Assessment of Third Generation Bioethanol Production Using Microalgae as Feedstock

Review Article

Austin Environ Sci. 2020; 5(1): 1045.

Assessment of Third Generation Bioethanol Production Using Microalgae as Feedstock

Suraj Katole, Sameer Sharma and Sibi G*

Department of Biotechnology, Indian Academy Degree College-Autonomous, India

*Corresponding author: Sibi G, Department of Biotechnology, Indian Academy Degree College- Autonomous, Bangalore, India

Received: May 02, 2020; Accepted: May 23, 2020; Published: May 30, 2020


Bioethanol from microalgal biomass have been recognized as a more promising alternative feedstock. The algal starch, cellulose or other accumulating carbohydrates can be used for the production of ethanol. Microalgae undergo a process consisting of pre-treatment, hydrolysis and fermentation to produce bioethanol. Strains having high biomass yield with high carbohydrates are taken into consideration for bioethanol production. The selection of appropriate pretreatment depends on their cost effectiveness. The exposure of the intracellular components of algae by using hydrolysis is crucial for bioethanol production. Similarly, the recovery of bioethanol varies on the microalgal species and the growth optimization is an effective way to maximize the bioethanol production. In other words, environmental and operational factors greatly influence bioethanol generation from algal biomass. Another important strategy is using the high yielding/immobilized co-cultures during the fermentation process. Further research optimization must be guided toward the development of cost-effective scalable methods to produce high bioethanol yield under optimum economy.

Keywords: Microalgae; Bioethanol; Hydrolysis; Fermentation; Pretreatment


Due to sharp increase in universal energy, there is a strong incentive to reduce the CO2 emissions and develop other energy sources as alternatives to fossil fuels [1]. In addition, increasing global population will lead to overexploitation of the resources and drives the scarcity of arable land to its limit [2]. It is a critical concern to develop the alternative energy resources and adopt policies to minimize the utilization of fossil reserves, maintain the environmental sustainability and cost-effective, and reduce the releases of greenhouse gas. The global demand for renewable energy sources has been continuously growing. Algae would be good candidates for renewable energy sources, receiving energy from the sunlight and building their biomass by eliminating CO2 from atmosphere through photosynthesis [3].

Biofuels are biological sources generally derived from primary fuels such as firewood, wood pellets, wood chips, animal waste, crop residues and landfill gas; while secondary fuels which consists of bioethanol, butanol, biodiesel, and biohydrogen [4,5]. Firstgeneration fuel which used the sources of food as feedstock but the large conversion of agricultural crops to biofuels has raised controversial debates [2]. Second-generation biofuels are mainly produced from lignocellulosic materials but involved difficulty and high costs to convert lignocellulosic biomass into biofuel [6-8]. Third-generation biofuels produced from microalgae as feedstocks have been recognized as a more promising alternative feedstock that do not require arable land, not competing with food cultures, high growth rate, high photosynthetic efficiency, potentially to cultivate in offshore marine environment and easy to be cultivated in larger quantity [9,7,10].

Bioethanol is produced from biomass by the fermentation of available carbohydrates, usually simple sugars, into bioethanol and carbon dioxide. In addition to bioethanol’s easy storage and distribution, the superior characteristics of bioethanol alone and blended with naturally occurring fossil fuels have made it a highly suitable automobile fuel. Un burned hydrocarbon and carbon monoxide emission levels of bioethanol combustion is significantly low when compared with gasoline combustion [11]. Bioethanol can be employed to replace gasoline, octane enhancers, and aromatic hydrocarbons, and has the advantage of being compatible with current infrastructure [12,13]. It is estimated that by 2050, liquid biofuels such as bioethanol is predicted to be on top of the ‘biofuel ladder’ due to their effectiveness in replacing gasoline for the transportation sector [14] Extensive cultivation of energy crops raises concerns regarding pollution of agricultural land with fertilizers and pesticides, soil erosion, reduced crop biodiversity, biocontrol ecosystem service losses and greenhouse gas emissions Several species of algae with high starch content are being tested to produce ethanol. Bioethanol of the third generation produced from microalgae biomass may represent an environmentally friendly fuel. It has many advantages in view of first- and second-generation biofuels produced from higher plants [15,16], mostly due to the rapid generation rate. Although plants have been used to produce bioethanol, alternative sources that do not require arable land should be considered [17]. Microalgae form a class of organisms that is likely to be adequate for producing third-generation bioethanol [18]. The algal starch, cellulose or other accumulating carbohydrates can be used for the production of ethanol after hydrolysis. Algae have higher photon conversion efficiency and can synthesize and accumulate large quantities of carbohydrate biomass for bioethanol production, from inexpensive raw materials [19,20]. The energy cost per carbon for triacylglycerol synthesis is 53% greater than for storage carbohydrate synthesis under standard conditions [21]; in this way, microalgae seem a truly viable feedstock for bioethanol production.

