Decomposers and natural
How elements are re-used
(k) describe the role of decomposers in the
decomposition of organic material;
(l) describe how microorganisms recycle nitrogen
within ecosystems. (Only Nitrosomonas, Nitrobacter
and Rhizobium need to be identified by name).
The syllabus says:
Loss to erosion or leaching
Soil nutrient pool
Forest: nutrients are put back into the soil slowly,
so organic material builds up
Tropical rain forest: decomposition is rapid so
there is very little organic build up
Result: if living material is removed from tropical rain
forests, the soil is nutrient poor to support new growth
Bacteria and archaeaBacteria and archaea
Breaking down organic material
Organisms that feed on dead organic matter, releasing molecules and minerals
which then become available to other organisms in the ecosystem
• Consumers feed on organisms or
particulate organic matter.
• Decomposers utilize complex compounds
in dead organic material.
• Bacteria and fungi are the main groups of
• Bacteria are the main feeders on animal
• Fungi feed primarily on plants, although
bacteria also are important in some plant
The Nitrogen Cycle
• All living things need nitrogen-containing compounds,
which include proteins and nucleic acids.
• 78% of the atmosphere is nitrogen gas, but most organisms
cannot use nitrogen in this form.
• The nitrogen cycle is an important nutrient cycle describing
how nitrogen is stored and transferred.
• There are five main processes in the nitrogen cycle:
1) Nitrogen fixation
nitrogen in the
nitrifying bacteria nitrifying bacteria
The Nitrogen Cycle
• This is the process whereby nitrogen in the atmosphere is converted
into a form plants can use.
• 78% of the atmosphere is nitrogen gas (N2):
The triple bond linking the two nitrogen atoms makes it a very stable
molecule, so it doesn't readily take part in chemical reactions and can’t
be used by plants or animals as a source of nitrogen.
• There are three main ways in which N2 is "fixed" into useful compounds:
1) Biological - through the action of bacteria (Enzymes)
2) Atmospheric - through chemical reactions caused naturally by
3) Industrial - through human-induced chemical reactions
• Nitrogen fixing bacteria reduce nitrogen gas (N2) to
ammonia (NH3) which then dissolves to form ammonium ions (NH4
• This reaction is catalysed by the enzyme nitrogenase and requires
lots of energy: 15 ATP molecules to fix each molecule of N2.
• Some nitrogen-fixing bacteria live freely in soil (Azotobacter), but
most live in colonies inside root nodules of leguminous plants such
as clover or peas (Rhizobium)
• This is a classic example of symbiosis, where both organisms
benefit. The leguminous plants gain a source of useful nitrogen from
the bacteria and the bacteria gain carbohydrates from the plant,
which they respire to make the ATP they need to fix nitrogen.
Rhizobium bacteria live in
nodules in the roots of legumes,
here they fix nitrogen.
• Atmospheric nitrogen (N2) can be oxidised to form nitrate
) and other oxides of nitrogen (N2O)
• This reaction happens naturally through the action of
• This is thought to have been a significant process in the
earth’s early atmosphere.
• Volcanic activity both in ancient times and now also
leads to increase in oxides of nitrogen in
• Nitrogen can be converted to ammonia by humans using
the Haber process:
N2 + 3H2 2NH3
• This reaction is used to make nitrate fertilisers.
• Today, almost a third of all nitrogen fixed is fixed in this
• Nitrification is the process by which ammonium is
converted into nitrates by bacteria in a series of oxidative
• The bacteria oxidise ammonia in two main stages, first
bacteria of the genus Nitrosomonas converts ammonia to
nitrite ions (N02
), then the bacteria of the genus Nitrobacter
convert nitrite ions to nitrate ions (NO3
• These oxidative reactions are exothermic. Energy is
released, which the nitrifying bacteria use to make ATP,
instead of using respiration.
• These bacteria are called chemoautotrophic bacteria.
• Assimilation is the building up of organic molecules
• Plants can make carbohydrates and lipids from CO2
and H2O, but to make proteins and nucleotides they need a
source of nitrogen.
• Plants acquire nitrogen in the form of dissolved nitrates. Plants
use active transport to accumulate nitrate ions in their root hair
cells against a concentration gradient.
• The supply of nitrates is a limiting factor in plant growth. That’s
why farmers add nitrate fertilisers to crops.
• Some plants living in extremely poor soils have developed an
unusual strategy to acquire nitrogen: they trap and digest insects.
• Animals get nitrogen by consuming plants and excrete
excess nitrogen as urea, ammonia or uric acid.
• Ammonification is the process in which nitrogen
compounds in the soil from dead animals and plants are
converted into ammonia.
• Microbial saprophytes (decomposers) break down proteins
in animal and plant matter to form ammonia in two stages
1) They digest proteins to amino acids using extracellular
2) They remove the amino groups from amino acids using
• The deaminated amino acids, containing Carbon, Hydrogen
and Oxygen, are respired by the saprophytes to CO2 and H2O
(this links in with the carbon cycle.)
• Denitrification releases nitrogen from nitrites and
nitrates into the atmosphere.
• Anaerobic denitrifying bacteria, which thrive in
waterlogged soil, convert nitrates to N2 (nitrogen)and
NOx (nitrous oxides) gases, which are then lost to the
• Nitrous oxides can increase global warming, they can
cause acid rain (nitric acid)
• Nitrous oxides also reduce the ozone layer which leads
to increases in UV rays which can cause skin cancer
• Another problem is that “useful” nitrogen is constantly
being lost from the soil.http://www.mhhe.com/biosci/genbio/casestudies/nitrogen.mhtml