Environmental Engineering Reference
In-Depth Information
Apart from the anthropogenic acetylene on Earth, naturally occurring acetylene
can be found among other prebiotic molecules in interstellar gas clouds [
8
]. Another
place where acetylene occurs naturally in greater abundance is on Saturn's moon
Titan. Titan's atmosphere is considered to be a cold model of our Earth's early
atmosphere approximately 4 billion years ago [
6
]. Photochemical processes
in Titan's upper atmosphere that create acetylene from methane are considered to
be the source of this acetylene [
9
]. Besides an atmospheric concentration of 3.5 ppm
acetylene [
6
], the Cassini/Huygens mission to Titan detected lakes consisting of
hydrocarbons (76-79 % ethane, 7-9 % propane, 5-1 % methane, 2-3 % hydrogen
cyanide, 1 % butene, 1 % butane, and 1 % acetylene) on Titan's surface [
10
,
11
].
The similarity between the atmosphere of Titan and the assumed composition of the
Earth's early atmosphere have led to speculations that photochemical reactions
along with other sources like volcanic eruptions may have provided the developing
life on Earth with sufficient amounts of acetylene to be a viable source of carbon
and energy [
6
].
Bacterial growth on acetylene is facilitated by its rather high solubility in water
of 47.2 mM (20
C; 1 atm or 101 kPa) compared to other gaseous compounds like
O
2
,H
2
or N
2
that have solubilities of around 1 mM under these conditions [
2
].
3 Bacteria Living on Acetylene
The energy richness of the C
C triple bond and the rather high solubility of
acetylene in water make it a suitable substrate for bacteria, provided an adequate
source is available. The first reports on bacteria living on acetylene were published
more than 80 years ago [
12
]. In 1979,
Nocardia rhodochrous
was grown on
acetylene as sole source of carbon and energy in the presence of dioxygen [
13
].
One year later, an acetylene hydratase activity was found in cell-free extracts from
Rhodococcus A1
grown on acetylene by anaerobic fermentation [
14
]. In these
cell-free extracts, acetylene was converted to acetaldehyde by addition of a water
molecule. Boiling of the extracts or addition of dioxygen inhibited this activity,
indicating that it originated from an oxygen-sensitive enzyme. In 1981, anaerobic
oxidation of acetylene to CO
2
was found in enrichment cultures from estuarine
sediments [
15
]. In this case, acetate was identified as major intermediate of the
process. Further studies revealed that two groups of bacteria were responsible for
the oxidation of acetylene to CO
2
. Fermenting bacteria converted acetylene to
acetaldehyde, which they dismutated to acetate and ethanol. These products were
then further oxidized by sulfate-reducing bacteria [
16
]. According to Culbertson
et al. [
16
], the morphology of these Gram-negative bacteria was described to be
similar to
Pelobacter acetylenicus
, an acetylene-fermenting bacterium, isolated
three years earlier by Schink [
17
]. After the isolation of the acetylene hydratase
from
P. acetylenicus
[
1
], Rosner et al. tested new isolates and bacterial strains from
culture collection in the presence of a similar enzyme by activity tests and antibody
cross reaction [
18
]. Acetylene hydratase activity was discovered in several cell-free
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