of algal biomass used in the generation of biofuels is directly associated with the emission of greenhouse gas and further with environmental and economic sustainability [102]. Algae are currently the most favored raw material for bioenergy, and they can cater to the growing demands for biofuels, food, feed, and valued compounds [98,99]. Several countries in Asia, Europe, and America are starting to produce bioenergy from algae commercially [103].

4.1: Biodiesel production

Most of the microalgal species are promising with regard to biodiesel production due to their high lipid content (50%–70%). The biomass of several algal species, including microalga Botryococcus braunii, can produce around 80% of oil, making biodiesel production appropriate [104–106]. Algae can yield up to 58,700 L/ha of oil, from which 121,104 L/ha biodiesel can be produced [101,107,108]. Alkaliphilic green alga C. vulgaris has been reported to be appropriate for biodiesel production. The highest productivity of biomass at 28–31 mg/L/day along with lipid content of 38% were achieved at a low light intensity (60–90 μmol/m²/s). The main fractions of the fatty acids C16:0, C18:1, and C18:2 were observed after lipid transesterification. Biodiesel production from the same strain of algae can also be energy efficient [109]. Economical algal biodiesel production is directly associated with high operational, maintenance, harvesting, and conversion costs. Wastewater has been used as a cost-effective medium for culturing algae and biodiesel production by using magnetic nanocatalyst si7_e /Fe3O4-Al2O3 [110]. The effect of storage temperature and time on the increase of the lipid composition of Scenedesmus sp. has been studied. This study found that the free fatty acid content in wet algal biomass increased from a trace to 62% when stored at 4°C. Preesterification and transesterification were used for a two-step catalytic conversion of algal oil having high free fatty acid content into biodiesel. A high conversion rate of triacylglycerols was obtained at a methanol-to-oil molar ratio of 12:1 during catalysis with 2% potassium hydroxide at 65°C for 30 min. According to Chinese National Standards, a biodiesel analysis confirmed the standard level after purification by bleaching earth [111]. Dairy farm wastewaters have been reported as potential resources for algae cultivation for biodiesel production. A consortium of native strains removes more than 98% of nutrients from the treated wastewater. Biomass production of 153.54 t ha/year and lipid content of 16.89% were achieved from the cultivated consortium in treated wastewater. Algal lipids of 72.70% obtained from the consortium can be converted into biodiesel [112]. A freshwater microalgae C. zofingiensis was cultivated in pilot-scale PBRs by using artificial wastewater as a medium for their growth and lipid and biodiesel production and treatment. The maximum removal rate of TN (92%) and TP (100%) was attained for a mixotrophic culture with acetic acid as a pH regulator. A high productivity of biomass (66.94 mg/L/day) and a specific growth rate of 0.260 mg/day were achieved at controlled pH. Higher productivity of lipid content (37.48 mg/L/day) was achieved at the optimal condition. A biodiesel yield of 19.44%, presenting a massive 16–18 carbon composition of FAME, was obtained. This study suggested that pH regulation with acetic acid is the most useful method for the growth of Chlorella zofingiensis in wastewater during winter for biodiesel production and the removal of nutrients [113]. Another study proposed a single-step subcritical methanol extraction (SCM) process for biodiesel production from C. pyrenoidosa. The highest yield of crude biodiesel (7.1 wt%) was achieved at 160°C in a 3-min reaction time along with an optimal (7 wt%) methanol ratio to algae. This study suggested that the SCM process does not require any pretreatment step or a catalyst making it economical and practical for large-scale biodiesel production from algae [114]. A study experimented with mono- or cocultivation of C. vulgaris and Scenedesmus dimorphus in media containing different sources of N for growth, lipid content, biodiesel production, and nutrient elimination. This study confirmed that algae cultivation in the media containing various sources of N indicated not only a high removal efficiency but also increased biomass, lipid productivities, and biodiesel production. A trend of high lipid content was observed in the mixed culture (as against their mono-culturing) of C. vulgaris and S. dimorphus in the media that had the same N source. The main fatty acids of C16:0, C16:1, C18:0, C18:2, and C18:3, accounting for 79.6%–90.6% of the total, were achieved. Further, biodiesel production in the range of 8.5–11.2 g biodiesel/100 g dry weight was achieved, which demonstrates that the N source can affect the nutrient removal efficiency and biodiesel production of both mono- and mix-cultured microalgae [115].

Biodiesel production from algal oil via transesterification using various acids, base catalysts, and supercritical fluids has been reported. These catalysts are toxic and pose many challenges related to environmental contamination. A lipase-based enzyme extracted from a novel fungal strain Cladosporium tenuissimum with a molecular weight of ∼ 46 kDa and specific activity of 37.2 U/mg has been reported for biodiesel production. A purified lipase as a biological catalyst was successfully used for transesterification of Nitzschia punctata oil into biodiesel. The highest conversion efficiency of 87.2% was achieved with lipase as biocatalyst, and it was 83.02% in the case of a conventional acid catalyst. Lipase has the potential for extensive scale applications to increase the transesterification process’ conversion efficiency [116]. A quick algal growth rate, biofixation of CO2, and no competition with food and high lipid content make them an appropriate feedstock for biodiesel production. Conversely, the high cost associated with dehydration, extraction, and biodiesel conversion can limit their application at the industrial level. Direct biodiesel production from pretreated wet algae C. vulgaris through esterification and transesterification has been reported. A high Fatty Acid Methyl Ester (FAME) yield of 80% was achieved with a small volume of methanol and catalyst (either HCL or NaOH) assisted by RF heating at 55°C for 20 min [117]. The ability of microalgal strain growth in various wastewaters to remove nutrients and accumulate lipid is demonstrated in Table 1.2.

Table 1.2