**Abstract**

The promise of algae to address the renewable energy and green-product production demands of the globe has yet to be realized. Over the past ten years, however, there has been a substantial investment and interest in realizing the potential of algae to meet these needs. Tremendous progress has been achieved. Ten years ago, the price of gasoline produced from algal biomass was 20-fold greater than it is today. Technoeconomic models indicate that algal biocrude produced in an optimized cultivation, harvesting, and biomass conversion facility can achieve economic parity with petroleum while reducing carbon-energy indices substantially relative to petroleum-based fuels. There is also an emerging recognition that algal carbon capture and sequestration as lipids may offer a viable alternative to direct atmospheric CO2 capture and sequestration. We review recent advances in basic and applied algal biomass production from the perspectives of algal biology, cultivation, harvesting, energy conversion, and sustainability. The prognosis is encouraging but will require substantial integration and field testing of a variety of technology platforms to down select the most economical and sustainable systems to address the needs of the circular economy and atmospheric carbon mitigation.

**Keywords:** algae, biofuel, biomass, carbon sequestration, carbon index

## **1. Introduction**

Over the last ten years (since 2010) there has been accelerated investment in research for the development of commercially viable algal biomass and coproduct production systems [1–5]. The challenge for algae biomass production systems has been that unlike crop biomass production systems having thousands of years of development history, algae until very recently were not the target of integrated research and development (R&D) strategies focused on efficient production of food, fuel, and coproducts [6]. Recent estimates indicate that there are globally more than 150,000 species of single cell and multicellular algae having polyphyletic origins, complex and diverse metabolic machinery, occupying vast environmental niches with immense ranges of biotic and abiotic stress tolerance, and having growth or biomass production rates that range over two magnitudes in yield compared to traditional agricultural production [7]. The challenge for the industry has been to identify the best algal production systems that are suitable for commercially viable industrial applications. Beginning with algal biology much effort has focused on identifying the best performing algal strains. The criteria for down-selecting the best performing strains have included, identifying algae with the greatest biomass production rates, optimizing algal growth media, CO2 exchange and culture

conditions, identifying algal strains that are the most resistant to pathogens and herbivory (minimizing pond crashes), and developing strains having enhanced performance characteristics through application of genetic engineering, breeding and genome editing tools [6]. Research and development for improved biomass production has also focused on developing enhanced cultivation, harvesting and biomass conversion technologies with the objective to achieve the lowest carbon emissions, recycle inorganic nutrients as efficiently as possible, minimize energy inputs at each stage in production, and integrate the algal biomass production systems into the existing energy infrastructure as seamlessly as possible.

In 2010, the US Department of Energy launched the largest government-funded integrated algal biomass, biofuels and bioproducts program carried out to date. The National Alliance for Advanced Biofuels and Bioproducts (NAABB) achieved notable advances in reducing the cost of producing biomass and making biofuels from microalgae. In three years NAABB developed and modeled a pathway to move the price point for producing a gallon gasoline equivalent (GGE) of fuel from microalgae from \$150 to \$8 a GGE [1–3]. More recently, the price point for a GGE produced from algal biomass has been reduced to < \$5. Based on Reliance's demonstration scale studies, the technoeconomic modeling (TEM) for a 10 k barrels/ day (bpd) scale production of crude oil from microalgae was estimated to be at 100\$/ barrel without any subsidy. The major factors contributing to the substantial cost reductions in producing fuel from algal biomass included, the discovery and development of more robust, high biomass producing algal strains for year-round consistent performance, identification of the best geographies to produce algal biomass, advance pond designs and improved culture mixing for effective light utilization, effective crop control methods that prevent pond crash and biomass loss, innovative harvesting techniques and effective water and nutrient recycling to maximize resource utilization. Also, advancements in biomass to biocrude conversion technologies including continuous flow hydrothermal liquefaction (HTL), the demonstration that algal biocrude coming from HTL could be used as a direct feedstock in existing oil refineries to produce fuels with performance characteristics similar to petroleum-based fuels, and the production of high value coproducts to offset the cost of producing fuels.

