185
The electron donor of many biogeochemically
important, phototrophic or chemotrophic bacteria
is also inorganic, such as H 2 , Fe
2+
or H 2 S. All
these organisms are called lithotrophic from
‘lithos’ meaning rock. In contrast, organotrophic
organisms such as animals use an organic electron donor:
CH 2 O + O 2 → CO 2 + H 2 O
(5.20)
Finally, organisms such as the green plants, but
also many lithotrophic bacteria, are autotrophs, i.e.
they are able to build up their biomass from CO 2
and other inorganic nutrients. Animals and many
bacteria living on organic substrates instead
incorporate organic carbon into their biomass and
are termed heterotrophs. Among the prokaryotes,
there is not a strict discrimination between
autotrophy and heterotrophy. Thus, heterotrophs
generally incorporate 4-6% CO 2 (mostly to convert
C 3 compounds to C 4 compounds in anaplerotic
reactions, in order to compensate for C 4
compounds which were consumed for biosynthetic purposes in the cell). This heterotrophic CO 2
assimilation has in fact been used to estimate the
total heterotrophic metabolism of bacterioplankton
based on their dark
14
CO 2 incorporation. As another
example, sulfate reducing bacteria living on acetate
are in principle heterotrophs but derive about 30%
of their cell carbon from CO 2 . Aerobic methane
oxidizing bacteria, which strictly speaking are also
living on an organic compound, incorporate 30-90%
CO 2 .
The three criteria in Table 5.3 can now be used
in combination to specify the main types of energy
metabolism of organisms. Green plants are
photolithoautotrophs while animals are chemoorganoheterotrophs. This detail of taxonomy may seem
an exaggeration for these organisms which are in
daily terms called photoautotrophs and heterotrophs. However, the usefulness becomes apparent
when we need to understand the function of, e.g.
chemolithoautotrophs or chemolithoheterotrophs
for the cycling of nitrogen, manganese, iron or
sulfur in marine sediments (see Sect. 5.5).
5.4.5
Energy Metabolism of
Microorganisms
The basic types of energy metabolism and representative organisms of each group are compiled in
Table 5.4. Further information can be found in
Fenchel et al. (1998), Ehrlich (1996), Madigan et al.
(1997), Canfield et al. (2005) and other textbooks.
Most photoautotrophic organisms use water to
reduce CO 2 according to the highly endergonic
reaction of Eq. 5.18. Since water has a high redox
potential it requires more energy than is available
in single photons of visible light to transfer electrons from water to an electron carrier, NADP
+
,
which can subsequently reduce CO 2 through the
complex pathway of the Calvin-Benson cycle.
Modern plants and algae, which use H 2 O as an
electron donor, consequently transfer the
electrons in two steps through two photocenters.
In photosystem II, electrons are transferred from
H 2 O which is oxidized to O 2 . In photosystem I
these electrons are transferred to a highly reducing primary acceptor and from there to NADP
+
.
More primitive phototrophic bacteria, which
predominated on Earth before the evolution of
oxygenic photosynthesis, have only one
photocenter and are therefore dependent on an
electron donor of a lower redox potential than
water. In the purple and green sulfur bacteria or
cyanobacteria, H 2 S serves as such a low-E H
electron donor. The H 2 S is oxidized to S
0
and
mostly further to sulfate. Some purple bacteria
are able to use Fe
2+
as an electron donor to
reduce CO 2 . Their existence had been suggested
many years ago, but they were discovered and
isolated only recently (Widdel et al. 1993;
Ehrenreich and Widdel 1994). This group is
geologically interesting, since direct phototrophic Fe(II) oxidation with CO 2 opens the theoretical possibility for iron oxidation in the
absence of O 2 some 2.0-2.5 billion years ago, at
the time when the great deposits of banded iron
formations were formed on Earth.
Some of the purple and green bacteria are able
to grow photoheterotrophically, which may under
some environmental conditions be advantageous
as they cover their energy requirements from light
but can assimilate organic substrates instead of
spending most of the light energy on the
assimilation of CO 2 . The organisms may either take
up or excrete CO 2 in order to balance the redox
state of their substrate with that of their biomass.
