1. 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 (N 2 ) (oxidation state =0) Most algae and plants require NO 3 ¯(+5) (NO 2 ¯ ) (+3) or NH 3 (NH 4 + ) (-3)to synthesize amino acids to make proteins N-fixing microorganisms can take up N 2 and convert it to NH 3 N 2 + 3H 2 → 2NH 3 Many plants have N-fixing mutualists (eg Azolla) Denitrifying bacteria can convert NO 3 ¯ back to N 2

2. 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.

4. 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 O 2 NO 3 - —the most oxidized form of N occurs highest in the water column where there is O 2 present NH 4 + or NH 3, the most reduced form is prevalent deep where O 2 is absent or nearly so N 2 O and NO 2 - —are intermediate oxidation states

5. 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

7. 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.