Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;


Ecology:  Ecosystems & Nutrient Cycles

 I.  Requirements.

     Recall that organisms exchange materials with the environment.  Essentially, living things must obtain two major materials from the environment:


A.  Energy (click here for notes)


B.  Proper elemental building blocks

II.  Elements are recycled (unlike energy!).  
The cycling or exchange of elements through the ecosystem is called a biogeochemical cycle.  The elements ["chemical"] cycle between biotic (living organisms, "bio-") and abiotic (rock, air, water; "geo-") reservoirs.


A.  General model for a biogeochemical cycle (diagram provided in class)

  1. organisms comprise the biotic reservoir

  2. An element can exist in various forms in the biotic reservoir.   

  3. Elements move in the biotic reservoir via food chains/webs

  4. Element movement through the biotic reservoir parallels energy flow.

  5. Nutrient elements return to the abiotic reservoir via death, excretion, wastes.

  6. There are various types of abiotic reservoirs (air - atmospheric cycle such as for carbon and nitrogen; water - hydrological cycle especially for hydrogen and oxygen; rock - sedimentary cycle as for phosphorus). 

  7. Elements exist in various forms in the abiotic reservoir.

  8. Geological activity moves materials through the abiotic reservoir.  

  9. Movement through the abiotic reservoir is typically much slower than through the biotic reservoir.

 B.  Points to consider.
    As you study each biogeochemical cycle, consider the following questions:

  1. In what form does the element exist in the abiotic and biotic reservoirs? 

  2. In what form does the element return to the abiotic reservoir (and by what process/es)?  

  3. How did the element enter the biotic reservoir and in what form?  

  4. How does the element move in the biotic and abiotic reservoirs?  

  5. Does the element move fast or slow?  

  6. Is it an atmospheric, sedimentary or hydrological cycle?

III.  The Hydrological cycle - the cycling of water (hydrogen, oxygen).  
    See diagram in text.  We won't discuss this one directly so check it out in the text.  Some important points to remember:  evaporation, runoff, precipitation, ground water, transpiration, watershed, aquifer.


IV.  Carbon cycle - an atmospheric cycle.  

     See diagram in text and provided in class.  Points to consider:

  1. Abiotic reservoir = carbon dioxide in air (0.033%) and dissolved in water (carbonates), rock/fossils = petroleum products, limestone, sediments.  

  2. Biotic reservoir - carbon is the basic structural building block for virtually all molecules (such as proteins, carbohydrates, lipids or fats, and nucleic acids).

  3. Carbon enters the biotic reservoir via photosynthesis by plants

  4. Carbon moves in the biotic reservoir via food chain

  5. Carbon returns to the abiotic reservoir via respiration, death, excretion

  6. Weathering, combustion, uplifting, etc., return carbon trapped in rock/petroleum to return to the atmosphere.

  7. Greenhouse Effect -  increased global temperature, like inside an automobile on a hot day, which is the result of increased levels of carbon dioxide and other greenhouse gases like CFC's (industrial byproduct, 15% of total), methane (microbial fermentations, especially in rice fields, cow guts and termite hills; 15%), and nitrous oxide (N2O, 10%).  They act like a transparent blanket that absorbs heat, but allows light to penetrate.  Carbon dioxide levels have increased dramatically in recent history because:  (a) increased combustion (fossil fuels, slash/burn agriculture in tropics) and (b) deforestation (decreased number of plants that can remove carbon dioxide).

  8. There is a little controversy about the relative importance of organisms in this cycle.  Some believe that the role of organisms is minor compared to cycling that occurs in the abiotic reservoir.


IV.   Nitrogen Cycle - another atmospheric cycle.  
    About 80% of air is made up of N2 gas.  Points to consider:

  1. Abiotic reservoir - N2 (air), ammonia (NH4), nitrate (NO3), & nitrite (NO2).  The latter three are mostly dissolved in water.

