The biosphere encompasses the part of the
Earth that is inhabited by life, and includes parts of the lithosphere,
hydrosphere, and atmosphere. Life evolved more than
3.8 billion years ago and has played an important role in
determining the planet’s climate and insuring that it does not
venture out of the narrow window of parameters that allow
life to continue. In this way, the biosphere can be thought of
as a self-regulating system that interacts with chemical, erosional,
depositional, tectonic, atmospheric, and oceanic processes
on the Earth.
Most of the Earth’s biosphere uses photosynthesis as its
primary source of energy, driven ultimately by energy from
the Sun. Plants and many bacteria use photosynthesis as their
primary metabolic strategy, whereas other microorganisms
and animals rely on photosynthetic organisms as food for
their energy and thus use solar energy indirectly. Most of the
organisms that rely on solar energy live, by necessity, in the
upper parts of the oceans (hydrosphere), lithosphere, and
lower atmosphere. Bacteria are the dominant form of life on
Earth (comprising about 5 × 1030 cells) and also live in the
greatest range of environmental conditions. Some of the
important environmental parameters for bacteria include
temperature, between –41°F to 235°F (–5°C to 113°C), pH
levels from 0 to 11, pressures between a near-vacuum and
1,000 times atmospheric pressure, and from supersaturated
salt solutions to distilled water.
Bacteria and other life-forms exist with diminished abundance
to several kilometers or more beneath the Earth’s surface,
deep in the oceans, and some bacterial cells and fungal
spores are found in the upper atmosphere. Life in the upper
atmosphere is extremely limited by a lack of nutrients and by
the lethal levels of solar radiation above the shielding effects
of atmospheric ozone.
Soils and sediments in the lithosphere contain abundant
microorganisms and invertebrates at shallow levels. Bacteria
exist at much deeper levels and are being found in deeper and
deeper environments as exploration continues. Bacteria are
known to exist to about 2 miles (3.5 km) in pore spaces and
cracks in rocks, and deeper in aquifers, oil reservoirs, and salt
and mineral mines. Deep microorganisms do not rely on photosynthesis
but rather use other geochemical or geothermal
energy to drive their metabolic activity.
The hydrosphere and especially the oceans are teeming
with life, particularly in the near-surface photic zone environment
where sunlight penetrates. At greater depths below the
photic zone most life is still driven by energy from the Sun, as
organisms rely primarily on food provided by dead organisms
that filter down from above. In the benthic environment
of the seafloor there may be as many as 10,000 million (1010)
bacteria per milliliter of sediment. Bacteria also exist beneath
the level that oxygen can penetrate, but the bacteria at these
depths are anaerobic, primarily sulfate-reducing varieties.
Bacteria are known to exist to greater than 2,789 feet (850
m) beneath the seafloor.
In the 1970s, a new environment for a remarkable group
of organisms was discovered on the seafloor along the midocean
ridge system, where hot hydrothermal vents spew heated
nutrient-rich waters into the benthic realm. In these
environments, seawater circulates into the ocean crust where
it is heated near oceanic magma chambers. This seawater
reacts with the crust and leaches chemical components from
the lithosphere, then rises along cracks or conduits to form
hot black and white smoker chimneys that spew the nutrient
rich waters at temperatures of up to 680°F (350°C). Life has
been detected in these vents at temperatures of up to 235°F
(113°C). The vents are rich in methane, hydrogen sulfide,
and dissolved reduced metals such as iron that provide a
chemical energy source for primitive bacteria. Some of the
bacteria around these vents are sulfate-reducing chemosynthetic
thermophyllic organisms, living at high temperatures
using only chemical energy and therefore existing independently
of photosynthesis. These and other bacteria are locally
so great in abundance that they provide the basic food source
for other organisms, including spectacular worm communities,
crabs, giant clams, and even fish.
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