Geoscience Reference
In-Depth Information
hv
Anoxygenic
photosynthesis
SO
4
2-
H
2
S
à
SO
4
2-
+ CH
2
O
Sulfate reduction
SO
4
2-
+ CH
2
O
à
H
2
S
Anaerobic
Methane oxidation
SO
4
2-
+ CH
4
à
H
2
S
CH
4
CH
2
O
à
CH
4
+ CO
2
Methanogenesis
H
2
S
Hydrothermal H
2
S
Figure 2.1. Possible workings of an ancient sulfur-based microbial ecosystem, and the likely
additional influence of microbial methane cycling. methane cycling will occur if some of the
sulfate produced during anoxygenic photosynthesis is lost from the system, as through river
runoff, for example. Also shown is the process of anaerobic methane oxidation, the oxidation
of methane with sulfate; this is of possible significance, but not discussed in the text.
redrawn from Canfield et al. (2006). CH
2
O indicates organic compounds.
bacteria would aid in organic matter decomposition and help generate
food for the sulfate-reducing bacteria. This type of ecosystem is known
as a “sulphuretum,” a term first introduced by Laurens Baas Becking in
1925 (Baas Becking would later initiate the field of geobiology in his
1934 book,
Geobiologie
) (
ig. 2.1)
. Such an ecosystem also cycles matter in
a way directly analogous to what happens in our modern oxygenated
biosphere. Just replace sulfide with water and sulfate with oxygen.
If some of the sulfate produced by the anoxygenic phototrophs
washed away with the flowing hydrothermal water, then there would
be insufficient sulfate to decompose all of the dead biomass via sulfate-
reducing bacteria. This deficiency would allow a community of methane
producers to develop and decompose the rest of the organic material,
adding even more complexity to our sulide-fueled terrestrial ecosys-
tem. If one could have visited these ancient ecosystems, one would have
marveled at the microbial stringers and colorful mats, somewhat analo-
gous to those found today. Life would have appeared bountiful in these
hydrothermal areas. However, with a general paucity of such areas on a
