DEFINITION OF BIOSPHERE

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.

See also ATMOSPHERE; BENTHIC; BLACK SMOKER CHIMNEYS;

SUPERCONTINENT CYCLE.

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