|Ph.D Student||Larom Shirley|
|Subject||Harnessing Photosynthesis for "Green" Energy Production:|
Building a Photosystem II-Based Electrochemical
|Department||Department of Biology||Supervisor||Professor Gadi Schuster|
|Full Thesis text|
Dwindling fuel resources and escalating ecological consequences, pose a major challenge: developing technologies to exploit alternative energy sources. The sun is an inexhaustible source of free and clean energy. Therefore, establishing a new technology to use solar energy extensively will provide a long term solution to fossil fuel dependency. Plants, algae and certain bacteria use sunlight to produce chemical energy and reducing power in a process called photosynthesis. In this natural solar energy conversion process, membrane-bound photosystems are utilized to catalyse light-induced electron flow. The purpose of our research is to build a photosynthesis-based photovoltaic cell that converts light into electron flow using water as a feedstock. The bio-hybrid system is designed to contain engineered photosynthetic apparatus interfacing a solid-state device and to produce both electricity and hydrogen gas.
The photosynthetic active, membrane-confined complexes, called photosystem II (PSII), constitute the solar energy capturing and converting site and facilitate electron transfer through multiple acceptor molecules in insulated surroundings. Thus, in order to integrate PSII in a photovoltaic cell, the inaccessibility of the membrane-entrapped electrons needs to be overcome.
In this work we show that specific modification of PSII-D1 protein in Synechocystis PCC 6803, changing lysine at position 238 to glutamic acid (Glu), resulted in the introduction of an alternative electron route to an external electron acceptor. The Glu strain, which grows photoautotrophically at nearly native rates, performs light dependent reduction of the soluble electron carrier cytochrome c. In addition, the presence of cytochrome c has a beneficial effect on the stability of the photosynthetic material as it inhibits photo-inactivation of the D1 protein in vitro. Integrating photosynthetic membranes, isolated from the Glu strain, with a gold electrode, resulted in direct electron transfer and photocurrent generation. The light-induced current, generated by the Glu membranes, was higher than the membranes isolated from the control strain in the presence of herbicide. However, although cytochrome c can efficiently be reduced by the Glu membranes, it acts as an inefficient mediator between the Glu membranes and the gold electrode and hinders the photocurrent generation. We also show that interfacing the Glu membranes with a charged gold surface can also provide protection against photoinhibition. Therefore, we have engineered a new electron transfer pathway in which electrons can be withdrawn by an electron acceptor without harming the normal function of PSII. Implementing the engineered PSII carrying the new pathway in a photocell, enables photo- current generation.