From systems biology to fuel—Chlamydomonas reinhardtii as a model for a systems biology approach to improve biohydrogen production
Introduction
There is an urgent need for the development of sustainable energy sources as an alternative to fossil fuels. Our high demand and consumption of fossil fuels causes two problems. Firstly, by burning fossil fuels the carbon dioxide generated has been recognized as the major contributor of global warming causing severe environmental and economic damage. In the recently published Stern review a limited time of 10–20 years to react on climate change was given. With the CO2 levels passing the 400 ppm threshold we already experience the destruction of reefs, landscapes and the frequency of storms and flooding is increasing causing severe damage (Stern, 2006). Secondly, the resources of fossil fuels are limited and our current demand does not allow the use of fossil fuels at the same level and for the same price in the future. From the fossil fuels, oil is the most important and the one with the lowest reserves. Despite an ongoing debate and different opinions about the true reserves there cannot be any doubt that we will run out of oil this century (Asif and Muneer, 2007). Consequently, alternative fuels are required that fulfill the following criteria: (i) no carbon dioxide should be released (or zero net carbon), (ii) a sustainable resource, (iii) suitable as fuel for the transportation sector, (iv) provides a major part of the global energy demand, (v) should be in an affordable price range (comparable to the current oil price).
This is a difficult task to achieve for any of the established alternatives to fossil fuels. Most systems (e.g. nuclear power, wind power and photo voltaics) mainly produce electricity. Suitable large-scale storage and transportation devices for electricity do not exist (while it can be converted into H2 following electrolysis of water, this process has around 30% conversion losses). However, around 60% of our energy demand is required as petrol (mainly for the transportation sector). Other systems, as they exist today, are far from covering this demand because their production efficiency is too low (e.g. biomass to liquid (BtL), conventional biodiesel, bioethanol). Hence we currently do not have an available alternative that covers the main part of our energy requirements.
Nature has developed photosynthesis, a very efficient light harvesting and conversion system, which uses sunlight to synthesize chemical energy carriers such as carbohydrates, lipids and proteins. Over millions of years nature has collected the sun's energy and stored it as fossil fuels such as oil, coal and natural gas.
Besides the properties of photosynthetically produced fossil fuels, the potential of current photosynthetic systems as a provider of clean CO2-neutral fuels, so called biofuels, has been recognized (Powledge, 2008, Boswall, 2006, Lofstrom, 2005). Although photosynthesis is the underlying mechanism for both kinds of fuels, fossil fuels are a concentrated storage form of former biomass. However, in order to produce biofuels one has to deal with the low energy density of sunlight, the low energy conversion efficiency and lower energy content of biomass (biomass: ∼15 GJ/t vs. oil: ∼45 GJ/t) to compete with fossil fuels as an energy source.
For biofuels to be widely accepted in the energy market, research must focus on adapting and improving photosynthetic organisms for biofuel production to meet this target. In this regard it is important to obtain a fundamental understanding of the physiological processes in photosynthetic organisms which are responsible for growth and synthesis efficiencies—we need to understand the organism as a whole. A newly designed systems biology approach helps to gain insights into the regulation and networking of an entire organism and not only isolated pathways and networks. With this knowledge, researchers will have the ability to perform a new level of biotechnology: devoting the entire organism to the synthesis of biotechnologically relevant products and thereby be able to tailor an organism suitable to produce biofuels at efficiency levels to compete with fossil fuels.
The aim of this review is to demonstrate the potential of the new discipline of systems biology for biotechnology and its application to the promising ‘new’ organism for biofuel (biohydrogen) production, Chlamydomonas reinhardtii.
Section snippets
Systems biology
Systems biology is the newly emerging discipline in modern biology. Over the last few years, more than 30 new institutes focused on this area have been established worldwide, working on a range of organisms from Arabidopsis thaliana to zebrafish.
A biological organism can be described as a system, a closed organization of interacting distinguishable components. However, this system is not isolated as it is able to communicate and interact with the environment. Eukaryotic organisms contain
Biofuels
A biofuel is a solid, liquid or gaseous fuel derived from any biological carbon source including treated municipal and industrial wastes (for an informative review see (Yuan et al., 2008)). With the problems we are now facing in terms of global warming due to burning of fossil fuels and depleting resources there is a serious renaissance of interest in renewable energy from biological sources (Gavrilescu and Chisti, 2005). A strategy to develop high performing organisms has to consider the
Systems biology and biotechnological biofuels production
Systems biology is not only a new discipline in modern biology but a method with the potential to significantly improve an organism's production flow. In the past many approaches to improve the product flow of organisms failed or ended far behind expectations because the product synthesizing pathways were considered in an isolated way rather than in the context of the organism as a whole, taking into account regulatory aspects and the minimal energy requirement for cell maintenance and
Chlamydomonas reinhardtii and biofuels
In the early 1940s Hans Gaffron discovered the ability of the green alga Scenedesmus to produce H2 under anaerobiosis (Gaffron and Rubin, 1943). Some years later the same mechanism was described for the genus Chlamydomonas (Frenkel, 1952). Since then the ability to produce H2 has been detected and described in many green algae and other microorganisms. However, H2-production levels were measured without standardized procedures (Kessler, 1974, Boichenko and Hoffmann, 1994), so comparisons of the
Conclusion
C. reinhardtii unifies several properties which make this green alga an ideal candidate for a systems biology based approach for the optimization of a biofuel producing organism and can serve as a proof of concept.
The fact that it is easy to handle and fast growing in combination with resources such as genetic knowledge and mutant collections provides the basis for this choice. Furthermore, its close relation to physiological processes in higher plants but also to other green algae (a large
Acknowledgements
The work is funded by the German Federal Ministry of Education and Research (BMBF). The author would like to thank Patrick May for the information on databases as well as Barbara Gaertner-Rupprecht and Kate Howell for critical proofreading. The work is supported by the GoFORSYS initiative as well as the Solar Biofuels Consortium.
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