5.4.6
Chemolithotrophs
The chemolithotrophs comprise a large and
diverse group of exclusively prokaryotic organisms, which play important roles for mineral
cycling in marine sediments (Table 5.4). They
conserve energy from the oxidation of a range of
5.4
Energy Metabolism of Prokaryotes
The electron donor of many biogeochemically
important, phototrophic or chemotrophic bacteria
is also inorganic, such as H 2 , Fe
2+
or H 2 S. All
these organisms are called lithotrophic from
‘lithos’ meaning rock. In contrast, organotrophic
organisms such as animals use an organic electron donor:
CH 2 O + O 2 → CO 2 + H 2 O
(5.20)
Finally, organisms such as the green plants, but
also many lithotrophic bacteria, are autotrophs, i.e.
they are able to build up their biomass from CO 2
and other inorganic nutrients. Animals and many
bacteria living on organic substrates instead
incorporate organic carbon into their biomass and
are termed heterotrophs. Among the prokaryotes,
there is not a strict discrimination between
autotrophy and heterotrophy. Thus, heterotrophs
generally incorporate 4-6% CO 2 (mostly to convert
C 3 compounds to C 4 compounds in anaplerotic
reactions, in order to compensate for C 4
compounds which were consumed for biosynthetic purposes in the cell). This heterotrophic CO 2
assimilation has in fact been used to estimate the
total heterotrophic metabolism of bacterioplankton
based on their dark
14
CO 2 incorporation. As another
example, sulfate reducing bacteria living on acetate
are in principle heterotrophs but derive about 30%
of their cell carbon from CO 2 . Aerobic methane
oxidizing bacteria, which strictly speaking are also
living on an organic compound, incorporate 30-90%
CO 2 .
The three criteria in Table 5.3 can now be used
in combination to specify the main types of energy
metabolism of organisms. Green plants are
photolithoautotrophs while animals are chemoorganoheterotrophs. This detail of taxonomy may seem
an exaggeration for these organisms which are in
daily terms called photoautotrophs and heterotrophs. However, the usefulness becomes apparent
when we need to understand the function of, e.g.
chemolithoautotrophs or chemolithoheterotrophs
for the cycling of nitrogen, manganese, iron or
sulfur in marine sediments (see Sect. 5.5).
5.4.5
Energy Metabolism of
Microorganisms
The basic types of energy metabolism and representative organisms of each group are compiled in
Table 5.4. Further information can be found in
Fenchel et al. (1998), Ehrlich (1996), Madigan et al.
(1997), Canfield et al. (2005) and other textbooks.
Most photoautotrophic organisms use water to
reduce CO 2 according to the highly endergonic
reaction of Eq. 5.18. Since water has a high redox
potential it requires more energy than is available
in single photons of visible light to transfer electrons from water to an electron carrier, NADP
+
,
which can subsequently reduce CO 2 through the
complex pathway of the Calvin-Benson cycle.
Modern plants and algae, which use H 2 O as an
electron donor, consequently transfer the
electrons in two steps through two photocenters.
In photosystem II, electrons are transferred from
H 2 O which is oxidized to O 2 . In photosystem I
these electrons are transferred to a highly reducing primary acceptor and from there to NADP
+
.
More primitive phototrophic bacteria, which
predominated on Earth before the evolution of
oxygenic photosynthesis, have only one
photocenter and are therefore dependent on an
electron donor of a lower redox potential than
water. In the purple and green sulfur bacteria or
cyanobacteria, H 2 S serves as such a low-E H
electron donor. The H 2 S is oxidized to S
0
and
mostly further to sulfate. Some purple bacteria
are able to use Fe
2+
as an electron donor to
reduce CO 2 . Their existence had been suggested
many years ago, but they were discovered and
isolated only recently (Widdel et al. 1993;
Ehrenreich and Widdel 1994). This group is
geologically interesting, since direct phototrophic Fe(II) oxidation with CO 2 opens the theoretical possibility for iron oxidation in the
absence of O 2 some 2.0-2.5 billion years ago, at
the time when the great deposits of banded iron
formations were formed on Earth.
Some of the purple and green bacteria are able
to grow photoheterotrophically, which may under
some environmental conditions be advantageous
as they cover their energy requirements from light
but can assimilate organic substrates instead of
spending most of the light energy on the
assimilation of CO 2 . The organisms may either take
up or excrete CO 2 in order to balance the redox
state of their substrate with that of their biomass.
5.4.6
Chemolithotrophs
The chemolithotrophs comprise a large and
diverse group of exclusively prokaryotic organisms, which play important roles for mineral
cycling in marine sediments (Table 5.4). They
conserve energy from the oxidation of a range of
5.4
Energy Metabolism of Prokaryotes