Geoscience Reference
In-Depth Information
time, and if you scrape your foot along a wet patch, you will typically
find a very interesting and repeatable layering of green, red, and black
(plate 2)
. If you stick your nose down close, you sense the faint smell of
hydrogen sulfide. The sulfide comes from sulfate reduction, a process
we met earlier, and the black color is produced by the reaction between
sulfide and small amounts of iron minerals in the sand. The green band
is colored by the oxygen-producing cyanobacteria we will explore in
detail in subsequent chapters. Important to our discussion here, the red
band is colored by anoxygenic photosynthetic organisms. These organ-
isms use the Sun's energy to convert sulfide to sulfate, and in the pro-
cess, they generate cell biomass from CO
2
.
hile the Bornholm sand provides a good environment for sulfide-
using anoxygenic phototrophic populations, these sands provide a
rather poor analogue to the ancient Earth. This is because the sulfide
used by this population of anoxygenic phototrophs is obtained, ulti-
mately, from the decomposition of organic material produced by the
cyanobacteria populating the upper layers of the sand. Take away the
cyanobacteria, and there would be no sulfide for the anoxygenic photo-
trophs to use. A more analogous environment to early Earth would be
the type of thermal springs found at Yellowstone National Park, on Ice-
land, or on North Island of New Zealand.
I visited Yellowstone National Park as a kid. I marveled at the hot
springs and I was mesmerized by “Old Faithful” but mostly, I was
absorbed by the hunt for bears from the safety of our car. I do remem-
ber the colors though, the beautiful browns, oranges, reds, and greens
spreading like abstract paintings from the seemingly bottomless throats
of the hydrothermal springs. Only years later did I learn that these col-
ors were formed from mats of bacteria, many of which contain large
populations of anoxygenic phototrophs that oxidize the hydrothermal
sulfide emanating from the springs. This, I think, is a decent analogue
to what we might have found on ancient Earth. As with deep-sea hy-
drothermal vents, we can imagine complex ecosystems developing. The
sulide-oxidizing anoxygenic phototrophs would produce sulfate, and
this would be used by sulfate-reducing bacteria to oxidize the organic
matter produced by the phototrophs. During sulfate reduction, sulfate is
reduced to sulfide, recycling sulfide for use again by the anoxygenic pho-
totrophs. As in the hydrothermal vent ecosystems, various fermenting