There has been a remarkable surge in research to investigate the utilization of microalgae as an advanced energy feedstock for bioethanol production [22,23,19]. Microalgae like Chlorella, Dunaliella, Chlamydomonas, Scenedesmus, Spirulina are known to contain a large amount (>50% of the dry weight) of starch and glycogen, useful as raw materials for ethanol production [24]. Microalgae can also assimilate cellulose which can also be fermented to bioethanol. This review attempts to summarize a thorough understanding of the significance of bioethanol production paves away for it suseasa versatile transportable fuel with excellent performance through microalgae.

A systematic search was carried out in PubMed, Scopus and Web of Sciences using a combination of Boolean operators. Peer reviewed papers in English on the bioethanol production using microalgae were retrieved and evaluated based on titles and abstracts. The retrieved papers were managed using Mendeley and the data were consolidated.

Bioethanol Production Process

Bioethanol production from algal biomass takes place by either the sugar or syngas pathway. Algae are directly fermented to produce bioethanol by the sugar pathway, while when processed via the syngas pathway, hydrocarbons of algal biomass are converted to syngas through gasification followed by fermentation of syngas to produce bioethanol. Starch-rich microalgae are extensively studied for the production of bioethanol via fermentation and different pretreatment methods have been evaluated to release the fermentable sugars from algal biomass in order to enhance bioethanol production. The production of bioethanol involves an extensive process. Like other feedstocks, algae undergo a process consisting of pre-treatment, hydrolysis and fermentation to produce bioethanol. Each step contains many variables, many distinctive methods exist, and the optimization of these steps is required to maximize the bioethanol yield.


The vast diversity of microalgae and derived products leads to a variety of pre-treatment. The selection of appropriate pretreatment depends on their cost-effectiveness. As far as microalgae pre-treatment is concerned, several studies have reported benefits of physical, chemical and biological pre-treatment methods in terms of product recovery.

Microwave pre-treatment: Microwave pre-treatment promotes starch digestibility which can enhance, depending on the conditions of the pre-treatment, the accessibility of enzymes to the pre-treated substrate [25]. The heating is performed by two mechanisms: 1) by the rotation of the dipoles where the polar molecules try to align in the electromagnetic field that changes rapidly by the microwaves and 2) by the ionic conduction consisting of the instant superheating of the ionic substance due to the friction of the ionic molecules generated by the movement that produces the electric field [26]. Since microalgae are grown in water and given the ionic nature of water, microwave radiation is well absorbed by the medium and consequently, it is an efficient and rapid way to carry out the pre-treatment.

Pyrolysis: Pyrolysis is widely used as a physical pre-treatment in which a high temperature is applied on the biomass for short time duration. However, the cost associated with its high energy consumption restricts its implementation at a commercial scale production [27]. Other physical methods including steam explosion and autoclaving rupture the microalgal cell wall, resulting in the release and recovery of bio components. The steam explosion method provides accessibility to the degradation of cellulose. Steam explosion is increasingly considered to be one of the most efficient, eco-friendly and cost-effective processes for commercial application and thus, it have been widely tested at the pilot scale for various biomasses [28].

Ultrasound pre-treatment: This type of technology can help break the cell wall of microalgae because when bubbles collapse on the surface of a solid, the pressure and elevated temperature create micro jets that allow the solvent to penetrate into the raw material and a rupture of the cell wall occurs [29]. This pre-treatment method is an alternative for cell disruption where water, acid, or alkalis could be used as catalysts for cell wall disruption of microalgal biomass. Ultrasonication effectively modifies the surface structure of biomass which lead to enhanced saccharification [30,31].

Pulse electric field pre-treatment: During this treatment, an effect called electroporation or electropermeabilization occurs [32,33]. When a critical electric field is applied, the electric forces cause a dielectric break that increases the permeability together with the formation of pores that are usually irreversible. This method of pre-treatment is used to extract sugars and high added value compounds without damaging or degrading the raw materials used (Figure 1).