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
Azolla, an aquatic fern used in rice culture
•The leaves of this aquatic fern
have cavities that harbour
filamentous cyanobacteria
Anabaena 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.
•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
The Nitrogen cycle involves many different oxidation states, and the
redox processes are facilitated by plants and wide variety of bacteria
Chemoheterotrophs (CH)
-3
CH
PA
0
+1
+3
Nitrite
CH
+5
Photoautotrophs (PA)
Chemoautotrophs(CA)
This graph shows Nitrate concentrations
In 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?? Explain
Prairie rivers and watersheds are ‘high’
in Nitrates and Nitrate export, even
though population density is low.