Nitrogen cycle in aquatic ecosystem...................................
1. A seminar on
Nitrogen cycle in aquatic ecosystem
Submitted by,
B.F.Sc. 1st yr 2nd sem
FS-06/13
2. Introduction:
Nitrogen is essential for many processes and is
crucial for any life on Earth.
It is a component in all amino acids, as
incorporated into proteins, and is present in the
bases that make up nucleic acids, such as DNA
and RNA.
The process of converting nitrogen into
compounds that can be used by plants and
animals is called the nitrogen cycle.
Although Earth’s atmosphere is an abundant
source of nitrogen, most is relatively unusable
by plants.
This trans- formation can be carried out through
both biological and physical processes.
3. Cycling of Nitrogen in Aquatic Ecosystem :
Four processes participate in the
cycling of nitrogen through the
biosphere as well as aquatic
ecosystem :
1) Nitrogen fixation
2) Decay
3) Nitrification
4) Denitrification
Microorganisms play major roles
in these processes
4. Fixation
Nitrogen fixation refers to the
conversion of N2 to either NO3
or NH4(Organic N)by bacteria.
Terrestrial systems: Soil
bacteria in root nodules of
legumes.
Aquatic systems: Blue green
algae( Anabaena, Nostoc,
Azolla)
Biological, meteorological,
industrial transformations
also occur.
5. 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 breakdown the molecules in excretions and dead
organisms into ammonia (NH3).
6. Contd…..
NH3 separated from organic protein via microbial
activity.
This process referred to as deaminification or
ammonification.
NH3 is released to water column (mineralization)
and assimilated into primary productivity (NH+ H+
3 --> NH+).
4
Ammonification is heterotrophic, under aerobic or
anaerobic conditions.
7. Nitrification
The term nitrification
refers to the conversion of
ammonium to nitrate
(pathway 3-4 opposite).
Responsible: nitrifying
bacteria known as
chemoautotrophs.
These bacteria gain their
energy by oxidizing NH3,
while using CO2 as a source
of carbon to synthesize
organic compounds.
8. Contd…..
First step by Nitrosomonas sp.
second step by Nitrobacter sp.
Both steps/reactions use NH4
+ and NO2
- as an energy
source, CO2 as a carbon source.
This is a non-photosynthetic type of growth. NH3
and NH4
+ are both either assimilated by aquatic
plants for growth or nitrified (oxidized) to NO3
-
(nitrate).
Nitrate can also be used as a growth substrate.
9. Two step process:
NH4
+ + 1.5O2
NO2
- + 2H+ + H2O
NO2
- + 0.5O2
NO3
-
These are oxygen-driven reactions. Reaction runs best at
pH 7-8 and 25-30oC.
However, under low DO, it runs in reverse.
It occurs in the hypolimnion under eutrophic (stagnant)
conditions.
10. Denitrification
By this process, NO3 in soil
or water is converted into
atmospheric N2, nitric
oxide or nitrous oxide.
This must occur under
anaerobic conditions
(anaerobic respiration).
Presence of O2 can reverse
the reaction.
It is again, mediated by
bacteria (Pseudomonas sp.,
Alkaligenes sp. and Bacillus
sp.)
11. Nitrogen: aqueous forms
Gaseous form of nitrogen (N2) is most prevalent.
Gaseous followed by: nitrite, nitrate, ammonia or
ammonium.
Ratio of NH3:NH4
+ rises with pH.
Unfertilized ponds: TAN (NH3 +NH4
+) = 0.05-0.075
mg/L.
Fertilized ponds: TAN = 0.5 mg/L, 0.075 mg NO3
-
12. Nitrogen Amendments
Nitrogen added as fertilizer to
ponds: urea
Immediately upon addition, it
starts to decline.
Only small portion detectable
from metabolic processes.
Plants typically take it up, die,
mud deposit.
Inorganic nitrogen typically
denitrified in the hypolimnion.
High afternoon pH = increased
volatization.
urea
13. Affects of Nitrogen Cycle In Aquatic Ecosystem :
Ammonia Toxicity :
Both NH3 and NH4
+ are toxic to fish/invertebrates.
In Blood/tissue of fishes if NH3 increases then blood pH also
increases.
It causes imbalance in enzyme activity, reduced membrane
stability.
Increases O2 consumption by tissues, gill damage, reduced O2
transport.
Reduced growth of fishes, histological changes in gills/other
organs.
Toxicity tolerance varies due to biological variability of
different strains of species, Eggs are most tolerant (fish),
Larvae least tolerant, olders are more tolerant.
NH3 is more toxic when DO levels are low.
14. Nitrate Toxicity :
Nitrite reacts with hemoglobin to form methemoglobin.
In this process, iron converted from ferrous (Fe2+) to ferric
(Fe3+) form.
Ferric form of iron cannot bind with oxygen,so blood
changes from red to brown.
Those fish having methemoglobin reductase enzyme can
convert iron moeity back to ferrous.
Recovery from nitrite toxicity usually occurs when fish are
transferred to better water.Complete recovery can occur in
24 h.
Nitrite is quickly transported across gill membrane by
lamellar chloride cells.So cells can’t distinguish between
NO- and Cl-
2
15.
16.
17. Conclusion:
Nitrogen Cycle play an important role as bio-geochemical
cycle.It carries on the balance of aquatic ecosystem.
Though free nitrogen is not present in aquatic
environment,but it present in aquatic ecosystem in a
combined form as NH+.
4
Chemical processing, or natural fixation are necessary to
convert gaseous nitrogen into compound forms by living
organisms, which makes nitrogen a crucial component of
food production.
The nitrogen cycle is the process by which nitrogen is
converted between its various chemical forms.
18. Reference:
WEBSITES :
www.google.com
www.wikipedia.com
www.ask.com
BOOKS :
“Fundamental of Aquatic Ecology” : Wiley,2nd
Edition.
“Freshwater Ecology”(2nd Edition) – Walter
K.Dodds and Matt R.Whiles.
“Introduction to Limnology”- Prof. S.Welch