Ethanol production by homoacetogenic Clostridia

Clostridium ljungdahlii is a homoacetogenic organism that is able to grow autotrophically on mixtures of H₂ and CO₂ or CO. These mixtures are called synthesis gas in the chemical industry. They are easily obtained by gasification of carbonaceous materials, such as coal, petroleum, biofuel, or biomass by converting the raw material at high temperatures with a controlled amount of oxygen and steam. As C. ljungdahlii produces large amounts of ethanol beside the acetate, it is possible to use this organism to convert synthesis gas to the biofuel ethanol with a high rate. This has already been demonstrated in various industrial pilot plants.

In order to optimize C. ljungdahlii for this very promising process it is essential to understand its physiology and to establish a molecular biology for this organism. Our lab was involved in the determination of the genome sequence of C. ljungdahlii and has developed a DNA microarray in order to analyze gene expression as a response to different growth conditions. Using this array we analyzed the growth of C. ljungdahlii on carbohydrates, various substrates and under autotrophic conditions. It is interesting to note that C. ljungdahlii, is able to grow by oxidation and reduction of pairs of substrates such as ethanol and betaine resulting in a complicated ethanol metabolism including synthesis and consumption, depending on the growth conditions. For a more detailed analysis of gene function in this organism we plan to analyze the transcriptional response and the phenotype of mutations. We will particularly focus on the energy metabolism as it is still poorly understood in autotrophic homoacetogens and might be the key to understand the ethanol production. Therefore we currently try to establish a molecular microbiology and methods for gene knock-out in C. ljungdahlii using the Clostron procedure and a markerless gene knock-out strategy.