From systems biology to fuel—Chlamydomonas reinhardtii as a model for a systems biology approach to improve biohydrogen production

Abstract

With economic wealth the need for energy is rising. Hence we are facing two problems: to satisfy the increasing energy demand and concomitantly deliver emission-free energy to avoid global warming. The process of photosynthesis offers a natural and highly efficient method to produce emission-neutral biofuels. However, using higher plants for such purposes causes several problems which are difficult to overcome and includes competition with food producing agriculture in terms of arable land, the need for fresh water, low process efficiency and the application of energy-intensive fertilizer in order to enhance growth performance. Photosynthetic microorganisms and, in particular, microalgae offer an alternative approach. In this case production sites in photo-bioreactors can be located on cheap, rural land and the organisms can be cultured in sea water rather than fresh water. However microorganisms are not naturally adapted as efficient producers of biofuels. Due to the complex regulatory network and mutual interaction of physiological processes and organelles, identifying the optimal production strategy is impossible without a greater understanding of the complex interplay of all cellular processes. Systems biology has emerged recently as a discipline to gain an understanding of these networks and their translation into a mathematical in silico model. An in silico model allows simulating optimization steps and, therefore, provides a useful method to identify targets for directed genetic/physiological modification to optimize the system for a biotechnological approach.

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.