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.
References:
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.
