Ben Hankamer

An economic and technical evaluation of microalgal biofuels

In her News Feature “Biotech’s green gold”, Emily Waltz details the ‘hype’ being propagated around emerging microalgal biofuel technologies, which often exceeds the physical and thermodynamic constraints that ultimately define their economic viability. Our calculations counter such excessive claims and demonstrate that 22 MJ m−2 d−1 solar radiation supports practical yield maxima of ∼60 to 100 kl oil ha−1 y−1 (∼6,600 to 10,800 gal ac−1 y−1) and an absolute theoretical ceiling of ∼94 to 155 kl oil ha−1 y−1, assuming a maximum photosynthetic conversion efficiency of 10%. To evaluate claims and provide an accurate analysis of the potential of microalgal biofuel systems, we have conducted industrial feasibility studies and sensitivity analyses based on peer-reviewed data and industrial expertise. Given that microalgal biofuel research is still young and its development still in flux, we anticipate that the stringent assessment of the technologys economic potential presented below will assist R&D investment and policy development in the area going forward.

Improved Photobiological H2 Production in Engineered Green Algal Cells

Oxygenic photosynthetic organisms use solar energy to split water (H2O) into protons (H+), electrons (e-), and oxygen. A select group of photosynthetic microorganisms, including the green alga Chlamydomonas reinhardtii, has evolved the additional ability to redirect the derived H+ and e- to drive hydrogen (H2) production via the chloroplast hydrogenases HydA1 and A2 (H2 ase). This process occurs under anaerobic conditions and provides a biological basis for solar-driven H2 production. However, its relatively poor yield is a major limitation for the economic viability of this process. To improve H2 production in Chlamydomonas, we have developed a new approach to increase H+ and e- supply to the hydrogenases. In a first step, mutants blocked in the state 1 transition were selected. These mutants are inhibited in cyclic e- transfer around photosystem I, eliminating possible competition for e- with H2ase. Selected strains were further screened for increased H2 production rates, leading to the isolation of Stm6. This strain has a modified respiratory metabolism, providing it with two additional important properties as follows: large starch reserves (i.e. enhanced substrate availability), and a low dissolved O2 concentration (40% of the wild type (WT)), resulting in reduced inhibition of H2ase activation. The H2 production rates of Stm6 were 5-13 times that of the control WT strain over a range of conditions (light intensity, culture time, ± uncoupler). Typically, ∼540 ml of H2 liter-1 culture (up to 98% pure) were produced over a 10-14-day period at a maximal rate of 4 ml h-1 (efficiency = ∼5 times the WT). Stm6 therefore represents an important step toward the development of future solar-powered H2 production systems.

Future prospects of microalgal biofuel production systems.

Climate change mitigation, economic growth and stability, and the ongoing depletion of oil reserves are all major drivers for the development of economically rational, renewable energy technology platforms. Microalgae have re-emerged as a popular feedstock for the production of biofuels and other more valuable products. Even though integrated microalgal production systems have some clear advantages and present a promising alternative to highly controversial first generation biofuel systems, the associated hype has often exceeded the boundaries of reality. With a growing number of recent analyses demonstrating that despite the hype, these systems are conceptually sound and potentially sustainable given the available inputs, we review the research areas that are key to attaining economic reality and the future development of the industry.

Selection, breeding and engineering of microalgae for bioenergy and biofuel production.

Microalgal production technologies are seen as increasingly attractive for bioenergy production to improve fuel security and reduce CO(2) emissions. Photosynthetically derived fuels are a renewable, potentially carbon-neutral and scalable alternative reserve. Microalgae have particular promise because they can be produced on non-arable land and utilize saline and wastewater streams. Furthermore, emerging microalgal technologies can be used to produce a range of products such as biofuels, protein-rich animal feeds, chemical feedstocks (e.g. bioplastic precursors) and higher-value products. This review focuses on the selection, breeding and engineering of microalgae for improved biomass and biofuel conversion efficiencies.

Photosynthetic biomass and H2 production by green algae: from bioengineering to bioreactor scale-up

The development of clean borderless fuels is of vital importance to human and environmental health and global prosperity. Currently, fuels make up approximately 67% of the global energy market (total market = 15 TW year(-1)) (Hoffert et al. 1998). In contrast, global electricity demand accounts for only 33% (Hoffert et al. 1998). Yet, despite the importance of fuels, almost all CO(2) free energy production systems under development are designed to drive electricity generation (e.g. clean-coal technology, nuclear, photovoltaic, wind, geothermal, wave and hydroelectric). In contrast, and indeed almost uniquely, biofuels also target the much larger fuel market and so in the future will play an increasingly important role in maintaining energy security (Lal 2005). Currently, the main biofuels that are at varying stages of development include bio-ethanol, liquid carbohydrates [e.g. biodiesel or biomass to liquid (BTL) products], biomethane and bio-H(2). This review is focused on placing bio-H(2) production processes into the context of the current biofuels market and summarizing advances made both at the level of bioengineering and bioreactor design.

