Environmental Engineering Reference
In-Depth Information
7.3
Microbiological Processes
7.3.1
Organisms and Processes
Various microbial processes can be exploited to utilize energy that has been stored in
biomass by photosynthesis. These processes can generate useful biofuels such as
hydrogen, butanol, and biogas. Moreover, biomass can be converted into ethanol; this
is commonly done using fungi but can also be achieved with bacteria. Finally, certain
algae can be used to produce biodiesel, as described in the following section.
7.3.2
Hydrogen Production
Hydrogen is the most abundant element in the universe and is also a very efficient
energy carrier. This extremely small molecule is metabolized by a variety of
organisms including bacteria, archaea, cyanobacteria, and green algae. The simple
conversion of protons into hydrogen gas is a reversible reaction in most organisms
that can perform it, and is catalyzed by a group of enzymes known as hydroge-
nases that have evolved a wide range of subunits to solve this problem under dif-
ferent circumstances. Thirteen different hydrogenases have been identified to date
[15], most of which perform some energy-related function. There are excellent
reviews on the biodiversity and functions of hydrogen-producing organisms (e.g.,
[15, 16]) and on the structure and function of the three known hydrogenase classes:
[Ni-Fe]-hydrogenases, [Fe-Fe]-hydrogenases, and [Fe]-hydrogenases [17].
The hydrogenase reaction has the following stoichiometric formula:
+
−
↔+
2
HHe
2
2
As indicated above, the process is reversible so hydrogen may either be produced
or consumed. Four categories of hydrogen-related processes and organisms that
perform them have been delineated as listed below:
1. photoautotrophic hydrogen production;
2. photoheterotrophic hydrogen production;
3. heterotrophic hydrogen production; and
4. heterotrophic hydrogen production coupled to photo-production.
Microorganisms that are capable of both hydrogen production and photosynthesis
are said to be phototrophic while those that metabolize hydrogen but cannot perform
photosynthesis are said to be heterotrophic. Phototrophic organisms can be divided
into photoautotrophs and photo-heterotrophs. The phototropic organisms are repre-
sented mainly by microalgae and cyanobacteria and are capable of using light as an
energy source and carbon dioxide as a carbon source. The photoheterotrophs are
cyanobacteria that, despite being able to use both light energy and carbon dioxide,
also require organic carbon to perform nitrogen fixation. The heterotrophic bacteria
utilize carbon compounds for their hydrogen production and are therefore excellent
catalysts for the decomposition of organic waste materials.
