OPINION ARTICLEpublished: 25 March 2014

doi: 10.3389/fchem.2014.00013

Synthetic sugar for sustainable power?Khaled Moustafa*

Institut Mondor de la Recherche Biomdicale - Institut National de la Sant et de la Recherche Mdicale, Crteil, France*Correspondence: khaled.moustafa@inserm.fr

Edited by:Wenzhen Li, Michigan Technological University, USA

Reviewed by:Xinhua Liang, Missouri University of Science and Technology, USAZhiyong Zhang, Oak Ridge National Laboratory, USA

Keywords: artificial photosynthesis, bioethanol, biofuels, carbon dioxide recycling, green environment, sustainable bioenergy, synthetic sugar

Energy is a major challenge for the futurewith important consequences on the eco-nomic and political stability, food security,water supply, energy independence, pol-lution, and global ecosystem. Owing toincreased industrial activities and grow-ing population, the reserve of oil andother fossil resources will become, atbest, rare, and costly for extraction andcommercialization.

For these reasons, sustainable energy ison the top interests of scientific and politi-cal programs in many countries where eco-nomic and industrial activities are maincontributors of the emission of green-house gases. High accumulation of thesegases is predictable to affect agricultureand to sharpen climate change in the nextdecades. To alleviate these effects, scien-tists and stakeholders need to addressesall potential hypotheses and approachesto develop sustainable energy systems thatminimize the pollution and the negativeeffects of carbon dioxide (CO2) on thebiosphere. Toward this objective, differentapproaches are currently under investiga-tion based on various platforms such aswater, microorganisms, plants and pho-tovoltaic devices. Splitting of water, forexample, is used to generate gaseoushydrogen using molecular catalysts such asmolybdenum-oxo complex (Karunadasaet al., 2010), copper oxide photocath-ode coated with an amorphous molybde-num sulphide (Morales-Guio et al., 2014)or Silicon/Hematite core/shell nanowirearray covered with gold nanoparticles(Wang et al., 2014). Various microorgan-ism genera are also investigated such asmicroalgae (Gong and Jiang, 2011), bac-teria (Kalscheuer et al., 2006), cyanobac-teria (Quintana et al., 2011) and yeast(Buijs et al., 2013). Other options include

genetically modified plants such as sugar-cane (Arruda, 2012), maize (Torney et al.,2007), sunflower and soybean (Dizge et al.,2009). Each of these platforms has itsown pros and cons. The production ofbiohydrogen for example encounters twomajor challenges relating to high pro-duction cost and low-yield (Gupta et al.,2013). Important sustainability issues alsoobstruct the production of biofuel fromalgae and plant platforms at industrialviable scales, admitting that a severe com-promise would be accepted for biofuelproduction from crops to the detriment offood production. Moreover, crops-basedbiofuels can provide only a small portionof the huge amounts of fuels needed annu-ally, estimated at the equivalent of 31 bil-lion barrels of crude oil (Rhodes, 2009).Plant-based biofuels also require precioussources (water and arable lands) that areincreasingly scarce, and it is much wiser tosave them for more vital needs than energy.Algae, on the other hand, have greaterpotential than plants to be good alternativeresources for renewable energy. However,algae require substantial amount of waterand fertilizer resources for scalable pro-duction of fuels (Edmundson and Wilkie,2013). Algal fuels also cannot be producedin large volumes in an environmentallysustainable manner, although crude oil hasbeen produced in various experimentalscales (Chisti, 2013). Moreover, there is apermanent need to fully recycle the nitro-gen and phosphorous nutrients that arenecessary for the sustainable algal biofuelsystem.

