Prolonged Direct Harvesting of Photosynthetic Electrons from Living Algal Cells
Authors: Hong Hyeonaug , Kim Yongjae , Han Myungjin , Yoo Gu , Song Hyun Woo , Chae Youngcheol , Pyun Jae-Chul , Grossman Arthur Robert , Ryu WonHyoung 
Published: February 2020

Plant cells produce photosynthetic electrons (PEs) by splitting water during photosynthesis. In the beginning of photosynthesis, antenna complex embedded in thylakoid membrane absorbs photon energy and converts them into excited electrons with high quantum efficiency. These excited electrons, known as PEs, are transferred through electron acceptors in order to grow cell or store surplus energy as a form of carbonhydrates. However, since the energy state of the PEs is lowered once they are stored as organic matter, many researchers have tried to extract PEs from photosynthetic electron transfer chain before their conversion into organic matter. Such previous works include collection of PEs by electrochemical oxidizing through mediator as an electron interceptor and use of electrical linker materials such as CNTs, electrically-conducting polymer matrix, or conducting linker to working electrodes between photosynthetic components and the surface of electrodes. Although these works demonstrated the feasibility of harvesting PEs in large scale, there are still issues including efficiency decrease from mediator use and limited stability or requirement of additional electron donors of isolated photosynthetic extracts. In our previous study, we demonstrated direct extraction of PEs from living algal cell, chlamydomonas reinhardtii, by inserting nanoelectrode (NE) into cell membrane. However, in spite of highly efficient extraction of PEs, the maximum harvesting period of PEs was shorter than 1 hour. In this study, we developed a cantilever type of NE system in order to find the optimal shape and size of NE for extraction of PEs for a longer period of time. Fabrication of cantilever NE system started by milling the end of commercial AFM cantilever tip by focused ion beam (FIB). Then, Au was deposited on the cantilever by sputtering as a working electrode. Si3N4 was coated on top of the Au-coated NE to minimize noise signals. Then, additional FIB milling was performed to expose the Au layer at the tip of the NE for localized PE signal measurement. With this cantilever NE, we firstly investigated a relationship between NE size and the viability of inserted cells. It was confirmed that algal cells remained stable for up to 7 days when NE diameters smaller than 500 nm were used. Through continuous observation of inserted algal cells, their viability was ascertained by cell proliferation. After high stability of NE inserted cells was confirmed, direct extraction of PEs was attempted without any mediator. Light-triggered currents from the cells inserted by NEs were observed and the photosynthetic origin of the currents was confirmed by application of DCMU and the subsequent disappearance of the light-responsive currents. Since exposure to highly intense and continuous illumination can damage the pigments of the photosystems, the effect of illumination condition on the PE extraction was investigated. Up to the light intensity of 132 μmol photons m-2s-1, no photobleaching was observed from the NE-inserted cells and the stronger light intensity produced the higher photosynthetic currents. When NE-inserted cells were incubated in a dark except during illumination for photosynthetic current harvesting, about 150 fA of photosynthetic currents were measured over 7 days with 9 μmol photons m-2s-1 of light intensity at 400 mV against Ag/AgCl electrode. As another comparison, when NE-inserted cells were exposed to continuous illumination with 16.5 μmol photons m-2s-1 for 60 hours, more than twice amount of PEs was measured compared to dark incubated cells. Finally, NE-inserted cells were exposed to continuous illumination and th e photosynthetic currents were monitored simultaneously. It was shown that PEs were extracted steadily during several hours of illumination. These results indicated that PEs could be extracted for an extended period of time under various illumination conditions. In this work, the maximum photosynthetic currents measured were 2.5 pA after continuous illumination on a single NE-inserted cell.