ELECTRON PARTITIONING IN CONTINUOUS RHODOBACTER SPHAEROIDES CULTURES

Abstract

Today?s global energy requirements are mostly dependent on fossil fuels, the utilization of which are causing global climate change as well as leading to the foreseeable depletion of these limited non-renewable energy sources (Jean-Baptiste, 2003). As a result of growing concern over environmental impacts and depletion of existing fossil fuels, the interest in developing renewable, sustainable ways of energy generation has greatly increased. Among the different renewable energy types, the so-called biofuels are one of the most promising and studied forms of energy generation. There is a growing trend towards employing modern technologies and efficient bioenergy conversion using a range of biofuels, which are becoming cost-wise competitive with fossil fuels (Demirbas, 2007). Some of these biologically based fuels which are considered or developed as renewable replacements for fossil fuels are ethanol, various next generation liquid transportation additives, or gases such as hydrogen (H2) or methane (CH4). The reason for the selection of hydrogen as the model biofuel is that the amount of energy produced during hydrogen combustion is higher than that released by any other fuel on a mass basis, with a low heating value (LHV) 2.4, 2.8 and 4 times higher than that of methane, gasoline and coal, respectively. Furthermore, hydrogen has a major advantage as a fuel because of the absence of CO2 emissions, as well as other pollutant emissions (thermal NOx) if it is employed in low temperature fuel cells. This is especially important for the transportation sector, which is responsible for 18% consumption of primary energy worldwide (Marban, 2007). Hydrogen can be produced by a number of processes, including electrolysis of water, thermocatalytic reformation of hydrogen-rich organic compounds, and biological processes. Currently hydrogen is produced, almost exclusively, by electrolysis of water or by steam reformation of methane, which involves fossil fuel consumption for hydrogen production, thereby minimizing the energy benefits of this technology. Biological production of hydrogen (bio-hydrogen), using microorganisms, is an exciting 2 area of technology development that offers potential production of usable hydrogen from a variety of renewable resources. Biological systems provide a wide range of approaches to generate hydrogen, and include direct biophotolysis, indirect biophotolysis, photo-fermentations, and dark fermentations (Levin, 2004). Among these systems, photo-fermentation is especially advantageous because photosynthetic bacteria can provide significant biologically-derived hydrogen as several species are able to generate large amounts of this fuel using energy from sunlight and renewable nutrients. More specifically, photoheterotrophic bacteria, which have the ability to simultaneously use energy derived from light and organic compounds to drive H2 production, can be used to develop solar-driven biofuel production processes for the utilization of renewable organic substrates in a biofuel economy (Kapdan, 2006). The objective of this work was to study the electron partitioning of the photosynthetic purple non-sulfur bacterium, Rhodobacter sphaeroides, under different environmental conditions (i.e., different growth rates). Quantifying the contribution of networks that impact solar powered hydrogen production can then be used for the development of a genome-scale metabolic network model of R. sphaeroides (Saheed et al, submitted). Previous research investigated the electron partitioning in batch cultures of R. sphaeroides (Yilmaz et al., 2010). However, for the purpose of metabolic model development, quantitative analysis of electron partitioning is needed on continuous steady-state R. sphaeroides cultures. The establishment and design of the metabolic model will increase the feasibility of using bio-hydrogen as an alternative fuel and will provide insights for practical applications in the future.