Biodiesel from algae: challenges and prospects

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Microalgae offer great potential for exploitation, including the production of biodiesel, but the process is still some way from being carbon neutral or commercially viable. Part of the problem is that there is little established background knowledge in the area. We should look both to achieve incremental steps and to increase our fundamental understanding of algae to identify potential paradigm shifts. In doing this, integration of biology and engineering will be essential. In this review we present an overview of a potential algal biofuel pipeline, and focus on recent work that tackles optimization of algal biomass production and the content of fuel molecules within the algal cell.

Introduction

With the need to reduce carbon emissions, and the dwindling reserves of crude oil, liquid fuels derived from plant material – biofuels – are an attractive source of energy. Moreover, in comparison with other forms of renewable energy such as wind, tidal, and solar, liquid biofuels allow solar energy to be stored, and also to be used directly in existing engines and transport infrastructure. Currently, bio-ethanol from, for example, corn starch, sugar cane or sugar beet, and biodiesel from oil crops such as palm and oilseed rape, are the most widely available forms of biofuel. However, there are two major issues over the sustainability of these first generation biofuels [1••, 2]. Firstly, to provide a significant proportion of transport fuel, the growth of these crops would compete for arable land with food crops. In 2008, the UK used an estimated 47 billion liters of transport fuel, 53% of which was diesel [3]. If this were met using biodiesel from oilseed rape, it would require 17.5 Mha (Table 1), more than half the land area of the UK. Secondly, the overall savings in energy and greenhouse gas emissions over the lifecycle of the biofuel may be less than anticipated; for example for biodiesel from oilseed rape [2] and soya [1••] the input of energy required over the life-cycle is ∼50% of the energy contained in the fuel.

Research into next-generation biofuels, such as ethanol from lignocellulose, offers the prospect of dealing with some of these concerns. In the past 2–3 years the production of biodiesel from algae has been an area of considerable interest [4, 5••]. This is because: (1) algae have higher productivities than land plants, with some species having doubling times of a few hours; (2) some species can accumulate very large amounts of triacylglycerides (TAGs), the major feedstock for biodiesel production (Box 1); and (3) high quality agricultural land is not required to grow the biomass. However, several challenges need to be tackled to allow commercial production of diesel from algae at a scale sufficient to make a significant contribution to our transport energy needs. In this review, we consider recent strategies devised to tackle some of these challenges, in particular the optimization of algal biomass production and TAG content.

Section snippets

Algal biofuel pipeline

Figure 1 shows a strategy for the production of algal biodiesel. At each stage, there are many factors to be considered and optimized, including energy and material inputs (e.g. nutrients, and energy for mixing during growth), and appropriate treatment of waste products, such as spent media and residual biomass. The major features in the pipeline are as follows.

Overall energy balance for algal biodiesel production

Life-cycle analysis (LCA; Box 1) is an essential element in designing an algal biofuel pipeline, since it quantifies, systematically, the environmental burdens at every stage of production, from growth of the biomass through to final use of the fuel. Of particular importance are the usages of fossil fuel in production and the concomitant releases of fossil-derived CO2. Energy inputs, such as the embodied energy in materials of plant construction and nutrients used, the electricity supplied from

Optimization of algal growth – the importance of light

Maximal production of algal biomass is essential to ensure the best possible outcome for the energy balance calculations. Although the biomass yield of many algal species is very high, it is important to note that this is constrained by the laws of thermodynamics [21]. Eight photons of photosynthetically active radiation (PAR) (∼48% of the incident solar flux) are required to fix one molecule of CO2 into carbohydrate, resulting in a maximum photosynthetic efficiency (not including respiration)

Maximizing the TAG content in algae

The yield of biodiesel from algae depends not just on the concentration of biomass achieved, but also on the oil content of the individual cells. Figure 3 provides a schematic of the biochemical pathways concerned. In general, productivity and lipid content are inversely correlated, and stress conditions such as deprivation of N or phosphate, which limit cell growth, also increase lipid content [12•, 32]. For example, while the lipid content of C. vulgaris grown under nutrient-sufficient

Conclusion and future prospects

We are still some way from realising the undoubted potential offered by algal biodiesel. Life-cycle analyses suggest that – using current methodologies – the process is marginal in terms of positive energy balance and global warming potential. Prospective schemes for the scale-up of algal production need to be informed by careful process modeling and LCA from the design stage. Without careful assessment of the energy balances and environmental impacts, there is a danger that many proposed

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We are grateful to many colleagues in the Bioenergy Initiative in Cambridge for stimulating discussions that have helped formulate our ideas. We apologize to colleagues whose work we could not cite owing to space limitations. Funding in our laboratories comes from the UK Biotechnology and Biological Sciences Research Council, Engineering and Physical Sciences Research Council, Natural Environment Research Council, and the Leverhulme Trust.

References (45)

  • J.E.W. Polle et al.

    Absence of lutein, violaxanthin and neoxanthin affects the functional chlorophyll antenna size of Photosystem-II but not that of Photosystem-I in the green alga Chlamydomonas reinhardtii

    Plant Cell Physiol

    (2001)
  • S.D. Tetali et al.

    Development of the light-harvesting chlorophyll antenna in the green alga Chlamydomonas reinhardtii is regulated by the novel Tla1 gene

    Planta

    (2007)
  • L. Wobbe et al.

    Cysteine modification of a specific repressor protein controls the translational status of nucleus-encoded LHCII mRNAs in Chlamydomonas

    Proc Natl Acad Sci U S A

    (2009)
  • K. Dehesh et al.

    Production of high levels of 8:0 and 10:0 fatty acids in transgenic canola by overexpression of ChFatB2, a thioesterase cDNA from Cuphea hookeriana

    Plant J

    (1996)
  • C. Jako et al.

    Seed-specific over-expression of an Arabidopsis cDNA encoding a diacylglycerol acyl-transferase enhances seed oil content and seed weight

    Plant Physiol

    (2001)
  • Z.T. Wang et al.

    Algal lipid bodies: stress induction, purification, and biochemical characterization in wild-type and starch-less Chlamydomonas reinhardtii

    Eukaryot Cell

    (2009)
  • P. Uphama et al.

    Substitutable biodiesel feedstocks for the UK: a review of sustainability issues with reference to the UK RTFO

    J Clean Prod

    (2009)
  • J. Hill et al.

    Environmental economic and energetic costs and benefits of biodiesel and ethanol biofuels

    Proc Natl Acad Sci U S A

    (2006)
  • A.L. Stephenson et al.

    Improving the sustainability of the production of biodiesel from oilseed rape in the UK

    Chem Eng Res Des

    (2008)
  • X.L. Miao et al.

    Biodiesel production from heterotrophic microalgal oil

    Bioresour Technol

    (2006)
  • J. Sheehan et al.

    US Department of Energy's Office of Fuels Development, July 1998. A look back at the US Department of Energy's Aquatic Species Program – Biodiesel from Algae, Close Out Report TP-580-24190

    (1998)
  • P. Metzger et al.

    Botryococcus braunii: a rich source for hydrocarbons and related ether lipids

    Appl Microbiol Biotechnol

    (2005)

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