ESBCO2 · Electrosynthesis of biofuels from gaseous carbon dioxide catalyzed by microbes: A novel approach/quest of microbe-electrode interactions

Electrosynthesis of biofuels from gaseous carbon dioxide catalyzed by microbes: A novel approach/quest of microbe-electrode interactions

For a strong European Research Area (ERA) it is necessary to develop bioenergy technologies dealing with pollution control. The European energy policy reform aims to make Europe the world leader in renewable energy and low-carbon technologies. The EU targets that 20% of EU energy consumption come from renewable resources. In addition Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewables aims to ensure that as we expand the use of biofuels in the EU only sustainable biofuels are used. These generate a clear and net greenhouse gas (GHG) saving and have no negative impact on biodiversity and land use. Therefore, there is a demand in Europe to improve the knowledge in energy technology dealing with pollution control. To accomplish this goal, this research proposal was designed as such that it contains both fundamental and applied aspects. The critical Research and Development problems relate to the mechanisms involved and the enhancement of electron exchange for the biofuel production. This will provide a tool for the better understanding of the mechanism involved and thus lead to a development of biosynthesis cell utilizing gaseous CO2. Furthermore, this will allow ERA to stay competitive with the rest of the world, which is investing massively in this area. The key aspects of this proposal addressed by means of novel and inter-disciplinary research training are mentioned below. More specific, biosynthesis and applied electron exchange has become a very timely, relevant and strategic field of bioscience and technology all over the world. The enhancement of competence and skill diversification of the beneficiary, the present proposal’s aim, had been progressively achieved through three main steps: - Establishment of complementary interdisciplinary expertise at an advanced level in the field of bioenergy and electron exchange; - Better understanding of mechanisms involved in electron transfer; - Electron exchange and fuel production aiming at practical applications. In the first step, Dr. Kumar had been introduced to the detailed aspects of the underlying engineering and the scientific challenges related to microbial electrosynthesis, such as electron transfer, and electroactive biofilms. In the second step, Dr. Kumar learned various modes, which are now-a-days considered to be the state-of-the-art methodologies to retrieve the intrinsic redox-active proteins, electron transfer, which is of crucial interest for understanding the involved mechanisms. In details, he investigated how an engineered strain exchanges electron. Electrotrophy characterizes a metabolic process of microorganisms that uses electrode routed electrons for the production of a diversity of organic commodities and fuels. Of all available, genetically tractable, pure cultures, Geobacter sulfurreducens is the most effective in direct microbe-electrode electron exchange. A previous limitation in employing G. sulfurreducens as an electrotrophy catalyst was its requirement for an organic carbon source during autotrophic growth. Metabolic modeling suggested that the requirement for an organic carbon source could be attributed to the lack of an ATP-dependent citrate lyase in the G. sulfurreducens genome. Expression of type II citrate lyase genes in G. sulfurreducens yielded a strain that could grow with hydrogen gas as the sole electron donor and carbon dioxide as the sole carbon source. Autotrophic growth was initially slow, but with continued transfer and adaptation, growth rates increased five orders of magnitude.The adapted autotrophic strain readily produced electrical current of 1.08 mA (166 mA/m2 of electrode surface) in microbial fuel cells with hydrogen gas as the sole electron donor and no organic carbon source. Furthermore, the autotrophic strain could consume current from a negatively poised cathode (-900 mV vs Ag/AgCl) with long-term maintenance of the cathode biofilm in the absence of an organic carbon source for biosynthesis. These results demonstrate that genetically tractable Geobacter species can function as electrotrophy catalysts under autotrophic conditions. During the second year of the project, Dr. Kumar devoted his efforts to the design and modelling for a better understanding of electron flow in biofilms for bioenergy applications at MIT. He examined charge transfer by developing a model. This value of diffusion coefficient (6.0 *10-6 cm2 s-1) is greater than that observed in traditional thin-film voltammetric studies. This study suggests that increased porosity improves electro-neutrality upon electrolysis. Furthermore, he choose Moorella thermoacetica to study growth by varying medium compositions. Preliminary results suggest that growth medium supplements have an impact on M. thermoacetica growth. As a result of this novel research project, Dr. Kumar gained a solid understanding related to electron transfer issues and modeling for bioenegry applications. In addition, he acquiring the necessary familiarity with this and related aspects as: research, management and time management skills, as well as negotiation strategies. Concerning data analysis methodologies, Dr. Kumar acquired the necessary knowledge of advanced methodologies utilized for electron exchange, fuel synthesis, electroactive biofilms, and microbial community analysis of the electroactive biofilms, which is now-a-days becoming a state-of-the-art methodology. In terms of scientific and technologically relevant achievement, the main results are: - G. sulfurreducens is capable of growing autotrophically. Potential-control induced biofilm growth of a modified G. sulfurreducens strain on graphite electrodes produced films capable of sustaining steady-state catalytic currents with hydrogen (166 mA/m2) fed cells. - Engineered G. sulfurreducens could readily accept electrons from graphite electrodes and reduce fumarate and cathode biofilms were ~24 µm thick which previously was not possible. Furthermore, current study shows succinate production rate of ~2 and 13 times higher than Gregory et al. (2004) and Strycharz et al. (2008), respectively. - This finding demonstrates an autotrophic G. sulfurreducens capable of producing sustained cathode biofilm with higher turnover rates for the first time. However, for better understanding transcriptomic studies will need to be done on shorter term biofilms. Furthermore, this work greatly expands the microbial toolbox available for designing microbial electrosynthesis strategies. - The microbe-electrode electron exchange has great potential and important applications for the production of biofuels and biochemicals.

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