Organisation at the Ecosystem Level 73
4.3.6 Sulphur cycle
Sulphur (S) is an essential component of some amino acids. Its cycle is
mostly realised in the aquatic environment and the soil, whereas gaseous
sulphur compounds, such as sulphur dioxide and hydrogen sulphide, have
a secondary part in its circulation in the biosphere. Sulphur’s basic source
for organisms is sulphide salts that are dissolved in the water. These are
absorbed by plants and are used for protein production. Sulphur’s organic
compounds circulate like the other substances through food chains and are
finally decomposed by special bacteria that generate hydrogen sulphide.
These microorganisms live in anaerobic conditions within the black silt
existing in the bottom of certain seas or lakes; many of them produce the
energy they need by the reduction of sulphide compounds. Conversely,
some autotrophic bacteria construct organic substances thanks to the
energy they produce from the oxidation of hydrogen sulphite to sulphur
compounds. Additionally, a part of S found in the bottom of the sea settles
in the sediments in the form of sulphide iron, whereas phosphorus is simultaneously released from the sediments, becoming soluble in the water.
This is a remarkable example of the combination and mutual regulation
of two different biogeochemical cycles. Finally, the biosphere is constantly
enriched by gaseous S compounds through volcanic activity, as well as
by human actions, as the combustion of fossil fuels, which have become
significant, especially in the last century.
4.4 EcoSyStEMS’ changE in tiME
4.4.1 Ecological Succession
Ecosystems are continuously changing under the influence of external
and internal factors. The time scale of the short-term change is that of the
organisms’ life span order. Long term, ecosystems’ changes correspond to
scales of hundreds, thousands and millions of years, following the changes
of geological conditions or of the climate, as well as the species’ evolution.
Moreover, a middle term change is observed in certain ecosystems in the
form of gradual maturation of biological communities. It is completed in
time scales of years, decades or centuries, at the most.
A piece of land that came to the surface by volcanic action or by the
recess of a glacier does not initially have traces of life. Gradually, as the
rock is exposed to the action of the wind, of the water and temperature
fluctuations, it will suffer erosion. Sooner or later, the first plants and
microorganisms will invade the area and they will accelerate the formation
of soil. The invasion of many plant species will follow, which will create
more soil through their roots and their waste. With the establishment of a
4.3.6 Sulphur cycle
Sulphur (S) is an essential component of some amino acids. Its cycle is
mostly realised in the aquatic environment and the soil, whereas gaseous
sulphur compounds, such as sulphur dioxide and hydrogen sulphide, have
a secondary part in its circulation in the biosphere. Sulphur’s basic source
for organisms is sulphide salts that are dissolved in the water. These are
absorbed by plants and are used for protein production. Sulphur’s organic
compounds circulate like the other substances through food chains and are
finally decomposed by special bacteria that generate hydrogen sulphide.
These microorganisms live in anaerobic conditions within the black silt
existing in the bottom of certain seas or lakes; many of them produce the
energy they need by the reduction of sulphide compounds. Conversely,
some autotrophic bacteria construct organic substances thanks to the
energy they produce from the oxidation of hydrogen sulphite to sulphur
compounds. Additionally, a part of S found in the bottom of the sea settles
in the sediments in the form of sulphide iron, whereas phosphorus is simultaneously released from the sediments, becoming soluble in the water.
This is a remarkable example of the combination and mutual regulation
of two different biogeochemical cycles. Finally, the biosphere is constantly
enriched by gaseous S compounds through volcanic activity, as well as
by human actions, as the combustion of fossil fuels, which have become
significant, especially in the last century.
4.4 EcoSyStEMS’ changE in tiME
4.4.1 Ecological Succession
Ecosystems are continuously changing under the influence of external
and internal factors. The time scale of the short-term change is that of the
organisms’ life span order. Long term, ecosystems’ changes correspond to
scales of hundreds, thousands and millions of years, following the changes
of geological conditions or of the climate, as well as the species’ evolution.
Moreover, a middle term change is observed in certain ecosystems in the
form of gradual maturation of biological communities. It is completed in
time scales of years, decades or centuries, at the most.
A piece of land that came to the surface by volcanic action or by the
recess of a glacier does not initially have traces of life. Gradually, as the
rock is exposed to the action of the wind, of the water and temperature
fluctuations, it will suffer erosion. Sooner or later, the first plants and
microorganisms will invade the area and they will accelerate the formation
of soil. The invasion of many plant species will follow, which will create
more soil through their roots and their waste. With the establishment of a