  2. Biotic reservoir - nitrogen used in many molecules especially in proteins and nucleic acids (DNA)

  3. Nitrogen fixation - converts nitrogen gas into ammonia.  This is the result of the action of:  (a) aquatic microbes like Anabaena and Nostoc, which are types of blue-green algae that are better called cyanobacteria); (b) electrical discharges ("the grass is always greener after a thunder storm"), (c) free-living soil microbes (Azotobacter); and (d) symbiotic nitrogen-fixing bacteria (Rhizobium) in nodules (bubbles) on the roots of legumes (called poor-man's persons, rich in protein cause they fix nitrogen) and a few other plants.

  4. Plants absorb ammonia (conifers, grasses) or nitrate (most others).

  5. Nitrification -  conversion of ammonia to nitrate by soil microbes (Nitrosomas converts ammonia to nitrite; Nitrobacter converts nitrite to nitrate).  Favored by warm temperature and neutral pH.

  6. Wastes and decayed organic materials decomposed by another set of bacteria into ammonia - called ammonification.  Various microbes are responsible, favored by cool temperatures, all pH's.

  7. Denitrification - closes the cycle.  Returns nitrogen to the atmosphere.  Conversion of nitrate, nitrite, and/or ammonia back to nitrogen gas by other microbes.

  8. Note the heavy reliance on microbes for the function of this cycle.

  9. Nitrogen-fixing plants can usually out compete others in nutrient poor soil

  10. Nitrogen fixation is "expensive" -  it requires a lot of energy (high metabolic cost)

  11. Legumes are excellent green manure

V.  Phosphorus Cycle - a sedimentary cycle. 
    Much slower than other cycles.

  1. Phosphate is the main form of phosphorus in the abiotic reservoir that is available to plants.  Also occurs as guano, rock, fossils.   

  2. Phosphorus is used in nucleic acids, membranes, energy metabolism

  3. Plants absorb phosphate

  4. Phosphatizing bacteria return phosphate back to the  abiotic reservoir when they decompose organic wastes and decaying materials

  5. Phosphate is relatively insoluble in water - it precipitates out of solution easily.  Ultimately, phosphate forms deposits in the  oceans.  

  6. Phosphate returns (moved) via weathering, volcanic activity, etc.

  7. Eutrophication often related to phosphorus availability - which is typically a limiting element.  Do you recall our lab last semester when we studied the water chemistry of East Gemini Lake?


VI.  Homeostasis exists in nutrient cycles
    In a stable ecosystem, biogeochemical cycles show homeostasis.  In other words, the cycles are intact and efficient.  Or more simply stated:  Input = Output.

      For example, Hubbard Brook Forest in the White Mountains (NH) is a massive project by Gene Likens, G. Bormann and colleagues that demonstrated the importance of an intact ecosystem for nutrient cycling.  Hubbard Brook is comprised of a series of forested valleys each with watershed that is drained by a single stream.  Thus, it was an ideal "contained" system.  Scientists could measure inputs and outputs before and after clear cutting the forest (Table 1).

Table 1:  Selected results from studies on nutrient cycling at Hubbard Brook Forest (arbitrary units)


Undisturbed (before clear cutting)

Disturbed (after clear cutting)



Net change

Output (Net change)































:  Runoff, nutrient loss (see Table) and erosion all increased dramatically after clear cutting.

:  Destruction of vegetation alters nutrient cycles.  (were you really surprised?)

VII.  Gaia.
    This idea was first proposed by James Lovelock and later championed by Lynn Margulis and others.  Lovelock has a couple of books out, check them out.  His ideas are considered somewhat skeptically by many ecologists - mostly because they are difficult to test scientifically. 



  1. The environment (i.e., temperature, moisture, radiation.) influences life;

  2. Life influences the environment.  For example:  oceanic producers use CO2   less greenhouse effect   cooler   ice age   oceanic life dies   CO2 increases   warmer   oceanic producers   etc.

  3. Life requires active biogeochemical cycles -  Plate tectonics.  Persistence of life was a puzzle to early geologists who predicted that nutrients should all wash into ocean sediments (Harold Morowitz)

  4. Maintenance of nutrient cycles (e.g., Hubbard Brook).


| Top| SGS Home | CSB/SJU Home | Biology Dept | Biol116  Section Home Page | Concepts Home Page | Disclaimer |

Last updated: April 22, 2004        � Copyright by SG Saupe