Geoscience Reference
In-Depth Information
global scale, sulide-based ecosystems would have probably contributed
only a little to the total activity of the biosphere.
To find big players on Earth before oxygen, we look to the skies and
recognize that ancient volcanoes would have spewed gases like H
2
, SO
2
(sulfur dioxide), CO
2
, and H
2
S into the atmosphere. As in the deep-
sea hydrothermal systems, methane-producing populations would have
been supported by the volcanic H
2
and CO
2
. One can imagine metha-
nogens living in saturated soils on land, in lakes, and in the sea, com-
bining CO
2
and hydrogen gas. More importantly, though, many anox-
ygenic phototrophic bacteria can convert hydrogen gas (H
2
) to water.
They couple this reaction to the generation of cell biomass from CO
2
.
11
These ancient hydrogen-utilizing phototrophs would have populated
ancient lakes, ponds, and the surface ocean; indeed, they would have
lived in any watery environment lit by the Sun where the H
2
and CO
2
from the atmosphere could dissolve. How productive might such an
ancient metabolism have been? Minik Rosing, Christian Bjerrum, and
I built on a model originally conceived by Jim Kasting and his group at
Penn State (we met Jim in the last chapter), and we estimated that an-
oxygenic phototrophs using hydrogen gas could have produced bio-
mass at a maximum rate of about 3 × 10
13
moles of organic carbon per
year (equivalent to 3.6 × 10
14
grams of carbon per year). This sounds like
a big number, but it is still one hundred times less than the rates of or-
ganic carbon production in the present biosphere, which is supported
by oxygenic photosynthesis.
To find what may have been the biggest player in the ancient pre-
oxygen biosphere, we take an imaginary dive with
Alvin
into the depths
of the ancient oceans. As we descend, we focus our attention at that
depth just where the Sun's light fades into darkness. Here, we might
well observe a dense population of bacteria, and curiously, an abun-
dance of iron oxide minerals, which you can view as basically rust. Our
chemical sensors would also detect an accumulation of dissolved ferrous
iron (Fe
2+
) in deep waters below where the rust particles were found.
hat's going on here?
We'll hear much more about iron in the chapters to follow, but basi-
cally, ferrous iron is the form of iron that persists in the absence of oxy-
gen, and it readily dissolves in water. Therefore, without oxygen in the
atmosphere, the ferrous iron coming from hydrothermal vents would
