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Photosynthetic microorganisms, such as micro-algae and cyanobacteria are able to harness low-intensity solar energy and store it as latent chemical energy in the biomass. This energy can then be released via biochemical conversion. The structural and storage carbohydrates in biomass have low energy content and it is necessary to concentrate the energy content further for fuel application. Anaerobic microbial fermentation is an efficient and widely used method for such conversion process. Useful renewable fuels produced by microorganisms include hydrocarbon, ethanol, methane and hydrogen. Biofuel cells which can release energy in fuel chemicals to generate electrical energy at ambient temperature have been developed.

Fuel produced by microbes has the potential for helping to meet world energy demands. Living organisms assimilate and concentrate energy in their biomass and products. Hence, biomass in its various forms is an attractive alternative source of energy (Lee, 2003). Through photosynthesis, biomass collects and stores low-intensity solar energy. This can then be harvested and released via biochemical conversion. Biological process are involved in both the harnessing of solar energy and upgrading of low energy feed stocks to biomass fuels.

Photo-biological hydrogen production: Chloroplast of some photosynthetic microorganisms such as the green alga chlorella in the presence of suitable electron acceptors is capable of producing H2 and O2 through direct photolysis of water. In the system, the substrate (electron donor) is water, sunlight as the energy source is unlimited, and the product (hydrogen) can be stored and is non-polluting. Moreover, the process is renewable, because when the energy is consumed, the substrate (water) is regenerated.

Conversion of biomass energy: Structural and storage carbohydrates in biomass having low energy content cannot be used as fuel directly. It is necessary to concentrate the energy content further for fuel applications. The use of microorganisms to produce commercially valuable fuels depends on getting the right microorganisms which can produce the desired fuel efficiently. The quantum of substrate they require for fermentation should be low and inexpensive (Tanaka et al., 1988). It is imperative that the production of synthetic fuels does not consume more of natural fuels than what they produce. Anaerobic microbial fermentation is an efficient and widely used route for such conversion processes.

a) Alcohol (ethanol) Production: The microbial production of ethanol has become an important source of a valuable fuel, particularly in regions of the world that have abundant supplies of plant residues. Fermentation production of fuel alcohol can be through microbial conversion of low cost agricultural substrates high in starch and sugar content. Numerous microorganisms are capable of producing ethanol, but not all are suitable for industrial processes. Yeast cultures, particularly saccharomyces, have been most extensively examined because they are very efficient in converting sugars into ethanol, i.e. cost competitive and are not as strongly inhibited by high ethanol concentrations as are other microbes.

The following equation illustrates the basic biochemical mechanism by which the ethanol is produced through the fermentation process.

The yeasts commonly used in industrial alcohol production include Saccharomyces cerevisiae (ferment glucose, fructose, maltose and maltoriose), S.uvarum, S.diataticus etc. The ethanol productivity ranges between 1 and 2 g ethanol/h/g cells.

Selected bacterial cultures were examined for use in ethanol production processes because of their higher temperature tolerance. However, their yield of ethanol was not as high as in yeast fermentation (Lee, 2003). Recently, the bacterium Zymomonas mobilis has been selected to achieve a high productivity of 2.5-3.8 g ethanol/h/g cells.

b) Methane production: Methane (CH4) is an energy-rich fuel that can be used for the generation of mechanical, electrical and heat energy. Large amounts of methane can be produced by anaerobic decomposition of waste materials. Efficient generation of methane can be achieved using algal biomass. In microbial production of methane, naturally occurring mixed anaerobic bacteria population is always used and cells are retained within the digester. During the fermentation process, a large amount of organic matter is degraded, with a low yield of microbial cells, while about 90% of the energy available in the substrate is retained in the easily purified gaseous products CH4. The end product is a mixture of methane gas and CO2 (also called biogas).

Fermentative bacteria hydrolyze the degradable primary substrate polymers such as proteins, lipids and polysaccharides and decompose to smaller molecules with the production to acetate and other saturated fatty acids, CO2 and H2 as major end products. The second group is the obligate H2 producing acetogenic bacteria, which metabolise low molecule organic acids (end products of the first group) to H2 and acetate (and sometimes CO2).

