Bioethanol production using carbohydrate-rich microalgae biomass as feedstock
Highlights
► A sugar-rich Chlorella vulgaris FSP-E strain was used as feedstock for ethanol production. ► Enzymatic and acidic hydrolyses can efficiently saccharify the microalgae biomass. ► SHF & SSF processes produced ethanol from the microalgae biomass with high yield. ► SSF process gave better ethanol production performance with a 92% theoretical yield.
Introduction
The fast growth of the world population and rapid development of a number of emerging economies have both led to sharp increase in global energy consumption (Harun et al., 2010). However, the use of fossil fuels is associated with environmental pollution, the greenhouse effect, and climate change (Ho et al., 2011, Sivakumar et al., 2010), and thus many countries are now increasing their efforts with regard to developing renewable energy sources, which are both more economic and environmentally friendly (Mussatto et al., 2010). Biomass is one of the most promising renewable resources used to generate different types of biofuels, such as biodiesel (Ho et al., 2010) and bioethanol (John et al., 2011). Currently, bioethanol is mainly derived from sucrose and starch crops (e.g., sugarcane and corn) as well as lignocellulosic materials (e.g., rice straw and switchgrass) (Nigam and Singh, 2011). However, using agricultural crops or agricultural waste as feedstock for bioethanol production still presents a number of problems, due to the increasing demands this makes on the limited arable lands and water supply, as well the high costs involved in converting lignocellulosic materials into ethanol. The major cause of the latter is the high lignin content in the lignocellulosic biomass, making the saccharification process very difficult (Sun and Cheng, 2002).
Microalgae have recently been considered as a third generation feedstock for biofuel production (Nigam and Singh, 2011), with the focal fuel in such processes being biodiesel (Chen et al., 2011, Chisti, 2007, Ho et al., 2010). However, since some microalgae species have high carbohydrate content, in terms of starch and cellulose, they are also excellent substrates for bioethanol production. Using carbohydrate-rich microalgal biomass for bioethanol production is advantageous, since microalgae grow faster and fix CO2 at a higher rate than terrestrial plants. In addition, microalgae-based carbohydrates are mainly in the form of starch and cellulose (with the absence of lignin), are thus much easier to convert to monosaccharides when compared with lignocellulosic materials (Harun et al., 2010, Ho et al., 2012, John et al., 2011). Microalgae like Chlorella, Chlamydomonas, Dunaliella, Scenedesmus, and Tetraselmis have been shown to accumulate a large amount of carbohydrates (>40% of the dry weight) (John et al., 2011). For example, many researchers have reported that the genus of Chlorella possess has a high carbohydrate content, especially the species of C. vulgaris, with carbohydrates being 37–55% of its dry weight (Brennan and Owende, 2010, Dragone et al., 2011, Illman et al., 2000).
The carbohydrates in green algae mainly come from starch in chloroplasts and cellulose/polysaccharides on cell walls (Domozych et al., 2012, Richmond, 2004), which are not readily fermentable for ethanol production by microorganisms. Therefore, prior to ethanol fermentation, the polysaccharides of microalgae should be hydrolyzed to fermentable sugars (Hahn-Hagerdal et al., 2007). In general, chemical (acid and alkaline) or enzymatic hydrolysis are common methods used for this purpose. While acid hydrolysis is faster, easier and cheaper than other types of hydrolysis, the acidic conditions may lead to decomposition of the sugars into unwanted compounds that inhibit the fermentation process (Girio et al., 2010, Harun et al., 2010, Moxley and Zhang, 2007). In contrast, enzymatic hydrolysis is slower and much more expensive than acidic hydrolysis (Lynd et al., 2002), but it is an environmentally benign process and can obtain higher glucose yields without producing inhibitory products. In addition to the cost issue, enzymatic hydrolysis has another drawback of requiring costly or energy-consuming physical or chemical pretreatments of the biomass to enhance the hydrolysis efficiency. To date, few studies have reported using microalgae for ethanol production (Harun and Danquah, 2011, Harun et al., 2010), while there are some examples describing the use of macroalgae (e.g., seaweed) for ethanol fermentation (John et al., 2011, Wargacki et al., 2012).
In this study, an indigenous microalgal isolate (identified as C. vulgaris FSP-E), with carbohydrates accounting for over 50% of its dry weight, was used as feedstock for producing ethanol via fermentation with Zymomonas mobilis. The effects of various hydrolysis methods and conditions on saccharification of the microalgal biomass were investigated. Several factors influencing the hydrolysis efficiency were examined, including hydrolysis strategies (enzymatic or acidic), hydrolytic enzyme composition, sulfuric acid concentration, hydrolysis time, and microalgae loading. The efficiency of converting the hydrolyzed microalgal biomass to bioethanol was also assessed by separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes. The aim of this work was to evaluate the feasibility of using high-sugar-content microalgal biomass as feedstock for bioethanol production.
Section snippets
Microalga strain and growth medium
Microalgal C. vulgaris FSP-E was isolated from a freshwater area located in southern Taiwan. Modified Basal medium was used for both pre-culture and culture of this strain, consisting of (g/L): urea, 0.56; KH2PO4, 1.25; MgSO4·7H2O, 1.00; CaCl2, 0.0835; H3BO3, 0.1142; FeSO4·7H2O, 0.0498; ZnSO4·7H2O, 0.0882; MnCl2·4H2O, 0.0144; MoO3, 0.0071; CuSO4·5H2O, 0.0157; Co(NO3)2·6H2O, 0.0049; EDTA·2Na, 0.50. Under the pre-culture condition, the microalgal strain was grown with continuous CO2 (2%) feeding
Effect of nitrogen starvation on carbohydrate accumulation in C. vulgaris FSP-E
To obtain a large amount of sugar-rich microalgal biomass as feedstock for bioethanol production at a low cost, a carbohydrate productivity of C. vulgaris FSP-E (i.e., high biomass productivity with sufficient carbohydrate content) much be obtained. How to trigger the synthesis of carbohydrates in microalgae has attracted considerable research interest worldwide. Several recent reports (Dragone et al., 2011, Ho et al., 2012, Yeh and Chang, 2011) have demonstrated that cultivation under nitrogen
Conclusions
This study demonstrated the feasibility of producing bioethanol from microalgal biomass. The self-made enzyme mixture from Pseudomonas sp. CL3 containing suitable amylase/cellulase composition could effectively hydrolyze C. vulgaris FSP-E biomass for subsequent ethanol production via the SHF and SSF processes. Ethanol production by the SHF process with dilute acid hydrolysis (1% H2SO4) of the microalgal biomass was more effective, producing 11.66 g L−1 of ethanol within 12 h with a yield of 87.59%
Acknowledgements
The authors gratefully acknowledge the financial support from the Core Research for Evolutional Science and Technology (CREST) of the Promoting Globalization on Strategic Basic Research Programs of the Japan Science and Technology Agency (JST). Support from the Top University Project of NCKU and by Taiwan’s National Science Council under grant numbers NSC 101-3113-P-110-002, NSC 101-3113-E-006-015, and NSC 101-3113-E- 006-016 is also acknowledged.
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