Cycles

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Cycles

• Developed by Adam F Sprague

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Carbon Cycle

• Carbon exists in the nonliving environment as
• carbon dioxide (CO2) in the atmosphere and
  dissolved in water (forming HCO3-)
• carbonate rocks (limestone and coral CaCO3)
• deposits of coal, petroleum, and natural gas
  derived from once-living things
• dead organic matter, e.g., humus in the soil

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Carbon enters the biotic world through the action
of autotrophs

• primarily photoautotrophs, like plants and algae,
  that use the energy of light to convert carbon
  dioxide to organic matter. and to a small extent,
  chemoautotrophs bacteria and arcahaens that do
  the same but use the energy derived from an
  oxidation of molecules in their substrate.

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Carbon returns to the atmosphere and water by

• respiration (as CO2)
• burning
• decay (producing CO2 if oxygen is present,
  methane (CH4) if it is not.

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The Nitrogen Cycle

• All life requires nitrogen-compounds, e.g.,
  proteins and nucleic acids.
• Air, which is 79 nitrogen gas (N2), is the major
  reservoir of nitrogen.
• But most organisms cannot use nitrogen in this
  form.
• Plants must secure their nitrogen in "fixed"
  form, i.e., incorporated in compounds such as
• nitrate ions (NO3-)
• ammonia (NH3)
• urea (NH2)2CO
• Animals secure their nitrogen (and all other)
  compounds from plants (or animals that have fed
  on plants).

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Four processes participate in the cycling of
nitrogen through the biosphere

• nitrogen fixation
• decay
• nitrification
• denitrification

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Nitrogen Fixation

• Three processes are responsible for most of the
  nitrogen fixation in the biosphere
• atmospheric fixation by lightning
• biological fixation by certain microbes alone
  or in a symbiotic relationship with plants
• industrial fixation

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Atmospheric Fixation

• The enormous energy of lightning breaks nitrogen
  molecules and enables their atoms to combine with
  oxygen in the air forming nitrogen oxides. These
  dissolve in rain, forming nitrates, that are
  carried to the earth.
• Atmospheric nitrogen fixation probably
  contributes some 5 8 of the total nitrogen
  fixed.

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Industrial Fixation

• Under great pressure, at a temperature of 600C,
  and with the use of a catalyst, atmospheric
  nitrogen and hydrogen (usually derived from
  natural gas or petroleum) can be combined to form
  ammonia (NH3). Ammonia can be used directly as
  fertilizer, but most of its is further processed
  to urea and ammonium nitrate (NH4NO3).

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Biological Fixation

• The ability to fix nitrogen is found only in
  certain bacteria.
• Some live in a symbiotic relationship with plants
  of the legume family (e.g., soybeans, alfalfa).
• Some establish symbiotic relationships with
  plants other than legumes (e.g., alders).
• Some nitrogen-fixing bacteria live free in the
  soil.
• Nitrogen-fixing cyanobacteria are essential to
  maintaining the fertility of semi-aquatic
  environments like rice paddies.

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Decay

• The proteins made by plants enter and pass
  through food webs just as carbohydrates do. At
  each trophic level, their metabolism produces
  organic nitrogen compounds that return to the
  environment, chiefly in excretions. The final
  beneficiaries of these materials are
  microorganisms of decay. They break down the
  molecules in excretions and dead organisms into
  ammonia.

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Nitrification

• Ammonia can be taken up directly by plants
  usually through their roots. However, most of the
  ammonia produced by decay is converted into
  nitrates. This is accomplished in two steps
• Bacteria of the genus Nitrosomonas oxidize NH3 to
  nitrites (NO2-).
• Bacteria of the genus Nitrobacter oxidize the
  nitrites to nitrates (NO3-).
• These two groups or autotrophic bacteria are
  called nitrifying bacteria. Through their
  activities (which supply them with all their
  energy needs), nitrogen is made available to the
  roots of plants.

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Denitrification

• The three processes above remove nitrogen from
  the atmosphere and pass it through ecosystems.
• Denitrification reduces nitrates to nitrogen gas,
  thus replenishing the atmosphere.
• Once again, bacteria are the agents. They live
  deep in soil and in aquatic sediments where
  conditions are anaerobic. They use nitrates as an
  alternative to oxygen for the final electron
  acceptor in their respiration.
• Thus they close the nitrogen cycle.

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Phosphorus Cycle

• Phosphorus is the key to energy in living
  organisms, for it is phosphorus that moves energy
  from ATP to another molecule, driving an
  enzymatic reaction, or cellular transport.
  Phosphorus is also the glue that holds DNA
  together, binding deoxyribose sugars together,
  forming the backbone of the DNA molecule.
  Phosphorus does the same job in RNA.

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Plants

• Again, the keystone of getting phosphorus into
  trophic systems are plants. Plants absorb
  phosphorous from water and soil into their
  tissues, tying them to organic molecules. Once
  taken up by plants, phosphorus is available for
  animals when they consume the plants.

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Water cycle

• When plants and animals die, bacteria decomposes
  their bodies, releasing some of the phosphorus
  back into the soil. Once in the soil, phosphorous
  can be moved 100s to 1,000s of miles from were
  they were released by riding through streams and
  rivers. So the water cycle plays a key role of
  moving phosphorus from ecosystem to ecosystem.

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Rocks

• In some cases, phosphorous will travel to a lake,
  and settle on the bottom. There, it may turn into
  sedimentary rocks, limestone, to be released
  millions of years later. So sedimentary rocks
  acts like a back, conserving much of the
  phosphorus for future eons.

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The Sulfur Cycle

• An important distinction between cycling of
  sulfur and cycling of nitrogen and carbon is that
  sulfur is "already fixed". That is, plenty of
  sulfate anions (SO42-) are available for living
  organisms to utilize.

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Given that Sulfur Is "Already Fixed", Why Bother
Studying the Sulfur Cycle?

• 1. Environmental impacts are diverse and
  important locally even on a human time scale
• a. Some of the reactions that occur in the sulfur
  cycle open up new environments to life. They
  support biological communities in unlikely places
  such as deep sea thermal vents, areas of low pH
  and areas of high temperature.
• b. On the other hand, certain reactions remove
  needed metabolites or produce wastes that make
  environments uninhabitable to some organisms.

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Sulfur

• Proteins are not only made from carbon and
  nitrogen, but many important proteins also
  contain sulfur. Sulfur is also an important
  component of coenzyme A, which is used to produce
  energy in cellular respiration. So the
  availability of sulfur is essential to
  maintaining life.

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2S H2O 3O2 ---gt 2H2SO4

• Just as plants can not convert N2 into something
  useful in the nitrogen cycle, neither can plants
  use elemental sulfur 2S. Again, plants are
  depended upon bacteria, in this case
  chemoautotrophic bacteria, which oxidizes
  elemental sulfur to sulfates, as in the above
  formula

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Sulfate

• Once in the form of sulfate (2H2SO4), plants can
  then incorporate the sulfur into proteins.

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H2S

• Sulfur shares other characteristics to the
  nitrogen cycle. 2H2SO4 can be converted into
  hydrogen sulfide (H2S) by sulfur bacteria, as can
  proteins when were broken down by decomposers.

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