Electricity from biofuel cells: Biofuel cells could release energy in fuel chemicals to generate electrical energy at ambient temperature. Fuel cells covert energy more efficiently than conventional power engines such as the internal combustion engine and produce almost no pollution. The basic-set up of the fuel cell is two electrodes placed in an electricity conducting electrolyte, separated by an ion exchange membrane. The arrangement allows the electrochemical equivalent of combustion to occur.

a) Generation of electricity from hydrogen gas: Hydrogen is a very attractive fuel because of its high energy content (18.7 kJ/g), which is about four-fold greater than ethanol and over two-fold higher than methane. A wide range of microorganisms produce hydrogen as a part of mechanisms for disposing of electrons that are generated during metabolic reactions:

The generation of hydrogen using microorganisms, or cell-free systems based on microbial components, is still very much in its infancy. However, there are three possible routes of production (Waites et al., 2001): Biophotolysis of water: It involves splitting water using light energy and does not require an exogenous substrate. In vivo, the energy generated is normally used to form reduced nicotinamide adenine dinucleotide phosphate (NADPH). In the presence of a bacterial hydrogenase and an appropriate electron carrier, molecular hydrogen can be generated. Photoreduction: it is a light dependent process of decomposition of organic compounds performed by photosynthetic bacteria. This is an anaerobic process requiring light and an exogenous organic substrate, which is inhibited by oxygen, dinitrogen and ammonium ions. Formation of hydrogen is attributed to a nitrogenase that can reduce protons as well as di-nitrogen. Members of the Chlorobioaceae, Chromatiaceae and Rhodospirillaceae, carry out photoreduction. Bacteria having higher potential are probably the purple non-sulphur bacteria, such as Rhodospirillium spp., which photometabolize organic acids. Fermentation: fermentation of organic compounds by many bacteria generates a small amount of hydrogen. For example, some enterobacteria produce hydrogen and CO2 by cleaving formate. In clostridia it is produced from reduced ferrodoxin.

The role of microorganisms in electricity generation may involve microbially produced gaseous and liquid fuels, such as ethanol or methane, being used to drive conventional mechanical generators. Possible routes are via intact microorganisms or microbial enzymes incorporated within fuel cells (Fig. 1).

Hydrogen gas as a fuel enters at one electrode and oxidant, usually oxygen from the air, at the other. The anode is coated with the enzyme hydrogenase, while the cathode with the laccase. On the anode, hydrogen molecules split into their constituent protons and electrons. The enzymes catalyze a reduction-oxidation reaction across the membrane, releasing energy, which pushes electrons round an external circuit. At the cathode, the protons and electrons combine with oxygen to form water. The fuel does not actually burn; therefore fuel cells do not produce pollutants associated with combustion, such as carbon oxides and oxides of nitrogen. Cells that use hydrogen generate only water compared to fossil fuel, which produce water and carbon dioxide as waste product. This type of fuel cells is called proton exchange membrane cell (PEM).

b) Generation of electricity form methanol:

In this case, the fuel is methanol. The direct methanol fuel cell (DMFC) runs on a dilute mixture of about 2% methanol in water. The methanol is converted into formate on the anode. The proton then reacts with oxygen as in a PEM cell. The metabolically active microorganisms, such as Proteus vulgaris and Anabaena variabillis immobilized in a biofuel cell could convert energy in their substrate (glucose for the former and light for the later) into electricity (Allen and Bennetto, 1993). A biofuel cell in which bacteria Proteus vulgaris and Escherischia coli were used as sulfate reduction catalysts was in operation for 5 years, demonstrating thus its long-term stability. The disadvantage of biofuel cell is that the power output is low (1 kW at 40 mA/cm2). Thus, it is used for specific purposes, such as small medical and military apparatuses used in the field and in space missions. Biofuel cells are considered to be ecofriendly and can be used as substitutes in order to reduce green houses gas emission.


Allen, R. M. and H. P. Bennetto, 1993. Microbial fuel cells: electricity production from carbohydrates. Appl. Biochem. Biotechnol., 39/40: 27-40.

Lee, Y.K., 2003. Microorganisms and production of alternative energy. In: microbial Biotechnology: Principles and Applications, (L.E. Kun, Ed.), World Scientific Publishing Co. Pte. Ltd., 655-670.

Tanaka, K., N. Kashiwagi and T. Ogawa, 1988. Effects of light on the electrical output of bioelectrochemical fuel-cells containing Anabaena variabilis M-2: Mechanism of the post- illumination burst. J. Chem. Technol. Biotechnol., 42: 235-240.

Waites, M. J., N. L. Morgan, J.S. Rockey and G. Higton, 2001. Industrial Microbiology: An introduction. Blackwell Science Ltd., 288 pp.

Published by: Surajit Das, P. S. Lyla and S. Ajmal Khan
CAS in Marine Biology
Annamalai University
Parangipettai - 608 502.

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