Diversity of Metabolism in Procaryotes (page 5)
(This chapter has 8 pages)
© Kenneth Todar, PhD
Lithotrophic Types of
Lithotrophy is the use of an inorganic compound as a source of
Most lithotrophic bacteria are aerobic respirers that produce energy in
the same manner as all aerobic respiring organisms: they remove
from a substrate and put them through an electron transport system that
will produce ATP by electron transport phosphorylation. Lithotrophs
happen to get those electrons from an inorganic, rather than an organic
Some lithotrophs are facultative lithotrophs, meaning they
able to use organic compounds, as well, as sources of energy. Other
do not use organic compounds as sources of energy; in fact, they won't
transport organic compounds. CO2 is the sole source of
for the methanogens and the nitrifying bacteria and a few other species
scattered about in other groups. These lithoautotrophs are
referred to as "chemoautotrophs", but the term lithoautotroph
a more accurate description of their metabolism. The lithotrophs are a
very diverse group of procaryotes, united only by their ability to
an inorganic compound as an energy source.
Lithotrophy runs through the Bacteria and the Archaea.
If one considers methanogen oxidation of H2 a form of
then probably most of the Archaea are lithotrophs. Lithotrophs
usually organized into "physiological groups" based on their inorganic
substrate for energy production and growth (see Table 5 below).
Physiological groups of lithotrophs
* The overall process of nitrification, conversion of NH3
to NO3, requires a consortium of microorganisms.
||oxidized end product
||H2S or S
The hydrogen bacteria oxidize H2 (hydrogen gas) as
an energy source. The hydrogen bacteria are facultative lithotrophs
as evidenced by the pseudomonads that fortuitously possess a
enzyme that will oxidize H2 and put the electrons into their
respiratory ETS. They will use H2 if they find it in their
even though they are typically heterotrophic. Indeed, most hydrogen
are nutritionally-versatile in their ability to use a wide range of
and energy sources. Some hydrogen bacteria possess an NAD-linked hydrogenase
that transfers electrons from H2 to NAD in a one-step
NAD then delivers the electrons to the ETS. Others have hydrogenase
that pass electrons to different carriers in the bacterial electron
The methanogens used to be considered a major group of
bacteria - until it was discovered that they are Archaea. The
are able to oxidize H2 as a sole source of energy while
the electrons from H2 to CO2 in its reduction to
methane. Apparently, H2 has more energy available than CH4,
for all you physical chemists out there. Metabolism of the methanogens
is absolutely unique, yet methanogens represent the most prevalent and
diverse group of Archaea. Methanogens use H2 and CO2
to produce cell material and methane. They have unique coenzymes and
transport processes. Their type of energy generating metabolism is
seen in the Bacteria, and their mechanism of autotrophic CO2
fixation is very rare, except in methanogens.
The carboxydobacteria are able to oxidize CO (carbon
to CO2, using an enzyme CODH (carbon monoxide
The carboxydobacteria are not obligate CO users, i.e., some are also
bacteria, and some are phototrophic bacteria. Interestingly, the enzyme
CODH used by the carboxydobacteria to oxidize CO to CO2, is
used by the methanogens for the reverse reaction - the reduction of CO2
to CO - during CO2 fixation by the CODH pathway (Figure 23).
The nitrifying bacteria are represented by two genera, Nitrosomonas
and Nitrobacter. Together these bacteria can accomplish the
of NH3 to NO3, known as the process of nitrification.
No single organism can carry out the whole oxidative process. Nitrosomonas
oxidizes ammonia to NO2 and Nitrobacter oxidizes NO2
to NO3. Most of the nitrifying bacteria are obligate
the exception being a few strains of Nitrobacter that will
acetate. CO2 fixation utilizes RUBP carboxylase and the
Cycle. Nitrifying bacteria grow in environments rich in ammonia, where
extensive protein decomposition is taking place. Nitrification in soil
and aquatic habitats is an essential part of the nitrogen cycle.
Lithotrophic sulfur oxidizers include both Bacteria (e.g.
and Archaea (e.g. Sulfolobus). Sulfur oxidizers oxidize
(sulfide) or S (elemental sulfur) as a source of energy. Similarly, the
purple and green sulfur bacteria oxidize H2S or S as an
donor for photosynthesis, and use the electrons for CO2
(the dark reaction of photosynthesis). Obligate autotrophy, which is
universal among the nitrifiers, is variable among the sulfur oxidizers.
Lithoautotrophic sulfur oxidizers are found in environments rich in H2S,
such as volcanic hot springs and fumaroles, and deep-sea thermal vents.
Some are found as symbionts and endosymbionts of higher organisms.
they can generate energy from an inorganic compound and fix CO2
as autotrophs, they may play a fundamental role in primary
in environments that lack sunlight. As a result of their lithotrophic
these organisms produce sulfuric acid (SO4), and therefore
to acidify their own environments. Some of the sulfur oxidizers are acidophiles
that will grow at a pH of 1 or less. Some are hyperthermophiles
that grow at temperatures of 115 degrees C.
Iron bacteria oxidize Fe++ (ferrous iron) to Fe+++
(ferric iron). At least two bacteria probably oxidize Fe++
a source of energy and/or electrons and are capable of lithoautotrophic
growth: the stalked bacterium Gallionella, which forms
rust-colored colonies attached to objects in nature, and Thiobacillus
ferrooxidans, which is also a sulfur-oxidizing lithotroph.
oxidations of nitrifying bacteria and sufide oxidizing bacteria