The Phosphorus Cycle
Human activities have played a major role in altering the balance of the global sulfur cycle. The burning of large quantities of fossil fuels, especially from coal, releases larger amounts of hydrogen sulfide gas into the atmosphere. As rain falls through this gas, it creates the phenomenon known as acid rain, which damages the natural environment by lowering the pH of lakes, thus killing many of the resident plants and animals. Acid rain is corrosive rain caused by rainwater falling to the ground through sulfur dioxide gas, turning it into weak sulfuric acid, which causes damage to aquatic ecosystems. Acid rain also affects the man-made environment through the chemical degradation of buildings. For example, many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years. These examples show the wide-ranging effects of human activities on our environment and the challenges that remain for our future.
The Biogeochemical Carbon Cycle
The Carbon Cycle
The six aforementioned elements are used by organisms in a variety of ways. Hydrogen and oxygen are found in water and organic molecules, both of which are essential to life. Carbon is found in all organic molecules, whereas nitrogen is an important component of nucleic acids and proteins. Phosphorus is used to make nucleic acids and the phospholipids that comprise biological membranes. Lastly, sulfur is critical to the three-dimensional shape of proteins.
Human activity can alter the nitrogen cycle by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers (which contain nitrogen and phosphorus compounds) in agriculture, which are then washed into lakes, streams, and rivers by surface runoff
Nitrogen enters the living world through free-living and symbiotic bacteria
incorporate nitrogen into their organic molecules
through specialized biochemical processes
Certain species of bacteria are able to perform nitrogen fixation
the process of converting nitrogen gas into ammonia
(NH3), which spontaneously becomes ammonium (NH4+)
Ammonium is converted by bacteria into nitrites (NO2−)
and then nitrates (NO3−)
At this point, the nitrogen-containing molecules are used by plants and other producers to make organic molecules such as DNA and proteins.
This nitrogen is now available to consumers.
the nitrogen that enters living systems is eventually converted from organic nitrogen back into nitrogen gas by bacteria
The process of denitrification is when bacteria convert the nitrates into nitrogen gas, thus allowing it to re-enter the atmosphere.
is associated with several effects on Earth’s ecosystems including the production of acid rain
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