Robert J. Conrado, Chad A. Haynes, Brenda E. Haendler, and Eric J. Toone

Abstract Biofuels are by now a well-established component of the liquid fuels market and will continue to grow in importance for both economic and environmen­tal reasons. To date, all commercial approaches to biofuels involve photosynthetic capture of solar radiation and conversion to reduced carbon; however, the low efficiency inherent to photosynthetic systems presents significant challenges to scaling. In 2009, the US Department of Energy (DOE) Advanced Research Projects Agency-Energy (ARPA-E) created the Electrofuels program to explore the potential of nonphotosynthetic autotrophic organisms for the conversion of durable forms of energy to energy-dense, infrastructure-compatible liquid fuels. The Electrofuels approach expands the boundaries of traditional biofuels and could offer dramati­cally higher conversion efficiencies while providing significant reductions in requirements for both arable land and water relative to photosynthetic approaches. The projects funded under the Electrofuels program tap the enormous and largely unexplored diversity of the natural world, and may offer routes to advanced biofuels that are significantly more efficient, scalable and feedstock-flexible than routes based on photosynthesis. Here, we describe the rationale for the creation of the Electrofuels program, and outline the challenges and opportunities afforded by chemolithoautotrophic approaches to liquid fuels.

R. J. Conrado • E. J. Toone (*)

US Department of Energy Advanced Research Projects Agency (ARPA-E), 1000 Independence Avenue, SW, Washington, DC 20585, USA e-mail: Eric. Toone@hq. doe. gov

C. A. Haynes • B. E. Haendler

Booz Allen Hamilton, 955 L’Enfant Plaza North, SW Suite 5300, Washington, DC 20024, USA

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_38, 1037

© Springer Science+Business Media New York 2013

1 Introduction

Virtually all transportation-over land, air and sea—utilizes the energy stored in carbon-carbon and carbon-hydrogen bonds to provide motive force. Our use of this stored pool of solar energy can only be described as rapacious, and although the magnitude of the remaining resource is difficult to estimate, it is finite and its extrac­tion will become increasingly complex. The USA currently consumes roughly 19 million barrels of oil per day, over 70% of which is used for transportation [58]. Nearly half the oil consumed in the USA is imported, accounting for a third of the Nation’s trade deficit. In recent years, the annual cost of imported oil has exceeded $300 billion, in current dollars the equivalent of funding the entire Apollo Program twice every year.[14] Even this staggering amount significantly underestimates the true cost of imported oil, since a significant fraction of both defense and nondefense Federal spending is devoted to ensuring a stable supply of imported oil. Alternative approaches to liquid fuels are both a national and global imperative.

The sustainable production of energy-dense, infrastructure compatible liquid fuels requires the conversion of a durable form of energy-most plausibly solar radiation, but also geothermal, nuclear, or other forms of renewable energy—into stored chemical energy. The biological production of carbon-based liquid fuels requires three distinct steps: the capture of energy and transduction of that energy to a usable form by an organism; reduction of inorganic carbon to a fungible metabolic intermediate, typically in an oxidation state at or below zero; and the formation of carbon-carbon bonds to provide a fuel with convenient physical properties. Although the reduction of inorganic carbon can be achieved chemically (i. e., abiotically), the high efficiency formation of carbon-carbon bonds remains a significant challenge for the field of chemistry and purely chemical approaches to liquid fuels are not currently economically feasible at scale.

Instead, the production of liquid fuels relies primarily on terrestrial photosynthe­sis, in which solar radiation is assimilated through Photosystems I and II, and the captured energy is used to reduce and fix carbon through the Calvin-Benson — Bassham (CBB) cycle. In the CBB cycle, inorganic carbon is converted to glyceral — dehyde-3-phosphate; although this intermediate is the source of reduced carbon for myriad products through both primary and secondary metabolism, it is primarily converted to glucose and polymerized to various structural and storage polymers. These photosynthetic products are converted to liquid fuels either fermentatively or thermally, producing a variety of fuel molecules.

The overall efficiency of this process—from solar photons to liquid fuel— depends on the nature of both the photosynthetic organism and the means of conversion, but is certainly less than 1%. While photosynthesis is operationally facile-plants and photosynthetic organisms are autonomous-the process competes with other forms of agriculture for resources, in particular land, fresh water and essential nutri­ents (NPK and trace metals). The scalable, sustainable production of liquid fuels would be greatly enhanced by the development of processes that do not share agri­cultural factors of production and that offer greater conversion efficiencies than those of photosynthesis.

In 2009, the US Department of Energy (DOE) Advanced Research Projects Agency-Energy (ARPA-E) created the Electrofuels program to explore nonphoto­synthetic autotrophic organisms for the conversion of renewable energy to energy dense liquid fuels for transportation [2] . The approach expands the boundaries of traditional biofuels and could achieve dramatically higher overall conversion efficiencies while providing massive reductions in both arable land and water usage. If a series of technical challenges can be overcome, an Electrofuels system could have a higher utilization of refinery capacity due to feedstock flexibility and be free from fluctuations in feedstock supply and cost inherent to photosynthetic biofuels approaches. To achieve this vision, the Electrofuels program leverages foundational work in microbiology, genomics, metabolic engineering, and synthetic biology to create microorganisms that assimilate energy, fix inorganic carbon, and produce fuel molecules without photosynthesis (Fig. 1).

