Basic Research Needs for Solar Energy Utilization

Figure 37 Anaerobic phototrophs

also produce hydrogen and are very

active nitrogen fixers.

Acetyl-

Dimethylamine

Mphen

CO 2

Formyl-methanofuran

Formyl-H 4 MPT

Methyl-S-CoM

CH 4

Ech hydrogenase

HS-CoB

Heterodisulfidereductase

HS-CoB+

HS-CoM

H 2 Fdred

F (^420) hydrogenase-non-reducing
H 2
2 H+
H+
Acetate
Acetyl-CoA phosphate
H 2
Methanogenesis Methanol
Trimethylamine
Mphen (^) Monomethylamine
Na+
CoM-S-S-CoB
Methenyl-H 4 MPT
Methylene-H 4 MPT
Methyl-H 4 MPT
F 420 H 2 F^420 H^2
F 420 H 2 F 420 H 2
H+
dehydrogenaseF^420 H^2
Figure 38 Mechanism of methanogenesis
as currently formulated
been made in identifying and characterizing the various enzymes involved in microbial digestion
of lignocellulose. However, in many cases, the enzymes or enzyme complexes found in nature
are not well-suited to industrial-scale processes for conversion of lignocellulose to fermentable
sugars and other useful chemicals. Progress in enzyme chemistry, structural biology, and
computational chemistry have created exciting new opportunities to greatly improve the
properties of enzymes for lignocellulose conversion. Additional investments in understanding the
structure and function of polysaccharide and lignin hydrolyases will create significant
improvements in the overall efficiency of lignocellulose conversion to liquid fuels. Additional
research is also required to improve the efficiency with which sugars other than glucose are
bioconverted to useful chemicals. Downstream processes for fermentation of cellulose
degradation products are extremely important for biofuel production, and more work on
maximizing these microbial processes is extremely important.

MICROBES AND SOLAR BIOFUELS

Globally, biological processes produce more than 250
metric tonnes of hydrogen per year. However, because
other organisms in the biosphere rapidly use most of the
metabolically produced hydrogen, this gas is not released
into the atmosphere and the phenomenon of biological
hydrogen evolution is not widely recognized. Algae and
cyanobacteria employ the same basic photosynthetic
processes found in green plants. They capture sunlight and
use the energy to split water, release oxygen, and fix
atmospheric carbon dioxide. All these microbes can adapt
their normal photosynthetic processes to produce
hydrogen directly from water using sunlight and the
enzymes hydrogenase or nitrogenase. In anaerobic photosynthetic bacteria (see Figure 37), there
are several enzymes that catalyze hydrogen metabolism and evolution, including reversible
hydrogenases and the nitrogenase complex. In addition, some of these organisms can even
couple the degradation of toxic halogenated
compounds and lignin monomers to hydrogen
production. While these capabilities have been known
as laboratory curiosities for many years, only recently
as the result of a number of advances in basic
physiology, enzymology, protein structure, and
molecular biology has the prospect of using these
unique metabolisms as the basis for new energy-
production technology become a possibility. The
emerging tools and modern plant biology hold
promise that significant amounts of global energy will
be supplied by algal farms that access desert and
coastal areas.

In addition to hydrogen production, biological

methane production (see Figure 38), is a well-