Stepping back, however, there remain many critical considerations that must be addressed if microalgal biomass is to be a commercial success in competition with other biomass sources in the world where the carbon energy index (g CO2 emitted/ kJ energy produced) and the environmental impacts of any biomass production system must also be considered along with economics [6]. Beginning with first principles it is critical to identify what the thermodynamically most efficient biological mechanisms are for producing algal biomass that also have the highest carbon capture efficiency. Recent thermodynamic models suggest that the greatest energy efficiency for carbon capture and biomass production is achieved in algae that utilize light most efficiently and accumulate chemical energy in the form of carbohydrate polymers, e.g., starch rather than those that store oils [8, 9]. Additionally, algae with rapid division rates and/ or the ability to grow substantially in volume are likely to be greater biomass producers [10]. While most algal biofuel programs have focused on producing biomass from high lipid accumulating strains due to ease of conversion of lipids into biocrude it is becoming apparent that algae accumulating starch as a metabolic storage end product have the highest biomass production rates and thermodynamic efficiency [8–10]. While lipids have greater energy density and are more readily converted into fuels, starches have a greater chemical energy density per carbon per photon captured during photosynthesis [8]. One of the microalgal strains achieving the highest known biomass yields in cultivation is *Pseudoneochloris* which stores starch as an energy reserve, achieves high cell numbers at stationary phase of growth, and can increase its cellular volume as it grows by greater than 100-fold [8].

**453**

*Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

bioreactors may be more difficult to eradicate.

over traditional agriculture for feedstock production [12, 13].

Many microalgal species are good source of proteins, carbohydrates, lipids and other high value bioactive molecules, such as enzymes, pigments and vitamins. By altering the cultivation conditions or through metabolic engineering approaches, composition of algae can be manipulated to accumulate the specific biomolecule(s) of interest. Considering the higher growth rate and ability to accumulate high lipid content (≥30%), it is reported that microalgae can yield 58,700 L of oil/ ha as opposed to 172 L/ha for corn, 446 L/ha for soybean, 1892 L/ha for Jatropha and 5950 L/ha for oil palm [14]. Thus, the projected ability to produce oil from algae is ~10 times more compared to highest oil producing crop plant. Likewise, algae biomass can be a potential feedstock for bioethanol production because of its ability to accumulate starch even higher than 50% (w/w) of biomass under optimal conditions. Absence of lignin in algal cell wall makes its processing easier compared to lignocellulosic agricultural waste and woody biomass, where lignin removal is an additional step before processing for bioethanol production [13]. Moreover, lack of structural parts like leaves and roots in algae makes algal biomass more homogenous and might be less energy intensive to process compared to crop plants [13]. In an estimate, net energy from sugarcane ethanol and bagasse was 143 GJ/ha/year as opposed to 928 GJ/ha/year from microalgae, indicating microalgae to be significantly more efficient feedstock [15]. Protein is another commercially important component of algae biomass. Algae protein is comparable to other high-quality plant and animal protein sources, however, protein yield from algae happen to be between 4 and 15 tons/ha/year, which is significantly higher than 0.6–1.2 tons/ha/year,

Cultivation systems are also a major cost factor in producing algal biomass. It is generally recognized that to produce low value algal biomass open pond production systems have the lowest capital and operating expenses and require less maintenance (to prevent fouling) than closed bioreactor cultivation systems. However, open pond systems require greater amounts of water to operate due to evaporation, have higher energy costs associated with concentrating more dilute cultures, and are more susceptible to contamination although biological contaminants in closed

Regardless of the constraints and challenges mentioned above and the necessity to input higher capital investment in cultivation and downstream processing, production of microalgae biomass still stands out advantageous on many fronts in comparison to agriculture crops for food and fuel. Microalgae have high photosynthetic efficiency and short division time, making them highly suitable candidates for generating more biomass in less time. Growth rates of several microalgae have been reported to be 5–10 times higher than agriculture crops [9]. Moreover, microalgae can grow on low economic and ecological value lands and can utilize marine, brackish or fresh water for cultivation, depending on the species being used. CO2 from industrial exhaust can be used for cultivation and nutrients from waste streams can be utilized for growth. Excess nutrients lost during harvesting process can be recycled back in the cultivation system, ensuring minimal wastage and maximum utilization [10, 11]. In contrast, agriculture depends on limited natural resources, like arable land and fresh water, with fresh water consumption being highest globally in agriculture. Over 80% of all water consumed globally is used for agricultural production. Agriculture also needs extensive application of fertilizers and pesticides to improve biomass productivity and prevent crop losses. However, nitrogen utilization is inefficient in crop plants, resulting in ~50% of nitrogen loss through leaching, soil erosion and gaseous evaporation [12]. Considering these facts, use of agricultural crops to meet growing biomass demands for food and energy will lead to land use change, environment pollution, loss of forest cover and biodiversity. Thus, from environment standpoint algal cultivation is much favored

