Geoscience Reference
In-Depth Information
we see the possibility of an ecosystem running on methane, with pri-
mary producers consisting of autotrophic methanogens, as well as con-
sumers, which include various fermenting bacteria and heterotrophic
methanogens.
This, I think, is a realistic view of the major microbial players popu-
lating ancient ecosystems at deep-sea hydrothermal vents before the
advent of oxygen-producing organisms. Other populations of microbes,
however, might also have been present, adding to ecosystem diversity.
For example, if sulfate was found in seawater, even in small concentra-
tions (we'll hear much more about sulfate in later chapters), a process
known as sulfate reduction could have been supported. In this process,
sulfate-reducing bacteria gain energy and grow through the reaction of
sulfate with organic matter or H
2
; sulfide and CO
2
are then produced.
8
These ancient sulfate reducers, then, could have lived off the organic
matter from other dead microbes, or from H
2
emanating from the hy-
drothermal vents. It seems probable, therefore, that ancient hydrother-
mal ecosystems housed quite diverse populations of microbes, but H
2
would have been the most likely primary source of energy.
9
Deep-sea hydrothermal systems would not, however, have been the
only place to encounter life on early Earth before oxygen. Indeed,
the early biosphere likely also saw considerable action on land and in
the upper reaches of the global ocean. There are many reasons for say-
ing this, but we start with the H
2
S (hydrogen sulfide) that was so un-
interesting for life in the deep, dark, oxygen-free ocean. On land, and in
the light of day, H
2
S suddenly becomes very interesting indeed. hen
available, this H
2
S would have been of great use to a group of photo-
synthetic organisms that evolved well before the oxygen producers we
know best. Since these photosynthesizers don't produce oxygen, they
are known as anoxygenic phototrophs.
10
I will say a good deal about
their evolution in the next chapter, but here I will introduce you to their
ecology. In fact, they are not rare.
I have always dreamed of living by the sea, and in an act of reckless
indulgence, my family agreed to purchase a small house on the Danish
island of Bornholm, where we vacation every summer. On the south-
eastern part of the island there are a series of beautiful white beaches
whose sand is so fine and regular that it was used to make hour glasses.
Because the sand is so fine, it dampens easily and stays wet for a long
