BIO321 Aquatic Biology
Nitrogen in Lake Ecosystems
Horne & Goldman Chapter 8 - Know and understand all figures
I. Why is Nitrogen important in aquatic ecosystems?
A. Life requires Nitrogen as it is the 4th most abundant material in living cells, making up 5% of living organisms by weight (C>H>O>N).
B. How is N used?
1. DNA, RNA, proteins, Hemoglobin, Chlorophyll
2. Breakdown products of metabolism
NH4, Uric acid, Urea - WHY is N the main waste form?
II. The nitrogen equation is really a complex cycle that is global in nature.
A. Review the Nitrogen Cycle (see Horne & Goldman)
B. Aquatic components of the cycle are similar to and connect into the larger terrestrial cycle.
C. Major Components
N2 or N=N is an intert component that makes up 80% of the atmosphere
NH4+-N (ammonium) is a reduced form readily used by plants in aquatic systems
NO3--N (nitrate)is an oxidized form also readily used by plants in aquatic systems
NO2--N (nitrite) is a minor intermediate form that is "toxic"
NH2-N is the most common biological form
III. How do these forms get into aquatic systems and what roles do they play?
A. N2 profiles are largely determined by physical factors:
1. is less soluble than O2
2. higher partial pressure
3. 1 & 2 result in the same concentration of dissolved N2 in water as O2
4. Temperature is the main factor determining the distribution of N2 in water, since outside of formation processes (denitrification) and breakdown processes (fixation) N2 is biologically inert.
Summer depth profiles
oligotrophic eutrophic
B. Breakdown processes = Nitrogen fixation
1. The equation: N2 NH4+
nitrogenase (with Mb& Fe factions)
2. Energy is required to break the triple bond
a. High energy via lightening, volcanoes, oil power
b. Biological energy under special low O2 conditions
blue green algae in the epilimnion
bacteria and attached blue green algae in the sediments
photosynthesis which requires O2 provides the energy and ironically fixation can only happen in the absence of O2, which reduces the high energy cost of breaking the triple bond. - a contradiction?
3. Nitrogenase only functions in a reducing atmosphere (anoxic conditions)
Such conditions are found under only special conditions or in special places
a. In the sediments O2 is often low enough for bacteria and attached bluegreen algae to fix N2
b. Bluegreen algae
i. Heterocyst formation provides a low O2 environment in the epilimnion while still allowing access to O2
If the C:N ration is > 8:1 N may be limiting and heterocyst formation is induced [conversely a negative feedback system exists in that high concentrations of NO3- or NH4+ inhibit nitrogenase synthesis bringing about a reduction in heterocysts in the population. Nutrition and nitrogenase flow through the polar canals from the smaller vegetative cells. The heterocyst is yellow because it lacks the O2 producing Photosynthesis II)
ii. Non-heterocyst forming BGA's can also fix N2 if they lower the O2 concentration via other methods like a mucilagenous matrix around a colonial or individual form. The mucus contains sulfhydryl compounds that absorb free O2
iii. Benthic BGA or BGA in an anoxic hypolimnion
iv. summer blooms of Anabaena, Aphanizomenon, Nostoc, Microcystis, Oscillatoria when N is low.
4. Nitrogen fixation is important in lakes.
a. In Eutrophic lakes fixation is a major process as first NH4+ then NO3- are used and BGA's begin to bloom as they can fix N2. In Clear Lake CA overy 1/2 of the annual N budget is provided by Fixation
b. In Oligotrophic fixation is minor as there are very few users of N. N is also very rarely the limiting factor for life in these lakes.
C. Biological sources of other nitrogen forms
1. NH4 comes as a major waste product from most aquatic physiologies. This form is rapidly used (NH4 à NH2) by algae and plants
a. oligotrophic lakes 0 - 5 mg/l
b. eutrophic lakes 5 - 10 & >mg/l
2. Ammonification is the breakdown of dead organic nitrogen(NH2) by heterotrophic bacteria to form NH4+
3. Nitrificaton is a two step process where NH4+ is broken down to NO3-
-NH2 NH4+ NO2- NO3- + energy
4. Plants then reconvert the NO3- to the form they use via the enzyme nitrate reductase
NO3- NO2- NH4+
Nitrate reductase nitrate reductase
Note: Since this process requires costly enzymes, plants prefer to use NH4+ when it is available
5. Since redox factors apply to these equations, NH4+ is the most available form of N in low O2 environments, while in high O2 environments, NO3- predominates
D. Seasonal patterns and depth profiles of nitrogen forms
See Horn and Goldman Figures 8-2, 8-3, 8-4, & 8-5.