**3. Genetic enhancement**

Given the aforementioned challenges to breed wild algal strains for improved yield performance traits and the fact that substantial progress has been made in algal genomics and the development of robust genetic transformation systems substantial research efforts have focused on engineering microalgae with improved biomass performance traits. Most algal genetic engineering efforts have focused on the manipulation of metabolic pathways for increased biomass and coproduct

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

having variable rates of cell division [35, 36].

apparatus further reducing biomass yields [42].

**3.1 Alterations in source strength**

production. The production and accumulation of biomass can be broadly divided into four phases known as source (push), sink (pull), storage (accumulate) and turnover (metabolism). Providing an over-riding template on this simplistic model of biomass accumulation is the genetic and developmental control of cell size and cell division or replication rates. Source strength is effectively the primary photosynthetic processes associated with light conversion into chemical energy and the fixation of carbon dioxide into storage products. Sink strength refers to the impact of downstream metabolic processes on biomass accumulation including metabolic feedback control of carbon flux from photosynthesis to production of carbon storage products. The carbon storage products must also be compartmentalized in the cell to support night-time respiration and biomass accumulation. In algae, starch is first primary carbon storage product and is stored in plastids. Algae may also accumulate high energy density hydrocarbons including triacylglycerols or oils. Oil is stored in specialized droplets packaged by outer membranes having surface displayed amphipathic proteins or oil droplet proteins. The extent of accumulation of these storage compartments can be regulated at the level of gene expression and thus is the subject of genetic manipulation impacting overall product yields. However, algal cell division rates and control of cell volume are among the more important determinants of algal biomass production. While many single celled algae have fixed cell volumes that determine the timing of cytokinesis some single celled algae are capable of over 100-fold increases in cell volume as they grow while

In the following paragraphs we focus on progress that has been made at the molecular level to engineer or breed algae with enhanced source and sink strength, increased storage product accumulation, and accelerated cell division rates leading to enhanced yields. As is evident from the success achieved to date two- to five-fold

The efficiency of solar energy conversion into chemical energy stored in biomass

by plants and algae ranges from 3 to 5% of available solar energy. Theoretically, efficiencies as high as 11% for conversion of solar energy into the chemical energy in biomass can be achieved utilizing just the photosynthetically active radiation (400–700 nm) in the solar spectrum. Maximum efficiencies of energy conversion as high as 30% can be achieved using just red light (~650–700 nm) which is most efficiently harvested by the photosynthetic pigments [8, 37, 38]. Thus, it is conceivable that 2- to 4- fold increases in biomass yields are feasible through improvements in photosynthetic efficiency. It has long been recognized that the greatest potential for increasing photosynthetic efficiency is through enhanced light use efficiency by the photosynthetic apparatus (**Figure 2**) [39–41]. During photosynthesis, light saturates in all plants and algae at approximately one quarter of full sunlight intensity [38, 41]. Thus 75% of the energy captured by the photosynthetic pigments does no productive work leading to biomass production. Since the excess energy captured by the photosynthetic pigments does not drive electron transfer and carbon fixation processes it must dissipate through non-productive energy emission and/ or energy conversion pathways (heat, fluorescence, production of reactive oxygen species (ROS)) some of which (ROS) can lead to substantial damage to the photosynthetic

One approach to deal with the challenge of excess light absorption by the photosynthetic apparatus has been to reduce the optical cross section of the light-harvesting antenna complex to better couple the rate of light capture with rate-limiting electron transfer processes, i.e., plastohydroquinone oxidation by the

increases in the rate of biomass production and yields are feasible.

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

*Biotechnological Applications of Biomass*

during nutrient deprived growth conditions [31–33].

An additional practical application of nutrient deprivation for oil production is that growth in nutrient depleted media may reduce competition from weedy algal species [34]. This observation has led to the application of nutrient pulse technology to simultaneously induce oil accumulation during nutrient stress and inhibit the growth of weedy algal species. Under ideal growth conditions the limiting nutrients are withheld until there is an impairment in growth. At this transition point a pulse of the limiting nutrient is added to the growth media to support continued high growth rates [34]. Overall, the ability to induce oil production if managed well can lead to sustained high growth rates while enhancing the energy density of the biomass and the increased accumulation of biofuel feedstocks such as TAGs.

