Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 132

gets deposited in sedimentary rocks. But in the anoxic oceans of the
Archaean, ferrous iron remained soluble and thus in ready supply,
with a continuous source weathered from the volcanic crust of the
early Earth.
Perhaps the surface of the early Earth was once pink-red from salt-
loving halophile Archaea. 78 There are also the thermophiles, found
growing in the scalding-hot water emerging from deep-sea hydro-
thermal vents. Or the methanogens that colonize regions with little
or no oxygen, and which today can be found in rice paddies or the
guts of cows and humans. Surprisingly, for the Archaea were once
thought to be specialists in colonizing oddball environments, they
also live side-by-side with bacteria and eukaryotes in oxygenated
waters with normal salinity. They are also widespread as microplank-
ton in the oceans, where they may act as a primary food source. And,
for a group of organisms supposedly specialized for heat tolerance,
they are abundant in the cold waters of the Antarctic. Nevertheless,
despite their modern environmental range, the genetic story of
these 'cold-adapted' Archaea suggests their origins lie in anoxic high-
temperature habitats, the same habitats that characterized early Earth.
Archaea are also phototrophic, using the energy of light to produce
chemical energy that drives metabolic processes in the cell. But this
process is not the photosynthesis used by cyanobacteria and higher
plants, and it does not liberate free oxygen. Ancient Archaea may also
have been heterotrophs, feeding on the inorganic carbon compounds
supplied by asteroid impacts; that is a narrow niche, though, with a
limited food supply even on the asteroid-bombarded early Earth.
Few places on Earth now resemble the ancient Archaean seas, but
Lake Matano on the Indonesian island of Sulawesi may distantly echo
those conditions. Matano is a deep lake, extending down 590 metres.
It is perhaps 4 million years old, and below its oxygenated surface
waters that teem with unique and exotic fish, the lake has no free