ARTIFICIAL AND BIONIC LEAF: A SUSTAINABLE AND RENEWABLE CYCLE FOR PRODUCING FOOD AND FUELS

Professor Daniel G. Nocera, Harvard University

Hawkin-Carlson Room-RRL

Hybrid biological | inorganic (HBI) constructs have been created to use sunlight, air and water as the only starting materials to accomplish carbon and nitrogen fixation, thus enabling distributed and renewable fuels and crop production.  The carbon and nitrogen fixation cycles begin with the artificial leaf, which was invented to accomplish the solar fuels process of natural photosynthesis – the splitting of water to hydrogen and oxygen using sunlight – under ambient conditions. To create the artificial leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts permit water splitting to be accomplished using any water source—which is the critical development for: (1) the artificial leaf, as it allows for the facile interfacing of water splitting catalysis to materials such as silicon and (2) the bionic leaf, as it allows for the facile interfacing of water splitting catalysis to bioorganisms. For the latter, using the tools of synthetic biology, a bio-engineered bacterium has been developed to convert carbon dioxide from air, along with the hydrogen produced from the catalysts of the artificial leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The HBI, called the bionic leaf, operates at unprecedented solar-to-biomass (10.7%) and solar-to-liquid fuels (6.2%) yields, greatly exceeding the 1% yield of natural photosynthesis.  Extending this approach, a renewable and distributed synthesis of fertilizer at ambient conditions has been created by coupling solar-based water splitting to a nitrogen fixing bioorganism in a single reactor. Nitrogen is fixed to ammonia by using the hydrogen produced from water splitting to power a nitrogenase installed in the bioorganism. The ammonia produced by the nitrogenase can be diverted from biomass formation to extracellular production with the addition of an inhibitor. The nitrogen reduction reaction proceeds at high turnover per cell and operates without the need for a carbon feedstock (other than the CO2 provided from air). This nitrogen fixing HBI can be powered by distributed renewable electricity, enabling carbon negative and sustainable crop production.  The science that will be presented will show that using only sunlight, air and water, a distributed system may be established to produce fuel (carbon neutral) and food (carbon negative). Such science is particularly useful to the poor of the world, where large infrastructures for fuel and food production are not tenable.