In this article, we describe the rationale for the Electrofuels program and con­sider the challenges to the economic viability of the approach.

2 Background

The USA is the global leader in the production of biofuels, in 2010 producing over 13 billion gallons of ethanol, or 9 billion gasoline gallon equivalents (GGE), from corn grain [48] . Further expansion of corn grain ethanol production is now con­strained by diminishing Congressional support for government subsidies (in 2010 the Volumetric Ethanol Excise Tax credit cost taxpayers approximately $6B), grow­ing concerns about the environmental impacts of increased corn production, and perceived impacts of converting food to fuel resources in the face of global popula­tion growth and rising food prices.

In order to address these and other issues, the Departments of Energy, Agriculture and Defense have made significant investments in advanced biofuels—biofuels derived from sustainable, nonfood resources such as agricultural residues, residues from forestry operations, food processing by-products, and municipal solid waste. In the near future, dedicated energy crops that grow on marginal or non-food- production land, including perennial grasses (e. g., switchgrass), woody species (e. g., willow), and aquatic macroalgae (e. g., Saccharina), will add to the supply of biomass feedstocks for the production of fuels.

Despite the many benefits offered by dedicated energy crops, the efficiency of energy capture and transduction by plants is remarkably low, calculated as the ratio

c

e-, H2

of incident solar radiation to stored energy in chemical bonds. Under optimized environmental conditions C4 plants can capture up to 6% of the available solar energy, while C3 plants convert a maximum of 4.5% [70]. Including in the calcula­tion the seasonal growth of plants, the diversion of plant matter for growth, and the conversion efficiency of fixed carbon to liquid fuels, overall annual photon-to-fuel efficiency stands at 0.18% for US corn ethanol (Supp Calc 1) and 0.20% for Brazil sugarcane ethanol (Supp Calc 2). As a result, land resources are vastly under — utilitized, even for perennial crops. Additionally, biofuels feedstocks and energy crops have significant competition in open markets, and suffer from price fluctuations that limit the viability of this approach.

Photosynthetic microorganisms represent an alternative to terrestrial plants for the production of biofuels, and several algal and cyanobacterial approaches to fuels have also been developed for use in closed systems [30, 31, 49]. Such microorgan­isms offer several advantages over terrestrial plants, including genetic tractability and the ability to secrete fuel products. Genetic tractability affords the opportunity to directly engineer pathways to produce fuel without the need for secondary pro­cessing, while the ability to secrete obviates the need for harvest or biomass manip­ulation. Still, diurnal and annual sunlight variation makes continuous algal production difficult to control, slows microbial growth, and reduces productivity, each of which results in an increase in the cost of capital and the final fuel product. Fresh water requirements are also potentially problematic, although these concerns can be ameliorated to some extent through the use of salt-tolerant species and aggressive water capture techniques. The deployment at scale of genetically modified microorganisms also raises issues of containment, both to prevent acci­dental release and adventitious infection by wild-type strains.

In its broadest conception, the term “biofuels” implies the action of living organ­isms in one or more of the steps required to reduce inorganic carbon to an energy — dense form: the capture of energy and transduction of that energy to a usable form; the reduction of carbon from the +4 oxidation state; and the elaboration of that reduced carbon into a final fuel molecule. The diversity of the microbial world— and especially of the deep oceans—is staggering: the ocean contains 300,000 times more bacteria than there are stars in the visible universe [59, 61]. Fewer than 1% of these microbes have been identified and fewer than 0.1% of marine microorganisms have been cultured [18, 29] . This astonishing store of diversity offers tremendous opportunities for many branches of science, including energy transduction and car­bon fi xation.

Photosynthesis is but one of the approaches to carbon fixation found in nature; myriad life forms exist in ecological niches where both reduced carbon and light are nonexistent. Such organisms assimilate energy from other energy rich (reduced) species, including H2, H2S, NH3 . and reduced metals ions. A group of so-called electrotrophs are capable of accepting reducing equivalents directly as electric current [39]. Many chemolithoautotrophs use carbon fixation cycles other than the CBB cycle, including the reductive acetyl-CoA, the reductive citric acid, and 3-hydroxypropionate-4-hydroxybutyrate cycles; some of these pathways offer significant advantages over the CBB cycle [5]. Such organisms might serve as factories for the high-efficiency production of liquid fuels from renewable forms of energy.

To address these fundamental questions the Electrofuels program seeks routes to biofuels that surpass the inherent limitations of photosynthesis. At the core of the program are chemolithoautotrophic microorganisms, organisms capable of fixing and reducing inorganic carbon but that derive energy from a variety of inor­ganic substrates. Such microorganisms might produce renewable biofuels from solar electricity, either directly or through the agency of a soluble mediator. This solar electricity could come from photovoltaic cells that even now capture >20% of the total solar spectrum [ 25] ; or the energy could come from other renewable sources (hydro, wind, wave, tidal), or non-fossil-based heat (geothermal, concentrating solar, nuclear). The approaches considered under the Electrofuels program tap the enormous and largely unexplored diversity of the microbial world, and may offer routes to advanced biofuels significantly more efficient, scalable and feedstock- flexible than routes based on photosynthesis.