Elsevier

Bioresource Technology

Volume 181, April 2015, Pages 330-337
Bioresource Technology

Prospective technology on bioethanol production from photofermentation

https://doi.org/10.1016/j.biortech.2015.01.090Get rights and content

Highlights

Abstract

The most important global demand is the energy supply from alternative source. Ethanol may be considered an environmental friendly fuel that has been produced by feedstock. The production of ethanol by microalgae represent a process with reduced environmental impact with efficient CO2 fixation and requiring less arable land. This work studied the production of ethanol from green alga Chlamydomonas reinhardtii through the cellular metabolism in a light/dark cycle at 25 °C in a TAP medium with sulfur depletion. The parameters evaluated were inoculum concentration and the medium supplementation with mixotrophic carbon sources. The combination of C. reinhardtii and Rhodobacter capsulatus through a hybrid or co-culture systems was also investigated as well. C. reinhardtii maintained in TAP-S produced 19.25 ± 4.16 g/L (ethanol). In addition, in a hybrid system, with medium initially supplemented with milk whey permeated and the algal effluent used by R. capsulatus, the ethanol production achieved 19.94 ± 2.67 g/L.

Introduction

The use of ethanol to replace oil is the most viable way to ensure a sustainable future. However, up to now more emphasis has been given on the yeast performance in reactors for bioethanol production (Andrietta et al., 2008). On the other hand, considering the advantages of microalgae culture such as rapid growth rate and productivity (Li et al., 2008) and their use to minimize contamination, since microalgae may to applied in wastewater treatment from inorganic salts (NH4+, NO3, PO43−) using them as nutrient materials (Mata et al., 2010), Chlamydomonas reinhardtii could be preferentially selected as a prospective biological system for bioethanol production instead of yeast.

Concerning on the application of microalgae, recent research have shown that a diversity of strains may be cultivated to produce biodiesel, hydrogen, methane and ethanol. Specifically to the ethanol production from microalgae, it can be directly synthesized by the cellular metabolism or from the fermentation of microalgae biomass, mainly cellulose and starch that are readily converted to fermentable products by enzymatic or acidic pretreatment technology (Oncel, 2013, Chen et al., 2013). Microalgae based ethanol may be considered as part of integrated process and a promising environmentally friendly alternative, since they could be present a potential for fixing CO2, high growth at high yields and low costs utilizing light as the energy source. Furthermore, they do not require fertile land and portable water as feedstock based biofuel (Chen et al., 2013, John et al., 2011, Hirano et al., 1997).

The unicellular green alga C. reinhardtii uses light to grow photoautotrophically or mixotrophically in the presence of small organic substrates (Goff et al., 2009). C. reinhardtii is widely studied for hydrogen production by biophotolysis of water. This method uses the same processes found in plants photosynthesis. Photosynthesis involves the absorption of light by two distinct photosynthetic systems operating in series: a water splitting and O2 evolving system (“photosystem II” or PSII) and a second photosystem (PSI), which generates the reductant used for CO2 reduction (Das and Veziroglu, 2001). It is important to note that green algae could produce hydrogen not only under light conditions, but under dark anaerobic conditions (Gaffron and Rubin, 1942). Nevertheless, if dark and anaerobic conditions are established on the microalgae cultures, hydrogen yield is quite low corresponding to about one-sixth of direct biophotolysis production (Kosourov et al., 2002). Besides hydrogen, the oxidative reaction of starch become incomplete and depending on the type of the microalga, carbon dioxide, ethanol, lactic acid, formic acid, acetic acid, malic acid, glycerol and other compounds are produced in varying proportions (John et al., 2011, Gfeller and Gibbs, 1984). In addition, the medium composition may be altered to induce anaerobic condition. Hemschemeier and Happe (2004) reported that, under sulfur depletion, C. reinhardtii stops growing and accumulates starch. The absence of sulfur forces the algae to reorganize the whole metabolism. Anaerobiosis is established and hydrogen and ethanol could be produced. These authors discussed that the accumulation of ethanol already indicate the activity of pyruvate formate-lyase (PFL). PFL cleaves pyruvate into acetyl-CoA and it can further be reduced to acetaldehyde by acetaldehyde dehydrogenase. Furthermore, ethanol can be formed from cleavage of pyruvate by pyruvate decarboxylase (PDC) producing acetaldehyde as intermediate that is converted to ethanol by alcohol dehydrogenase (ADH).

Previous work suggested it is possible to produce ethanol from metabolism of C. reinhardtii maintained in a basal medium supplemented with the mixotrophic carbon source (Costa et al., 2014). And, the purpose of this work was at first to verify the possibility of C. reinhardtii produce ethanol using a basal medium with sulfur depletion added mixotrophic carbon source such as milk whey permeate (rich in lactose) and sodium acetate. In the second step, the algae association with the purple non sulfur bacterium, a Rhodobacter capsulatus, to improve ethanol content into the medium was also evaluated by hybrid system (two stages) and co-culture. The hybrid systems and co-cultures approaches are strategies used in order to improve the yield. The purpose is to integrate microorganisms with distinct biological routes. Thus, the metabolites produced by one type of microorganism may be the substrate to the second type.

Section snippets

Algal biomass

C. reinhardtii CC-124 was purchased from the Canadian Culture Collection, the Chlamydomonas Resource Center. The green alga was maintained in the basal medium Tris Acetate Phosphate (TAP) (Andersen, 2005) at initial pH of 7.0. In order to guarantee enough amounts of cells for the fermentation assays, the algal inoculum was subcultured with the addition of 250 mL of fresh TAP medium in 250 mL of growing culture. The alga was kept in Erlenmeyer (500 mL) at 25 °C under light cycle (night/day) of 12 h

Results and discussion

Photobiological production of biofuel by green algae has great potential to be used for generating renewable fuel from clean resources. As reported by Oncel (2013), microalgae can also excrete ethanol directly through the cell walls by means of intracellular processes under dark. In fact, the degradation of intracellular starch, which is the main endogenous carbon source stored during aerobic phototrophic metabolism, into pyruvate is accomplished by the Embden–Meyerhof–Parnas and pentose

Conclusion

In this study the effect of inoculum concentration and carbon source to C. reinhardtii, as well as the influence of hybrid system and co-culture (C. reinhardtii and R. capsulatus) on the photofermentative ethanol production were investigated. Maximum ethanol content 19.94 ± 2.67 g/L (0.17 ± 0.02 g/L h) was achieved by hybrid system in which the effluent of C. reinhardtii containing organic acids was used as substrate to R. capsulatus. The results from this work are beneficial to comprehend the

Acknowledgements

The authors wish to thank Brazilian agencies CNPq, CAPES and FAPEMIG (Grant No. APQ-01020-12) for financial support.

References (32)

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