Oxygen cycle

Oxygen cycle

The term oxygen cycle or oxygen cycle means the transport and storage of oxygen in the earth's atmosphere , biosphere and lithosphere . This cycle is a biogeochemical cycle . The most important driving factor in the oxygen cycle is photosynthesis , which is responsible for the modern earth's atmosphere (see Great Oxygen Disaster ). The oxygen cycle is connected to the other material cycles through oxidations and reductions . These take place particularly quickly with many other elements because of the high reactivity of oxygen.

Storage

The greatest oxygen storage of the earth is in the silicate - and oxide - minerals of the earth's crust and mantle (99.5%). Only a small part is released into the biosphere (0.01%) and the atmosphere (0.36%) as free oxygen. The main source of atmospheric free oxygen is photosynthesis, which creates free oxygen from water :

${\displaystyle \mathrm {6\ CO_{2}+12\ H_{2}O+Energie\longrightarrow C_{6}H_{12}O_{6}+6\ O_{2}+6\ H_{2}O} }$

Coupling of the C, H and O cycle in the metabolism of photosynthetic plants (oxygen is initially a "waste product" of the photolysis of the water)

Photosynthetic organisms (photoautotrophic) are primarily plants, especially land plants and the phytoplankton of the oceans. Photoautotrophic bacteria and archaea also exist . For example, the tiny marine cyanobacterium Prochlorococcus is blamed for more than half of the photosynthesis in the open ocean. It wasn't discovered until 1986.

An additional source of free atmospheric oxygen is photolysis , in which high-energy ultraviolet radiation breaks atmospheric water and nitrous oxide into atoms . The free H and N atoms ( radicals ) react with other compounds in the atmosphere, or the hydrogen escapes into space:

${\displaystyle \mathrm {2\ H_{2}O+Energie\longrightarrow 4\ H+O_{2}} }$
${\displaystyle \mathrm {2\ N_{2}O+Energie\longrightarrow 4\ N+O_{2}} }$

Free oxygen can disappear from the atmosphere through respiration and decomposition , mechanisms in which heterotrophic organisms such as B. Animals and bacteria consume oxygen and release carbon dioxide.

The lithosphere also consumes free oxygen through chemical weathering and surface reactions. An example is the formation of iron oxide (rust):

${\displaystyle \mathrm {4\ FeO+O_{2}\longrightarrow 2\ Fe_{2}O_{3}} }$

Capacities and flows

The following tables show estimates of oxygen cycle reservoirs and flows .

Table 1: Important reservoirs involved in the oxygen cycle:

Storage	capacity	Flow (kg O 2 per year)	Dwell time
the atmosphere	1.4 * 10 18	30,000 * 10 10	4,500
biosphere	1.6 * 10 16	30,000 * 10 10	50
Lithosphere	2.9 * 10 20	60 * 10 10	500,000,000

Table 2: Annual gains and losses of atmospheric oxygen (units of 10 ¹⁰ kg O ₂ per year):

Photosynthesis (land) Photosynthesis (ocean) Photolysis of N 2 O Photolysis of H 2 O	16,500 13,500 1.3 0.03
all in all	~ 30,000
Losses - breathing and putrefaction
aerobic respiration microbial oxidation combustion of fossil fuels (anthropogenic) photochemical oxidation fixation of N 2 by lightning strikes fixation of N 2 by industrial oxidation of volcanic gases	23,000 5,100 1,200 600 12 10 5
Losses - weathering
chemical weathering surface reaction of O 3	50 12
all in all	~ 30,000

Ozone-oxygen cycle

→ Main article : ozone-oxygen cycle

The presence of atmospheric oxygen has led to the formation of ozone (O ₃ ) and the ozone layer in the stratosphere . The ozone layer is very important as it absorbs the harmful UV rays : ${\displaystyle \mathrm {O_{2}+(Strahlung<240nm)\longrightarrow 2O} }$
${\displaystyle \mathrm {O+O_{2}\longrightarrow O_{3}} }$

literature

• P. Cloud, A. Gibor: The oxygen cycle. In: Scientific American. September 1970, pp. 110-123.
• J. Fasullo: Substitute Lectures for ATOC 3600: Principles of Climate, Lectures on the global oxygen cycle .
• RM Morris: OXYSPHERE - A Beginners' Guide to the Biogeochemical Cycling of Atmospheric Oxygen. ( Memento from April 15, 2009 in the Internet Archive )

Individual evidence

1. ↑ oxygen cycle . GeWeb.de, accessed on December 30, 2012 . 
2. ↑ Steve Nadis: The Cells That Rule the Seas. ( Memento of October 12, 2007 in the Internet Archive ) In: Scientific American . Nov. 2003. (English)
3. ↑ JCG Walker: The oxygen cycle in the natural environment and the biogeochemical cycles. Springer-Verlag, Berlin 1980.