The Enzymatic Conversion of Major Algal and Cyanobacterial Carbohydrates to Bioethanol

REVIEW published: 04 November 2016 doi: 10.3389/fenrg.2016.00036 The Enzymatic Conversion of Major Algal and Cyanobacterial Carbohydrates to Bioethanol Qusai Al Abdallah1*, B. Tracy Nixon2 and Jarrod R. Fortwendel1 1 Department of Clinical Pharmacy, University of Tennessee Health Science Center, Memphis, TN, USA, 2 Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, USA The production of fuels from biomass is categorized as first-, second-, or third- generation depending upon the source of raw materials, either food crops, lignocellulosic material, or algal biomass, respectively. Thus far, the emphasis has been on using food crops creating several environmental problems. To overcome these problems, there is a shift toward bioenergy production from non-food sources. Algae, which store high amounts of carbohydrates, are a potential producer of raw materials for sustainable production of bioethanol. Algae store their carbohydrates in the form of food storage sugars and structural material. In general, algal food storage polysaccharides are composed of glucose subunits; however, they vary in the glycosidic bond that links Edited by: the glucose molecules. In starch-type polysaccharides (starch, floridean starch, and Arumugam Muthu, Council of Scientific and glycogen), the glucose subunits are linked together by α-(1→4) and α-(1→6) glycosidic Industrial Research, India bonds. Laminarin-type polysaccharides (laminarin, chrysolaminarin, and paramylon) are Reviewed by: made of glucose subunits that are linked together by β-(1→3) and β-(1→6) glycosidic Wenjie Liao, bonds. In contrast to food storage polysaccharides, structural polysaccharides vary in Sichuan University, China Sachin Kumar, composition and glycosidic bond. The industrial production of bioethanol from algae Sardar Swaran Singh National requires efficient hydrolysis and fermentation of different algal sugars. However, the Institute of Renewable Energy, India hydrolysis of algal polysaccharides employs more enzymatic mixes in comparison to *Correspondence: terrestrial plants. Similarly, algal fermentable sugars display more diversity than plants, Qusai Al Abdallah [email protected] and therefore more metabolic pathways are required to produce ethanol from these sugars. In general, the fermentation of glucose, galactose, and glucose isomers is Specialty section: carried out by wild-type strains of Saccharomyces cerevisiae and Zymomonas mobilis. This article was submitted to Bioenergy and Biofuels, In these strains, glucose enters glycolysis, where is it converted to pyruvate through a section of the journal either Embden–Meyerhof–Parnas pathway or Entner–Doudoroff pathway. Other mono- Frontiers in Energy Research saccharides must be converted to fermentable sugars before entering glycolysis. In Received: 12 August 2016 contrast, microbial wild-type strains are not capable of producing ethanol from alginate, Accepted: 17 October 2016 Published: 04 November 2016 and therefore the production of bioethanol from alginate was achieved by using genet- Citation: ically engineered microbial strains, which can simultaneously hydrolyze and ferment Al Abdallah Q, Nixon BT and alginate to ethanol. In this review, we emphasize the enzymatic hydrolysis processes of Fortwendel JR (2016) The Enzymatic Conversion of Major Algal and different algal polysaccharides. Additionally, we highlight the major metabolic pathways Cyanobacterial Carbohydrates to that are employed to ferment different algal monosaccharides to ethanol. Bioethanol. Front. Energy Res. 4:36. Keywords: bioethanol, algal carbohydrates, food reserves, structural polysaccharides, enzymatic hydrolysis, doi: 10.3389/fenrg.2016.00036 fermentation Frontiers in Energy Research | www.frontiersin.org 1 November 2016 | Volume 4 | Article 36 Al Abdallah et al. Bioethanol from Algae INTRODUCTION Lewis, 2007). Because of their importance for biofuel production, this review will cover cyanobacteria as well. During the last decade, growing concerns over depleting fossil Algae vary dramatically in size and morphology from micro- oil reserves and increasing greenhouse gas emissions have led to scopic unicellular phytoplanktons to 50-m long seaweeds. Based the use of food crops as biomass for making what is called first- on their morphology and size, algae are classified into microalgae generation biofuel. Nevertheless, expansion in biofuel production and macroalgae. Currently, microalgae are the major source for from food crops has drawn attention to several environmental third-generation biofuels. In contrast, only small amount of impacts, such as the conversion of agricultural land from produc- cyanobacterial biomass are utilized for bioethanol production. ing food crops to producing