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The Cycling of Nitrogen
N is an important nutrient that frequently limits primary productivity in aquatic ecosystems
It is rare in the earth’s crust, but makes up 79% of the atmosphere (N2)(oxidation state =0)
Most algae and plants require NO3¯(+5) (NO2 ¯) (+3) or NH3 (NH4
+) (-3)to synthesize amino acids to make proteins
N-fixing microorganisms can take up N2 and convert it to NH3
N2 + 3H2 → 2NH3
Many plants have N-fixing mutualists (eg Azolla)
Denitrifying bacteria can convert NO3¯ back to N2
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Azolla, an aquatic fern used in rice culture
•The leaves of this aquatic fern have cavities that harbour filamentous cyanobacteriaAnabaena azollae
•The large cells (heterocysts) are specialized for N-fixation
•Traditional rice farming in many countries involve planting Azolla to build up N concentrations in rice paddy.
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•Nutrients like N and P tend to accumulate in the hypolimnion during summer stratification—sedimentation.
•In eutrophic lakes the deep layers become very depleted in O2
•NO3-—the most oxidized form of
N occurs highest in the water column where there is O2 present
•NH4+ or NH3, the most reduced
form is prevalent deep where O2 is absent or nearly so
•N2O and NO2-—are intermediate
oxidation states
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The Nitrogen cycle involves many different oxidation states, and the redox processes are facilitated by plants and wide variety of bacteria
-3
+3
+5
0
+1
Nitrite
Photoautotrophs (PA) Chemoautotrophs(CA)
CH
Chemoheterotrophs (CH)
PA
CH
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This graph shows Nitrate concentrationsIn large rivers as a function of human population density
This graph shows Nitrate export from large river watersheds as a function of human population density
Question?? ExplainPrairie rivers and watersheds are ‘high’ in Nitrates and Nitrate export, even though population density is low.