A seminar on

Nitrogen cycle in aquatic ecosystem

Submitted by,Asik Ikbal

B.F.Sc. 1st yr 2nd semFS-06/13

AEM:124

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.

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

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.

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).

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 (NH3 + H+ --> NH4

+). Ammonification is heterotrophic, under

aerobic or anaerobic conditions.

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.

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.

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.

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.)

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

-

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

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.

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 NO2

- and Cl-

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 NH4

+.

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.

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

Thank

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