#### *Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

*Biotechnological Applications of Biomass*

value coproducts to offset the cost of producing fuels.

conditions, identifying algal strains that are the most resistant to pathogens and herbivory (minimizing pond crashes), and developing strains having enhanced performance characteristics through application of genetic engineering, breeding and genome editing tools [6]. Research and development for improved biomass production has also focused on developing enhanced cultivation, harvesting and biomass conversion technologies with the objective to achieve the lowest carbon emissions, recycle inorganic nutrients as efficiently as possible, minimize energy inputs at each stage in production, and integrate the algal biomass production systems into the existing energy infrastructure as seamlessly as possible.

In 2010, the US Department of Energy launched the largest government-funded integrated algal biomass, biofuels and bioproducts program carried out to date. The National Alliance for Advanced Biofuels and Bioproducts (NAABB) achieved notable advances in reducing the cost of producing biomass and making biofuels from microalgae. In three years NAABB developed and modeled a pathway to move the price point for producing a gallon gasoline equivalent (GGE) of fuel from microalgae from \$150 to \$8 a GGE [1–3]. More recently, the price point for a GGE produced from algal biomass has been reduced to < \$5. Based on Reliance's demonstration scale studies, the technoeconomic modeling (TEM) for a 10 k barrels/ day (bpd) scale production of crude oil from microalgae was estimated to be at 100\$/ barrel without any subsidy. The major factors contributing to the substantial cost reductions in producing fuel from algal biomass included, the discovery and development of more robust, high biomass producing algal strains for year-round consistent performance, identification of the best geographies to produce algal biomass, advance pond designs and improved culture mixing for effective light utilization, effective crop control methods that prevent pond crash and biomass loss, innovative harvesting techniques and effective water and nutrient recycling to maximize resource utilization. Also, advancements in biomass to biocrude conversion technologies including continuous flow hydrothermal liquefaction (HTL), the demonstration that algal biocrude coming from HTL could be used as a direct feedstock in existing oil refineries to produce fuels with performance characteristics similar to petroleum-based fuels, and the production of high

Stepping back, however, there remain many critical considerations that must be addressed if microalgal biomass is to be a commercial success in competition with other biomass sources in the world where the carbon energy index (g CO2 emitted/ kJ energy produced) and the environmental impacts of any biomass production system must also be considered along with economics [6]. Beginning with first principles it is critical to identify what the thermodynamically most efficient biological mechanisms are for producing algal biomass that also have the highest carbon capture efficiency. Recent thermodynamic models suggest that the greatest energy efficiency for carbon capture and biomass production is achieved in algae that utilize light most efficiently and accumulate chemical energy in the form of carbohydrate polymers, e.g., starch rather than those that store oils [8, 9]. Additionally, algae with rapid division rates and/ or the ability to grow substantially in volume are likely to be greater biomass producers [10]. While most algal biofuel programs have focused on producing biomass from high lipid accumulating strains due to ease of conversion of lipids into biocrude it is becoming apparent that algae accumulating starch as a metabolic storage end product have the highest biomass production rates and thermodynamic efficiency [8–10]. While lipids have greater energy density and are more readily converted into fuels, starches have a greater chemical energy density per carbon per photon captured during photosynthesis [8]. One of the microalgal strains achieving the highest known biomass yields in cultivation is *Pseudoneochloris* which stores starch as an energy reserve, achieves high cell numbers at stationary phase of growth, and can increase its cellular volume as it grows by greater than 100-fold [8].

**452**

Cultivation systems are also a major cost factor in producing algal biomass. It is generally recognized that to produce low value algal biomass open pond production systems have the lowest capital and operating expenses and require less maintenance (to prevent fouling) than closed bioreactor cultivation systems. However, open pond systems require greater amounts of water to operate due to evaporation, have higher energy costs associated with concentrating more dilute cultures, and are more susceptible to contamination although biological contaminants in closed bioreactors may be more difficult to eradicate.

