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NIRE Annual Report
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1998

Conversion of Solar Energy and Organic Substrates to
Electricity using Photosynthetic Microorganisms

Biomass Division
Global Warming Control Department
Objectives
Photosynthesis in plants and photosynthetic microorganisms is a sophisticated conversion device of light to electricity and also an environmental purification system for waste water and waste gas.
This report presents a study on a conversion of light energy and organic substrates to electricity in an electrochemical cells using cyanobacteria and 2-hydroxy-1,4-naphthoquinone (HNQ), in which electrons generated from oxidation of water by photosynthesis and oxidative degradation of substrates by respiration were trapped and were transferred to electrode by HNQ as shown in Fig. 1.
Results
1) Conversion efficiency Under optimized solution conditions in an electrochemical cells using a cyanobacterium, Synechococcus sp. UTEX2380, the energy conversion efficiency was 3.3%. The energy loss arose remarkably from the reaction in the electron transfer chain and the high internal resistance of the electrochemical cell.
2) Repeated discharge (dark) / culture (light) cycles Fig.2 shows current vs time curves of an electrochemical cell running under repeated discharge/culture cycles, under which Anabaena variabilis M-3 was removed from the electrochemical cells at regular intervals and cultured in the light with CO2. Although the duration time of the current outputs was about 12 hours under continuous running, it was extended by at least 10 times by the repeated discharge/culture cycles.
3) Effect of glucose on the current outputs A part of cyanobacteria such as Synechocystis sp. M-203 and PCC6714 have a glucose transport system and can grow chemoheterotrophicallyand photoheterotrophically in the presence of a inhibitor. When such cyanobacteria was applied to an electrochemical cells, the current outputs increased by the addition of glucose both in the light and the dark.

Selected Publications
1) Behavior of glucose degradation in Synechocystis sp. M-203 in bioelectrochemical fuel cells. Bioelectrochem. Bioenerg., 43, 177-180, 1997.
2) Effects of intensity of incident light and concentrations of Synechococcus sp. and 2-hydroxy-1,4-naphthoquinone on the current output of photosynthetic electrochemical cell. Solar Energy, 61, 347-353, 1997.

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