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oxygen and is rich in ferrous iron sourced from the weathering of
lateritic soils in its immediate hinterland. Below the oxygen-rich sur-
face waters of Lake Matano live phototrophic bacteria using ferrous
iron for energy, mimicking their ancient ancestors from long ago.
One group of Earth's early bacterial organisms, the cyanobacteria,
started a revolution. They began to use water for photosynthesis,
combining this with carbon dioxide to make sugars for energy, and as
a by-product of the process releasing free oxygen. When this process
first happened remains contentious. There is some fossil evidence for
cyanobacteria as early as 3.5 billion years ago. However, analysis of
the rate at which genetic change has occurred in cyanobacteria, using
their internal 'molecular clock', and a wider analysis of the fossil
record
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suggests the rise of cyanobacteria occurred around the
boundary between the Archaean and Proterozoic eons of the Precam-
brian, at a time referred to by geologists as the Great Oxygenation
Event, or GOE. This is the point of time, 2.4 billion years ago, when
oxygen began to accumulate in the atmosphere and surface of the
oceans. It was a toxic poisonous gas for the organisms of the Archaean,
but it would have large implications for the evolution of a new ocean
state that heralded the Proterozoic Eon.
Stinking Seas
The Earth's oceans have evolved through at least three major states.
Most importantly for this story, transitions between these states
occurred as a direct response to their interaction with life.
The banded iron formations (BIFs) of the Archaean represent the
first ocean state, of ancient anoxic seas full of ferrous iron weathered
from the Earth's volcanic crust. Gradually this iron was removed from
the oceans by the actions of anoxygenic phototrophs (microbes that
photosynthesized but did not release free oxygen), or possibly by
some early cyanobacteria. This iron was deposited on the seabed in a
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