Regardless of the constraints and challenges mentioned above and the necessity to input higher capital investment in cultivation and downstream processing, production of microalgae biomass still stands out advantageous on many fronts in comparison to agriculture crops for food and fuel. Microalgae have high photosynthetic efficiency and short division time, making them highly suitable candidates for generating more biomass in less time. Growth rates of several microalgae have been reported to be 5–10 times higher than agriculture crops [9]. Moreover, microalgae can grow on low economic and ecological value lands and can utilize marine, brackish or fresh water for cultivation, depending on the species being used. CO2 from industrial exhaust can be used for cultivation and nutrients from waste streams can be utilized for growth. Excess nutrients lost during harvesting process can be recycled back in the cultivation system, ensuring minimal wastage and maximum utilization [10, 11]. In contrast, agriculture depends on limited natural resources, like arable land and fresh water, with fresh water consumption being highest globally in agriculture. Over 80% of all water consumed globally is used for agricultural production. Agriculture also needs extensive application of fertilizers and pesticides to improve biomass productivity and prevent crop losses. However, nitrogen utilization is inefficient in crop plants, resulting in ~50% of nitrogen loss through leaching, soil erosion and gaseous evaporation [12]. Considering these facts, use of agricultural crops to meet growing biomass demands for food and energy will lead to land use change, environment pollution, loss of forest cover and biodiversity. Thus, from environment standpoint algal cultivation is much favored over traditional agriculture for feedstock production [12, 13].

Many microalgal species are good source of proteins, carbohydrates, lipids and other high value bioactive molecules, such as enzymes, pigments and vitamins. By altering the cultivation conditions or through metabolic engineering approaches, composition of algae can be manipulated to accumulate the specific biomolecule(s) of interest. Considering the higher growth rate and ability to accumulate high lipid content (≥30%), it is reported that microalgae can yield 58,700 L of oil/ ha as opposed to 172 L/ha for corn, 446 L/ha for soybean, 1892 L/ha for Jatropha and 5950 L/ha for oil palm [14]. Thus, the projected ability to produce oil from algae is ~10 times more compared to highest oil producing crop plant. Likewise, algae biomass can be a potential feedstock for bioethanol production because of its ability to accumulate starch even higher than 50% (w/w) of biomass under optimal conditions. Absence of lignin in algal cell wall makes its processing easier compared to lignocellulosic agricultural waste and woody biomass, where lignin removal is an additional step before processing for bioethanol production [13]. Moreover, lack of structural parts like leaves and roots in algae makes algal biomass more homogenous and might be less energy intensive to process compared to crop plants [13]. In an estimate, net energy from sugarcane ethanol and bagasse was 143 GJ/ha/year as opposed to 928 GJ/ha/year from microalgae, indicating microalgae to be significantly more efficient feedstock [15]. Protein is another commercially important component of algae biomass. Algae protein is comparable to other high-quality plant and animal protein sources, however, protein yield from algae happen to be between 4 and 15 tons/ha/year, which is significantly higher than 0.6–1.2 tons/ha/year,

reported for soybean [16]. Clearly, microalgae supersede traditional agriculture on multiple aspects, however, biomass harvesting is an area which is well established in case of crop plants but highly energy intensive in case of algae due to its small size and low biomass density [10].

Regarding algal biomass harvesting systems the general objective has been to develop algae harvesting and concentrating systems that have parasitic energy consumption values of less than 10% of the total algal biomass energy content [6]. To reduce the costs of fuel production, recent efforts have focused on the direct conversion of harvested algal biomass into separate fuel and coproduct fractions in a continuous flow system while efficiently recycling water and nutrients. One of the more promising technology developments in this sector has been the development of two-stage HTL which allows for the separate recovery of coproducts and biocrude feedstock while recycling water and nutrients back to the pond thus avoiding the energy intensive step of drying the algal biomass before biomass to fuel conversion. The appropriate selection of what high value coproduct(s) to produce from algal biomass is critical for economic viability when coproduct production is coupled with fuel production. From this perspective the coproduct should have sufficient value based on biomass yields to be economically sustainable without saturating markets to the point of driving coproduct prices so low as to be economically untenable. As modeled by the US-DOE PACE algal biofuels consortium a fully integrated algal cultivation, harvesting, co-product and fuel production system with integrated water and nutrient recycling has the potential to recover over 60% of the energy content of the algae as biocrude while producing valuable coproducts that have a large global market demand (**Figure 1**).

