Geology Reference
In-Depth Information
builds up, so that only the thin slime at the top is alive—perhaps in this way the bacteria
keep themselves as close to the light as possible. Stromatolites are found today only in
places where the sea water is so saline that bacterial predators can't survive, but in the
long distant past of the Archean, 3,500 million years ago, these predators were absent
and stromatolites were everywhere. Their significance for Gaia was their generous gift
of oxygen, which gradually accumulated in the atmosphere, and their fixing of carbon-
rich sugars locked into their microbial bodies.
Figure 35: Living stromatolites at Shark Bay, Western Australia.(
photo © Reg Morrison
)
Yet the newly invented photosynthetic pathway could have quickly frozen the nascent
Gaia to death by removing all the carbon dioxide from the atmosphere. This dreadful
fate was prevented thanks to the efforts of the decomposing bacteria living in the sedi-
ments at the bottom of the ocean. These beings digested the corpses of photosynthesisers
when they reached the sediments from the upper sunlit regions, releasing methane gas
which bubbled its way upwards into the air, where its inordinate penchant for reflecting
heat from the Earth's surface kept the planet warm.
At the ocean surface, wherever photosynthesis had made oxygen locally abundant, a
new bacterial metabolic opportunity opened up: respiration. Respirers are extraordinar-
ily cheeky, as they use oxygen to digest the very same photosynthesisers that produced
the oxygen in the first place. In a sense, respiring creatures such as humans and the
primeval bacterial respirers run photosynthesis in reverse, by using oxygen to break
down the complex sugar molecules inside the bodies of their prey, releasing carbon di-
oxide and water in the process. Because photosynthesis and respiration balance more or
