Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 40

charcoal piles from where several of these soil microbes have been isolated
[ 15 ]. CO is also consumed by pathogenic Mycobacteria, like the tubercle bacillus
Mycobacterium tuberculosis , which can grow on CO as sole source of carbon and
energy [ 16 ]. Thus, biological processes are relevant to remove CO and keep it at
low trace gas concentrations.

1.2.2 Use of Carbon Monoxide under Aerobic
and Anaerobic Conditions

CO gains increasing attention in biology and medicine as a signaling molecule,
acting at trace gas concentrations [ 10 , 11 ]. Ironically, the toxic CO participates
in various regulatory processes in vasodilatory action, oxygen-sensing, and
neurotransmission [ 10 ] and may be used as therapeutic and bactericidal
agent [ 10 , 11 ].
CO is also a key metabolite in many microorganisms, which are using it as a
source of energy and carbon [ 9 , 17 - 19 ]. CO-utilizing microbes convert CO to CO 2
and a pair of reducing equivalents as products. The enzyme catalyzing CO oxida-
tion is carbon monoxide dehydrogenase (CODH, CO:acceptor oxidoreductase).
CODHs of aerobic and anaerobic microorganisms have different structures and
contain different metals in their active sites (see Section 2 )[ 2 ]. The CO-consuming
microorganisms employ diverse metabolic pathways for coupling CO oxidation to
various pathways of energy conservation and carbon fixation [ 9 , 17 ].

1.2.2.1 Fates of Carbon Monoxide under Aerobic Conditions

Some aerobic CO-utilizing microorganisms are able to grow with CO as sole
carbon source by fixing the CO 2 formed along the Calvin-Benson-Bassham cycle.
We can distinguish between carboxydothrophs, which grow at elevated CO
concentrations (over 10 %) and carboxydovores, which use CO at concentrations
below 1000 ppm [ 9 ]. Aerobic carboxydothrophs -proteobacteria,

firmicutes, actinobacteria, while aerobic carboxydovores include

include ʱ

ʱ -, ʲ -, and

ʳ -proteobacteria. Both groups of microorganisms fuel CO into their cellular

metabolism via the Calvin-Benson-Bassham cycle [ 16 , 20 - 23 ]. Some aerobic
carboxydovores are unable to use CO 2 as sole carbon source as they lack the
enzymes required for CO 2 fixation along the established pathways like the Calvin-
Benson-Bassham cycle or the reverse tricarboxylic acid cycle [ 9 ]. These organisms,
which include -proteobacteria like Stappia stellulata and Ruegeria (previously

Silicibacter ) pomeroyi apparently use CO only as a source of energy [ 22 , 24 ].
Thus, CO may be used in different ways.
Enzymatic CO oxidation supplies microorganisms with powerful reducing
equivalents. The reducing equivalents generated are channeled into a respiratory
chain via ubiquinones and cytochromes leading to the reduction of the terminal
electron acceptor. Commonly molecular oxygen serves as terminal electron

ʱ