Soils are rich in organics, particularly those that support growth of plants. These organics are possible sources of sustainable energy, and a microbial fuel cell (MFC) system can potentially be used for this purpose. Here, we report the application of an MFC system to electricity generation in a rice paddy field. In our system, graphite felt electrodes were used; an anode was set in the rice rhizosphere, and a cathode was in the flooded water above the rhizosphere. It was observed that electricity generation (as high as 6 mW/m(2), normalized to the anode projection area) was sunlight dependent and exhibited circadian oscillation. Artificial shading of rice plants in the daytime inhibited the electricity generation. In the rhizosphere, rice roots penetrated the anode graphite felt where specific bacterial populations occurred. Supplementation to the anode region with acetate (one of the major root-exhausted organic compounds) enhanced the electricity generation in the dark. These results suggest that the paddy-field electricity-generation system was an ecological solar cell in which the plant photosynthesis was coupled to the microbial conversion of organics to electricity.

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Algae have contributed greatly to the creation of the Earth's environment and the development of human civilization. Currently, microalgae are considered to be among the most promising sources for biofuel. Most microalgae accumulate triacylglycerols; however, fatty acid methyl esters produced from triacylglycerols by transesterification have critical end-use issues. Hydrocarbons produced by Botryococcus and Aurantiochytrium are the most suitable algal oils for replacing existing transportation fuels and are highly compatible with existing petroleum infrastructure. Over the years, many technologies have been investigated for achieving sustainable biofuel production using oleaginous microalgae; however, existing techniques of algal fuel production are suitable mainly for small-scale procedures or for the recovery or removal of high-value products. This situation strongly influences life cycle assessment studies for algal fuel production, and published life cycle assessments show different and discrepant results-reliable data on inputs and outputs from industrial-scale experiments are needed for solving these problems. The estimated cost of algal fuel production is still high compared with that of fossil crude oils. The integration of water treatment and algal biomass production, in a coupled hybrid production system comprising of phototrophic and heterotrophic algae, has tremendous potential for improving the economy of future algal fuels.

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