Given the aforementioned challenges to breed wild algal strains for improved yield performance traits and the fact that substantial progress has been made in algal genomics and the development of robust genetic transformation systems substantial research efforts have focused on engineering microalgae with improved biomass performance traits. Most algal genetic engineering efforts have focused on the manipulation of metabolic pathways for increased biomass and coproduct

density carbohydrates to high energy density hydrocarbons including triacylglycerol (TAG) and/ or polyterpenoids [20]. The metabolic shift from carbohydrate to hydrocarbon accumulation is typically induced by nutrient depravation. Upon shifting from a nitrogen-, sulfur- and/ or micronutrient-rich condition to a nutrient poor condition many algae will facultatively shift the metabolism of energy storage product accumulation from carbohydrates (starch) to hydrocarbons [21–25]. Hydrocarbons have more than 60% the energy density per fixed carbon of carbohydrates. Importantly, the facultative shift to hydrocarbon production allows algae to continue to generate and utilize reducing energy generated by the photosynthetic apparatus. Significantly, the accumulation of triacylglcerols may not only involve de novo synthesis but the remodeling of existing chloroplast membrane lipids into more fully reduced TAGs [26–29]. Given the desirability of hydrocarbons as a feedstock for biocrude production the ability to shift metabolism from carbohydrate to hydrocarbon production has been exploited to produce hydrocarbon rich biofuel feedstocks. The challenges with this strategy (nutrient deprivation) for facultative hydrocarbon production is that it can also lead to reduced rates of cell division and overall biomass accumulation. In a comprehensive empirical analysis of the impact of nitrogen deprivation on cell division rates, TAG accumulation, lipid remodeling, biomass accumulation and total caloric or biochemical energy accumulation in the green alga, *Chlorella sorokiniana*, it was demonstrated that upon shifting algae to a nitrogen-free growth medium there was a substantial increase in TAG accumulation and a redistribution of total cellular fatty acid profile to more energy dense saturated fatty acids [30]. Under the two-week nitrogen deprivation period employed in this study there was no statistically significant reduction in the rates of cell division or biomass (dry weight) accumulation. However, during the nutrient deprivation period the total chemical energy accumulated in biomass increased by greater than 60% associated with a 20-fold increase in TAG content. It is perhaps surprising that the two-week nitrogen deprivation period did not impair cell division and biomass accumulation suggesting that the alga had the capability to sequester nutrients and/ or catabolize and remodel existing nitrogen rich (proteins) molecules [30]. Not all algal species, however, exhibit similar responses to nutrient deprivation. For many algal species growth rates and biomass accumulation are substantially impaired

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**3. Genetic enhancement**

production. The production and accumulation of biomass can be broadly divided into four phases known as source (push), sink (pull), storage (accumulate) and turnover (metabolism). Providing an over-riding template on this simplistic model of biomass accumulation is the genetic and developmental control of cell size and cell division or replication rates. Source strength is effectively the primary photosynthetic processes associated with light conversion into chemical energy and the fixation of carbon dioxide into storage products. Sink strength refers to the impact of downstream metabolic processes on biomass accumulation including metabolic feedback control of carbon flux from photosynthesis to production of carbon storage products. The carbon storage products must also be compartmentalized in the cell to support night-time respiration and biomass accumulation. In algae, starch is first primary carbon storage product and is stored in plastids. Algae may also accumulate high energy density hydrocarbons including triacylglycerols or oils. Oil is stored in specialized droplets packaged by outer membranes having surface displayed amphipathic proteins or oil droplet proteins. The extent of accumulation of these storage compartments can be regulated at the level of gene expression and thus is the subject of genetic manipulation impacting overall product yields. However, algal cell division rates and control of cell volume are among the more important determinants of algal biomass production. While many single celled algae have fixed cell volumes that determine the timing of cytokinesis some single celled algae are capable of over 100-fold increases in cell volume as they grow while having variable rates of cell division [35, 36].

In the following paragraphs we focus on progress that has been made at the molecular level to engineer or breed algae with enhanced source and sink strength, increased storage product accumulation, and accelerated cell division rates leading to enhanced yields. As is evident from the success achieved to date two- to five-fold increases in the rate of biomass production and yields are feasible.