Other alternative energy resourcesinclude nuclear and solar-based plat-forms. Nuclear is a highly risk adventureat local and global level, especially underunpreventable environmental disasters

and uncontrollable climate change. Theother safer approach is sunlight and itsapplications with photovoltaic devices.An ideal energy system, however, shouldoffer the possibility to produce sustain-able power while reducing CO2 emissionsat the same time. An enhanced artificialphotosynthesis protocol that uses CO2,water and sunlight to produce fermentableorganic matter (sugar), in the same waythat plants do for their own natural pho-tosynthesis, would be an ultimate, or atleast a complementary solution to otherpotential bioenergy alternatives, for boththe production of biofuels and the reduc-tion of CO2 levels. Figure 1 illustratesa streamlined scheme for a conceptualenergy system inspired from photosynthe-sis process, starting by capturing sunlightand CO2 and ending by the synthesis ofsugars. The synthetized sugar can then beconducted to a chamber (i.e., fermentationreservoir) where it could be converted tobioethanol by fermentation. Once realized,such an approach would not only offer theadvantage to save valuable lands, freshwa-ter and crops, but also to reduce CO2 levelsand make industrial activities as desirableactivities rather than culprit operations,because the CO2 emitted will be recy-cled permanently to feed the artificialsystem and produce bioethanol. In otherterm, the more CO2 emitted by indus-trial activities, the more sugar synthesized,and the more bioethanol produced. Toenable maximum fixation of CO2, sucha system is expected to be implementednear industrial factories that emit CO2intensively.

The composition, size, chemical,and physical properties of appropriatedevices and materials intended for sucha technology need to be scrutinized and

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Moustafa Synthetic sugar for sustainable power?

FIGURE 1 | General scheme of an artificial biosynthesis for theproduction of bioethanol from synthesized sugar. To save valuable crops(e.g., maize, beet) and water for more vital applications, an artificialphotosynthesis can be used to produce fermentable sugar from CO2 andwater in a kind of artificial plastids (shown here as Chlorophotocell) as aphoto bioreactor for sustainable energy production. An artificialphotosynthesis system could be composed of a large unit with artificialplastids to capture solar light and atmospheric CO2 and then convert theminto sugar by chemical reaction. Produced sugar will be collected in another

unit (fermentation unit) where microorganisms can ferment the sugar andproduce bioethanol in optimized fermentation conditions. To insure CO2auto-feedback, in case of insufficient atmospheric CO2 concentration, aportion of the synthesized sugar can be conducted to another fermentationchamber to produce CO2 by yeast and store it to feed the system at theneeded moment. Adding natural plant debris as a source of minerals formicroorganisms would be necessary to improve the fermentation efficiency.An adapted reservoir can be designed to capture and filter the rain water tosupply the requirements of water.

custom-made for the specific projectedtask. This will entail efforts at the inter-face of multidisciplinary fields includingbiotechnology, chemistry, physics, engi-neering and plant biology. Moreover, inan ideal conception, a good sustainableenergy platform should combine artificialphotosynthesis with photovoltaic devicesto take advantages from both settings in allcircumstances.

Unlike the natural photosynthesis,whose rate is relatively low and depends

on plant species and ambient conditions,the artificial system should enable bettercontrol and management of the bioenergyyield. In fact, natural photosynthesis yieldis not necessarily low, but just adaptedto the needs of photosynthetic organ-isms. Artificial photosynthesis, on theother hand, is needed to offer more flex-ibility on the control of the productionyield and to satisfy great energy demands.Yet, one of the foremost challenges ofsunlight-based technology is what to do

when there is little or no sun? To addressthis intermittent nature of sunlight, anefficient photosynthesis system shouldmimic plants in their perfect absorptionand tackling the light spectrum for theirown photosynthesis. From hot and wet tocold and rainy environments, plants cap-ture light efficiently and develop intensevegetation with important organic mat-ter produced (e.g., equatorial and borealforests). Similarly, an artificial photosyn-thesis should be optimized to operate

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Moustafa Synthetic sugar for sustainable power?

in all circumstances of light shifts andintensity. To achieve such an objective,future multidisciplinary advancementsand doubled efforts will be necessary tobuild an effective artificial photosynthesisthat could be scalable for industrial pro-duction of bioenergy. The conversion ofCO2 sugarbioethanol may result inloss of some energy efficiency. To avoidsuch potential losses and take advantageof a shorter way to convert CO2 to fuel,another option would be the conversionof CO2 directly to ethanol (United StatesPatent 8,313,634).

Some other drawbacks may alsoencounter t