Robert Ludwig

Professor Emeritus of MCD Biology
B.S., University of Michigan
Ph.D., Yale University
Postdoctorate, Massachusetts Institutes of Technology

Renewable bioenergy: hydrogen production by direct photoconversion

Biocatalytic processes designed to yield renewable biofuels will prove increasingly important in the production of renewable energy. Among such biofuels, hydrogen offers unique advantages as its global use mitigates both increased greenhouse gas emissions and terrestrial warming. However, production of all renewable biofuels confronts an inescapable thermodynamic paradox: their synthesis itself requires energy. Problematically, on a global scale, where is that considerable energy to be had? An obvious, renewable energy resource is solar radiation. Indeed, photosynthesis, the nexus of biocatalytic processes of living organisms, is inextricable from life itself on earth. While associated with contemporary higher plants and algae, photosynthesis as a biocatalytic process evolved in and was perfected by ancestral phototrophic bacteria in the ancient, reducing biosphere. Also prevalent in this early microbial world were anaerobic bacteria capable of efficient production and use of hydrogen. Yet as biocatalytic processes of phototrophic bacteria, concurrent photosynthesis and hydrogen production were disadvantageous, lest solar energy be converted and stored as a gaseous product, hydrogen, able to readily dissipate and the energy required for its synthesis be lost. The primary objective of experiments ongoing in my laboratory is to accomplish what Nature scrupulously avoids: to genetically engineer phototrophic bacteria able to convert light energy and output hydrogen as a directly coupled biocatalytic process.

Nitrogen Recycling during Photorespiration

‘Plastidic’ glutamine synthetase activity is critical to photorespiration, the light dependent uptake of O2 and release of CO2, a process common to all oxygenic photosynthetic organsisms. As it acts contrary to photosynthesis, photorespiration has remained conceptually problematical since its discovery some fifty years ago. During photorespiration, glycine is combusted in leaf mitochondria at high rates yielding free ammonium which must be reassimilated to allow glycine resynthesis. We are also studying whether glutamine synthetase activity, which mediates the first step in ammonium assimilation, need occur in leaf mitochondria and/or chloroplasts by genetically modifying the GLN2 gene to yield variants targeted to a specific organelle and assessing the consequences for photorespiration.

Molecular Biology of Plant-Microbe Interactions

The symbiotic legume root nodule with Rhizobium bacteria as endosymbionts offers a wealth of problems to be analyzed at the molecular level, including those of symbiosis itself. Because nodulated legume plants grow in very nitrogen-stressed environments, this symbiosis is of great agronomic importance. As the symbiotic nodule develops, there follows an elaborate morphogenesis in which rhizobia finally persist as differentiated, specialized, nitrogen-fixing organelles inside plant cells. Our research seeks to understand the molecular regulation of Rhizobium growth and differentiation during symbiosis. We are attempting to reconstruct this process using genetics, biochemistry, physiology, and recombinant DNA technology. With improved understanding of symbiotic nitrogen fixation, we hope to maximize both the yields of the world's legume crops and the efficiencies of cultivation, and to minimize adverse consequences to soil ecosystems, with particular relevance to sustainable agriculture.

Selected Publications

Please follow this link to find these publications in the National Library of Medicine PubMed database.

Ng, G., Tom, C. G. S., Park, A. S., Zenad, L. and Ludwig, R. A. (2009) “A novel endo-hydrogenase activity recycles hydrogen produced by aerobic dinitrogen fixation”, PLoS ONE 4(3): e4695. doi:10.1371/journal.pone.0004695.

Taira, M., U. Valtersson, B. Burkhardt, and R.A. Ludwig (2004) “Arabidopsis thaliana GLN2-encoded glutamine synthetase is dual-targetted to leaf mitochondria and chloroplasts”, Plant Cell 16: 2048-2058.

Scott, J.D., and R.A. Ludwig (2004) “Azorhizobium caulinodans electron-transferring flavoprotein-N electrochemically couples pyruvate dehydrogenase complex activity to N2 fixation”, Microbiology 150: 117-126.

Ludwig, R.A. (2004) “Microaerophilic bacteria transduce energy via oxidative metabolic gearing”, Research In Microbiology 155: 61-70.

Pauling, D.C., Lapointe, J.P., Paris, C.M. and R.A. Ludwig. "Azorhizobium caulinodans pyruvate dehydrogenase activity is dispensable for aerobic but required for microaerobic growth." Microbiology 147: 2233-2245 (2001).

Kaminski, P.A, Kitts, C.L., Zimmerman, Z. and R.A. Ludwig. "Azorhizobium caulinodans Uses Both Cytbd (Quinol) and Cytcbb3 (Cytc) Terminal Oxidases for Symbiotic N2 Fixation." J. Bacteriology 178: 5989-5994 (1996).

Loroch, A.I., Nguyen, B. and R.A. Ludwig, "FixLJK and NtrBC signals interactively regulate Azorhizobium nifA transcription via overlapping promoters." J. Bacteriology 177: 7210-7221 (1995).

Kitts, C.L. and R.A. Ludwig, "Azorhizobium respires with at least four terminal oxidases." J. Bacteriology 176: 886-895 (1994).

Ludwig, R.A. "Arabidopsis chloroplasts dissimilate L-arginine and L-citrulline for use as N source." Plant Physiology 101: 429-434 (1993).

Kitts, C.L., Lapointe, J.P., Lam, V.T. and R.A. Ludwig, "Elucidation of the complete Azorhizobium nicotinate catabolism pathway." J. Bacteriology 174: 7791-7798 (1992).

Lazo, G.L., Stein, P.A., and R.A. Ludwig. "A transformation competent Arabidopsis genomic library in Agrobacterium." Bio/Technology 9: 963-971 (1991).