Perspectives and advances of biological H2 production in microorganisms

The rapid development of clean fuels for the future is a critically important global challenge for two main reasons. First, new fuels are needed to supplement and ultimately replace depleting oil reserves. Second, fuels capable of zero CO2 emissions are needed to slow the impact of global warming. This review summarizes the development of solar powered bio-H2 production processes based on the conversion of photosynthetic products by fermentative bacteria, as well as using photoheterotrophic and photoautrophic organisms. The use of advanced bioreactor systems and their potential and limitations in terms of process design, efficiency, and cost are also briefly reviewed.

Improvement of light to biomass conversion by de-regulation of light-harvesting protein translation in Chlamydomonas reinhardtii

The efficient use of microalgae to convert sun light energy into biomass is limited by losses during high light illumination of dense cell cultures in closed bioreactors. Uneven light distribution can be overcome by using cell cultures with smaller antenna sizes packed to high cell density cultures, thus allowing good light penetration into the inner sections of the reactor. We engineered a new small PSII antenna size Chlamydomonas reinhardtii strain with improved photon conversion efficiency and increased growth rates under high light conditions. We achieved this goal by transformation of a permanently active variant NAB1* of the LHC translation repressor NAB1 to reduce antenna size via translation repression. NAB1* expression was demonstrated in Stm6Glc4T7 (T7), leading to a reduction of LHC antenna size by 10-17%. T7 showed a approximately 50% increase of photosynthetic efficiency (PhiPSII) at saturating light intensity compared to the parental strain. T7 converted light to biomass with much higher efficiencies with a approximately 50% improved mid log growth phase. Moreover, T7 cultures reached higher densities when grown in large-scale bioreactors. Thus, the phenotype of strain T7 may have important implications for biotechnological applications in which photosynthetic microalgae are used for large-scale culturing as an alternative plant biomass source.

Two-dimensional structure of plant photosystem II at 8-A resolution

The photosystem II complex, which is the most abundant membrane protein in chloroplasts, comprises the light-harvesting complex II and a reaction-centre core. The reaction centre uses the solar energy collected by the light-harvesting complex II to withdraw electrons from water, releasing oxygen into the atmosphere. It thus generates an electrochemical potential, providing the energy for carbon dioxide fixation and the synthesis of organic molecules, which make up the bulk of the biosphere. The structure of the light-harvesting complex II has been determined at 3.4-Å resolution by electron crystallography, but the high-resolution structure of the photosystem II reaction centre and other core components remained unknown. We have grown well-ordered two-dimensional crystals of a sub-core complex containing the reaction centre from spinach thylakoid membranes and used electron crystallography to obtain a projection map of its structure at 8-Å resolution. The features reveal the likely location of the key components that are active in electron transport, and suggest a structural homology and evolutionary links, not only with the purple bacterial reaction centre but also with the reaction centre of photosystem I.

The Metabolome of Chlamydomonas reinhardtii following Induction of Anaerobic H2 Production by Sulfur Depletion

The metabolome of the model species Chlamydomonas reinhardtii has been analyzed during 120 h of sulfur depletion to induce anaerobic hydrogen (H2) production, using NMR spectroscopy, gas chromatography coupled to mass spectrometry, and TLC. The results indicate that these unicellular green algae consume freshly supplied acetate in the medium to accumulate energy reserves during the first 24 h of sulfur depletion. In addition to the previously reported accumulation of starch, large amounts of triacylglycerides were deposited in the cells. During the early 24- to 72-h time period fermentative energy metabolism lowered the pH, H2 was produced, and amino acid levels generally increased. In the final phase from 72 to 120 h, metabolism slowed down leading to a stabilization of pH, even though some starch and most triacylglycerides remained. We conclude that H2 production does not slow down due to depletion of energy reserves but rather due to loss of essential functions resulting from sulfur depletion or due to a build-up of the toxic fermentative products formate and ethanol.

Crystal structure of a central stalk subunit C and reversible association/dissociation of vacuole-type ATPase.

The vacuole-type ATPases (V-ATPases) exist in various intracellular compartments of eukaryotic cells to regulate physiological processes by controlling the acidic environment. The crystal structure of the subunit C of Thermus thermophilus V-ATPase, homologous to eukaryotic subunit d of V-ATPases, has been determined at 1.95-Å resolution and located into the holoenzyme complex structure obtained by single particle analysis as suggested by the results of subunit cross-linking experiments. The result shows that V-ATPase is substantially longer than the related F-type ATPase, due to the insertion of subunit C between the V1 (soluble) and the Vo (membrane bound) domains. Subunit C, attached to the Vo domain, seems to have a socket like function in attaching the central-stalk subunits of the V1 domain. This architecture seems essential for the reversible association/dissociation of the V1 and the Vo domains, unique for V-ATPase activity regulation.