Download Introduction to Biogeochemical Cycles

The following slides are provided by
Dr. Vincent O’Flaherty.
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Symbiotic nitrogen fixation
1. Legume symbioses
Most NB examples of nitrogen-fixing symbioses
are the root nodules of legumes (peas, beans,
clover, etc.).
Bacteria are Rhizobium species, but the root
nodules of soybeans, chickpea and some other
legumes are formed by small-celled rhizobia
termed Bradyrhizobium
Bacteria "invade" the plant and cause the
formation of a nodule by inducing localised
proliferation of the plant host cell
Chemicals called lectins act as signal molecules
between Rhizobium and its plant host - v.
specific
Bacteria form an “infection thread” and
eventually burst into the plant cells - cause cells
to proliferate - form nodules
Bacteria always separated from the host
cytoplasm by being enclosed in a membrane
In nodules - plant tissues contain the oxygenscavenging molecule - leghaemoglobin
Function of this molecule is to reduce the
amount of free O2, protects the N-fixing enzyme
nitrogenase, which is irreversibly inactivated
by oxygen
Bacteria are supplied with ATP (80%),
substrates and an excellent growth
environment by the plant -carry out Nfixation
Bacteria provide plant with fixed N major advantage in nutrient poor soils
Other symbiotic associations
2. Frankia form nitrogen-fixing root nodules
(sometimes called actinorhizae) with several
woody plants of different families, such as alder
3. Cyanobacteria often live as free-living
organisms in pioneer habitats such as desert
soils (see cyanobacteria) or as symbionts with
lichens in other pioneer habitats
The nitrogen cycle
Diagram shows an overview of the
nitrogen cycle in soil or aquatic
environments
At any time a large proportion of the
total fixed nitrogen will be locked up in
the biomass or in the dead remains of
organisms
So, the only nitrogen available to support new
growth will be that which is supplied by
NITROGEN FIXATION from the atmosphere
(pathway 6)
or by the release of ammonium or simple
organic nitrogen compounds through the
decomposition of organic matter (pathway 2
(AMMONIFICATION/MINERALISATION)
Other stages in this cycle are mediated by
specialised groups of microorganisms NITRIFICATION AND DENITRIFICATION
Nitrification
Nitrification - conversion of ammonium to
nitrate (pathway 3-4)
Brought about by the nitrifying bacteria,
specialised to gain energy by oxidising
ammonium, while using CO2 as their source of
carbon to synthesise organic compounds
(chemoautotrophs)
The nitrifying bacteria are found in most soils
and waters of moderate pH, but are not active
in highly acidic soils
Found as mixed-species communities
(consortia) because some - Nitrosomonas sp. are specialised to convert ammonium to nitrite
(NO2-) while others - Nitrobacter sp. - convert
nitrite to nitrate (NO3-)
Accumulation of nitrite inhibits Nitrosomonas,
so depends on Nitrobacter to convert this to
nitrate, and Nitrobacter depends on
Nitrosomonas to generate nitrite
Nitrate leaching from soil is a serious problem
in Ireland
Denitrification
Denitrification - process in which nitrate is converted to
gaseous compounds (nitric oxide, nitrous oxide and N2).
Several types of bacteria perform this conversion when
growing on organic matter in anaerobic conditions
Use nitrate in place of oxygen as the terminal electron
acceptor. This is termed anaerobic respiration and can
be illustrated as follows:
In aerobic respiration (as in humans), organic
molecules are oxidised to obtain energy, while
oxygen is reduced to water:
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy
In the absence of oxygen, any reducible
substance such as nitrate (NO3-) could serve the
same role and be reduced to nitrite, nitric
oxide, nitrous oxide or N2
Conditions in which we find denitrifying organisms: (1)
a supply of oxidisable organic matter, and (2) absence
of oxygen but availability of reducible nitrogen sources
Common denitrifying bacteria include several sp. of
Pseudomonas, Alkaligenes and Bacillus. Their activities
result in substantial losses of N into the atmosphere,
roughly balancing the amount of nitrogen fixation that
occurs/year
Microbial NFixation
Other Biogeochemical cycles P
and S
Other major nutrient cycles include S
and P
Sulfur cycle involves the cycling of
elemental Sulfur (So), Sulphate (SO42-)
and hydrogen sulphide (H2S) and organic
matter (-SH)
The Sulfur Cycle
Some major steps in the sulfur cycle include:
1.Assimilative reduction of sulfate (SO42-) into SH groups in proteins (cysteine) carried out by
virtually all bacteria
2.Release of -SH to form H2S during excretion
and decomposition
3.Oxidation of H2S by chemolithotrophs to
form sulfur (So) and sulfate (SO42-)
4.Dissimilative reduction of sulfate (SO42-) by
anaerobic respiration of sulfate-reducing
bacteria.
5.Anerobic oxidation of H2S and S by
anoxygenic phototrophic bacteria (purple and
green bacteria)
The Sulfur Cycle
The sulfur cycle includes more steps.
Sulfur compounds undergo some
interconversions due to chemical and
geologic processes (slow flux)
Human impact on the S-cycle is through
the production of SO2 through fossil fuel
combustion
The Phosphorus Cycle
The major reservoir of P is locked in a slow
geochemical flux between rocks n sediments
and soils release slowly over millennia
The ecosystem phase of the phosphorus cycle
moves faster than the sediment phase. All
organisms require phosphorus for synthesizing
phospholipids, NADPH, ATP, nucleic acids, and
other compounds. Plants absorb phosphorus v.
quickly, and herbivores get phosphorus by
eating plants
Carnivores get phosphorus by eating
herbivores. Eventually both of these organisms
will excrete phosphorus as a waste
Then DECOMPOSITION will release
phosphorus into the soil. Plants absorb the
phosphorus from the soil and they recycle it
within the ecosystem