biofuel crops and the deforestation of Additionally, development of methods that overcome obstacles hundreds of thousands of acres (Groom et al., 2008; Searchinger in using macroalgae would greatly improve harvesting bioethanol et al., 2008; Naik et al., 2010). from natural, renewable biomaterials. The advantages and dis- To overcome these environmental problems, there is a shift advantages of relevant algal sources are summarized in Table 1. toward the production of biofuel from non-food biomass sources, such as lignocellulosic and algal biomass sources, which are also Microalgae Are the Current Source for known as the second- and third-generation biofuel crops, respec- Third-generation Biofuels tively (Badger, 2002; Zheng et al., 2009; Brennan and Owende, Microalgae are microscopic in size (measured in micrometers) 2010). However, commercial production of second-generation and exist as single cells; or unspecialized multicellular filaments biofuels is only limited to countries with large agricultural and and colonies (Satyanarayana et al., 2011). They are highly diverse forestry lands. Therefore, algal biomass is an emerging alternative including 40,000 species that belong to nearly all major algal for the production of biofuels. groups with the exception of brown algae [reviewed by Metting The production of biofuel from algae has several advan- (1996), Dahiya (2015), and Kim (2015)]. tages over the first- and second-generation of biomass sources Microalgae exhibit several features that favor using them for [discussed by John et al. (2011)]. First, algae serve as non-food industrial production of biofuel. First, they lack specialized tis- feedstock, which does not compromise our food security. Second, sues and structures, which simplify the cultivation and harvesting algae grow in aquatic habitats and thereby do not compete with processes. In addition, microalgae exhibit high rates of asexual food crops on agricultural land, or cause deforestation. Third, growth and yield huge amount of biomass from low inoculum algal biomass can be used to produce two types of biofuel (Packer, 2009; Chen et al., 2010). Furthermore, microalgae accu- (bioethanol and biodiesel) since they accumulate high amounts mulate large amounts of polysaccharides and triacylglycerols – of carbohydrates and lipids. Finally, the fresh water requirement storage lipids and energy sources, and thereby they are suitable for algal growth is significantly lower than plant demands to for simultaneous production of bioethanol and biodiesel (Mata produce the same volume of biofuel. Nevertheless, there are et al., 2010; Singh et al., 2011; Suutari et al., 2015). several constraints that restrict the production of biofuel from The commercial production of microalgal biomass is algae [discussed by Hannon et al. (2010), Singh et al. (2011), and obtained from cultivating the freshwater algae Chlorella and Behera et al. (2015)]. Haematococcus, and marine algae, such as Dunaliella, The hydrolysis of algal carbohydrates to basic sugars is Phaeodactylum, and Tetraselmis (Lee, 1997; Wikfors and Ohno, primarily carried out using chemical and enzymatic methods. 2001; Carlsson et al., 2007; Benemann, 2013; Borowitzka, 2013). Although the chemical method yields high concentrations of Additionally, other microalgae have been shown to be a potential fermentable sugars in a short time, this method requires harsh source for third-generation biofuels due to their high oil and reaction conditions producing byproducts, which might inhibit carbohydrates contents (Singh et al., 2011). the fermentation process and require costly disposal processes. One of the challenges for commercial cultivation of microal- In contrast, enzymatic hydrolysis produces high amounts of gae is the economic feasibility. In their marine natural habitats, fermentable sugars under mild conditions without producing the productivity of microalgae is very low, not exceeding 10% of inhibitory byproducts (Chen et al., 2013). that for macroalgae under the same conditions. Such low yield Algae produce a wide spectrum of polysaccharides that are of microalgal biomass is not sufficient for the industrial produc- specific to an algal group, family, or species. The enzymatic tion of bioethanol. To improve the yield, microalgae should be hydrolysis of algal polysaccharides requires a wider range of cultivated in artificial systems (Lüning and Pang, 2003). The most enzymatic mixtures, compared to plants. This review focuses on common two methods for the cultivation of microalgae are the the enzymatic hydrolysis steps of the major algal carbohydrates outdoor open pond system and the closed photobioreactor [for and their fermentation process to ethanol. Since the scope of this reviews, refer to Brennan

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