Optimizing algal biomass production and carbon sequestration also has the potential to address the existential threat of global climate change associated with greenhouse gas emissions. Currently, biological carbon capture and sequestration (BCCS) is one of the more feasible means to remediate the earth's atmosphere. As a BCCS system, algae are particularly attractive not only for their high areal

#### **Figure 1.**

*PACE consortium working model for the integrated co-production of biofuels and co-products (green chemicals, polysaccharides (guar), and methane) from algae. Inorganic nutrients and wastewater are recycled. Algae are preloaded with nutrients (nutrient pulse) and grown in minimal media to reduce weedy species competition and continuously harvested at mid-log phase growth. HTL, hydrothermal liquefaction; CHG, catalytic gasification.*

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*Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

mass and has a density of 0.91 g/cm3

as a solid has a density of 1.96 g/ cm3

chemical, and environmental challenges.

to other biomass production systems.

**2. Algal strains**

rates of carbon capture but also for their potential storage of carbon as lipids while recycling inorganic nutrients and water [17]. While not generally considered as a carbon sequestration material, lipids have several advantages over solid CO2 as a carbon sequestration material [17]. Triacylglycerol (C55H98O6) is 77% carbon by

that is 32% greater than solid CO2. Furthermore, being a liquid and not readily convertible to a gas, the ability of lipids to escape from deep geological sequestration is substantially less than CO2 reducing potential long-term risk to aerobic organisms [17]. Overall, algae have great potential to address simultaneously fuel, food, green

In the following sections we will review recent advances in the sustainable production of algal biomass and coproducts for fuels and economic competitiveness with petroleum and non-algal coproduct production systems. Substantial achievements have been realized from an industry that has a truly short history compared

Substantial efforts have focused on the identification of algal strains having maximum biomass yields under cultivation. Ideal biomass production strains must not only have fast growth rates but also must be robust and tolerate well abiotic (temperature, salinity, light) and biotic (pathogen, herbivore and weedy algae) stress conditions to minimize pond crashes and downtime in algal cultivation. There have been several large-scale algal surveys of wild algal species to identify those strains that perform well in cultivation [18]. In addition, screening systems for identifying strains with elevated performance characteristics in high light environments among others have led to some success in the identification of high performing algal strains [19]. Given that there as many as 150,000 species of algae have been identified and that limited resources have been available to screen algae for high biomass production, there remains a significant number of algae that remain to be assessed for biomass productivity in select environments [7]. In addition, substantial potential to improve algal productivity may also be achieved in traditional and molecular assisted breeding practices. Algae breeding efforts, except for laboratory strains such as *Chlamydomonas*, have been limited, however. This is because the means to induce gametogenesis to identify sexual mating types in most algae is not well understood. If the increased yield achieved through plant breeding are to serve as a prognosticator of the potential to enhance algal productivity it can be anticipated that algal breeding programs may enhance yields in the field by as much as ten-fold.

**2.1 Modulating cultivation conditions to impact oil and carbohydrate yields**

Given the fast rates of cell division and the absence of dedicated higher-order cellular structures including tissue and organs it is not unexpected that microalgae have an enhanced capability to metabolically remodel cellular functions under different growth conditions. Algae frequently live boom and busts cycles in the nutrient deserts of lakes and the open oceans. Thus, it is imperative that algae have flexible metabolic systems to survive in unpredictable and ever-changing environments and be unencumbered by programmed cell fates associated with the differentiation and organization of cells into higher order tissues and organs. One of the manifestations of this metabolic flexibility is the ability to shift the biochemistry of the major cellular energy storage products from low energy

. In contrast, CO2 is 27% carbon by mass and

. Thus, lipids have a volumetric carbon density

*Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

rates of carbon capture but also for their potential storage of carbon as lipids while recycling inorganic nutrients and water [17]. While not generally considered as a carbon sequestration material, lipids have several advantages over solid CO2 as a carbon sequestration material [17]. Triacylglycerol (C55H98O6) is 77% carbon by mass and has a density of 0.91 g/cm3 . In contrast, CO2 is 27% carbon by mass and as a solid has a density of 1.96 g/ cm3 . Thus, lipids have a volumetric carbon density that is 32% greater than solid CO2. Furthermore, being a liquid and not readily convertible to a gas, the ability of lipids to escape from deep geological sequestration is substantially less than CO2 reducing potential long-term risk to aerobic organisms [17]. Overall, algae have great potential to address simultaneously fuel, food, green chemical, and environmental challenges.

In the following sections we will review recent advances in the sustainable production of algal biomass and coproducts for fuels and economic competitiveness with petroleum and non-algal coproduct production systems. Substantial achievements have been realized from an industry that has a truly short history compared to other biomass production systems.
