Harvesting solar energy as a source of power
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.
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.
Ultimately, the sun is the only large renewable source of energy.
• We have a lot, but it is diffuse and not in a form we can use of most things for which we need energy.
Useful energy is in electrons!
• So, the goal is get the electrons from renewable, but diffuse sources into energy forms easily used by society: e.g., electricity,
Photosynthetic Microorganisms
• Fast growing - doubling time 0.5-1 day
• Do not require arable land
• Growth year-round
• High areal production
• Homogeneous (all cells are the same)
• Water-efficient; can recycle minerals • Not lignocellulosic
Converting solar energy to lots of high-energy biomass using the cyanobacterium Synechocystis
– Areal yield is high enough to replace all of the world’s fossil fuel in the area of
Texas
Higher yield per volume; an estimated hundred-fold increase over current biofuels. The reason: bacteria double in volume every 24-48 hours — faster than any plant can grow.
Does not require arable land; tube “crops” can be located anywhere there is sunlight
Does not compete with food or commodity crops
Requires less water than plant-based biofuels
Does not require fertilizer, so eliminates soil depletion/contamination concerns
Has a simpler genetic structure than plants, resulting in higher quality control and virtually no waste
Allows less costly processing
Is carbon-neutral. Like a plant, the bacteria use carbon dioxide for growth
Can be located in urban as well as rural areas, reducing transportation costs and associated environmental impact.
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.