BIO226 : Ecosystem : Nutrients : Lesson

Glossary terms that are important in this lesson: Ammonification, anaerobic, anoxic, assimilatory, biochemical oxygen demand, BOD, carbonic acid, cation, chemoautotrophs,denitrification, dissimilatory, epilimnion, eutrophic, eutrophication, external loading, HNLC, hyphae, hypolimnion, internal loading, laterite, leaching, luxury consumption, methanogenesis, mycelium, mycorrhizae, nitrification, nitrogen fixation, oligotrophic, oxic, oxidation, photoautotrophs, redox potential, reduction, thermocline, upwelling, watershed.

I. Elements in the Ecosystem

1. Introduction
   • Nutrients cycle over and over within an ecosystem
   • Nutrient elements are biochemically transformed
   • Chemical and energy transformations go hand in hand

2. Energy transformation and element cycling
   • Assimilatory processes transform elements to biological forms, e.g. photosynthesis transforms inorganic carbon to organic carbon in carbohydrates
   • Dissimilatory processes complement assimilatory processes with the reverse transformation and a release of energy, e.g., respiration
   • Not all transformations are biologically mediated, e.g. weathering of bedrock, oxidation of N by lightning, sedimentation
   • Coupled transformations
     • Oxidation occurs when an atom gives up electrons and releases energy
     • Reduction occurs when an atom accepts electrons and gains energy
       • Excess energy supplied by oxidation is lost as heat: thermodynamically inefficient

3. Compartment models
   • Various parts of an ecosystem are considered compartments which interact with each other by inputs and outputs
     • Compartments may contain subcompartments, e.g., plants and animals within the organic carbon compartment
   • Rate of energy movement varies among compartments: a few minutes for some organic forms; centuries for geologic forms

4. Water cycle
   • Most of the water movement through the ecosystem occurs by the physical processes of evaporation, transpiration, and precipitation powered by light energy from the sun
   • Evaporation of water to vapor requires energy and condensation to clouds releases energy
   • Precipitation exceeds evaporation over land; opposite occurs over oceans
   • Global hydrologic cycle takes 25% of the sun's radiation hitting the earth
   • Evaporation determines the rate of water movement through the ecosystem

5. Redox potential
   • Capacity of an atom to accept electrons (become reduced) drives chemical energy transformations
   • O₂ is a strong oxidizer: readily accepts electrons, will reduce other substances
   • C is a weak oxidizer (strong reducer)
   • Some other biologically important electron acceptors and donors: NO₃ , H₂S, NH₃, Fe⁺³, SO₄

6. Carbon cycle
   • Photosynthesis and respiration: 10¹⁷g C/year
     • Photosynthesis changes the oxidation state of C from +4 to 0: gains 4 electrons and increases in energy; reversed by respiration
     • Anaerobic conditions and bacteria convert some organic carbon into methane: cattle and rice paddies
   • CO₂ dissolves in oceans, lakes and streams and links terrestrial and aquatic ecosystems
     • Oceans contain about 50 times as much CO₂ as the atmosphere
     • Carbonate forms a buffer in aquatic systems
   • Carbonate compounds, after CO₂ dissolves in water and forms carbonic acid, precipitate and form sediments
     • Sedimentation removes about 1% of the annual cycling of carbon

7. Nitrogen cycle
   • Biological nitrogen originates from N₂ in the atmosphere
   • Nitrogen fixation facilitated by free-living Azotobacter, Rhizobium symbiotic with legume roots, and cyanobacteria, converts atmospheric N₂ into ammonia
     • Conversion requires energy: reduction of nitrogen takes energy from the oxidation of organic carbon
     • Nitrogen fixers are early colonists in nitrogen-poor habitats
   • Ammonification produces ammonia from proteins and amino acids
     • Releases some energy
     • Energy potential of the nitrogen atom does not change
   • Nitrification converts ammonia to nitrite (by Nitrosomonas) and from nitrite to nitrate (by Nitrobacter)
     • Releases energy
     • Anaerobic conditions use nitrate and nitrite as electron acceptors
   • Denitrification under anaerobic conditions converts nitrate and nitrite (by Pseudomonas denitrificous) to NO and N₂ which leave biological circulation

8. Phosphorus cycle
   • Phosphorus is a major constituent of nucleic acids, cell membranes, energy transfer systems, bones, and teeth
   • Phosphorus limits plant productivity
   • Phosphorus has no gaseous form
   • pH greatly affects availability of phosphorus to plants

9. Sulfur cycle
   • Sulfur occurs in amino acids cysteine and methionine
   • Sulfur has many oxidation states, forms hydrogen sulfide under anoxic conditions
   • Sulfur oxidizes (with help of Thiobacillus) and forms sulfuric acid with water

10. Microorganisms in element cycles
    • Microbes are critical to nutrient cycling, especially under anoxic conditions
    • Microbes compete with plants for nitrogen in the soil
    • Autotrophs
      • Photoautotrophs use sunlight for energy
        • Cyanobacteria use H₂O as an electron donor (reducing agent) and are aerobic
        • Purple and green bacteria have light absorbing pigments, use hydrogen sulfide or organic compounds as electron donors, are anaerobic
      • Chemoautotrophs
        • Use CO₂ as carbon source
        • Use energy from oxic oxidation of inorganic substrates to reduce carbon
        • Base of food chain at hydrothermal vents in ocean

II. Nutrient regeneration

1. Introduction
   • Terrestrial habitats: most elements cycle through detritus at the soil surface, where plant roots have access to nutrients
   • Aquatic habitats: most regeneration occurs in the sediments, usually separated from sites of primary production
   • Biochemical processes in soils, water, and sediments influence productivity of ecosystems and cycling of elements

2. Terrestrial ecosystems
   • Weathering of bedrock supplies new nutrients
     • Measured indirectly by measuring inputs and outputs to nutrient budgets of undisturbed ecosystems
     • Basic cations: calcium, potassium, sodium, and magnesium - dissolve in water and leave the ecosystem in streams where they are measured
     • Cation budgets of watersheds
       • Measure inputs and outputs from a watershed
       • Example: Hubbard Brook Forest in New Hampshire for annual calcium budget (kg/ha)
         • Precipitation input (2) - Stream loss (14) = System loss (12)
         • Assimilation into plants and detritus
         • Bulk of nutrient taken up by plants comes from regeneration within the soil profile; only 10% provided by weathering of bedrock
       • Plants assimilate cations with water uptake
   • Detritus and nutrient regeneration
     • Nitrogen, phosphorus, and sulfur are important nutrients that support plant production through regeneration from detritus
     • Leaf litter breakdown on the forest floor
       • Leaching of soluble minerals and small organic compounds by water: 10-30%
       • Consumption and maceration by large (soil) invertebrates: assimilate 30-45% energy in leaves, less in wood
         • Feeding increases the surface area of detritus for subsequent microbial feeding
       • Breakdown of woody components by fungi
       • Decomposition by bacteria
     • Decomposition rate depends on organic composition
       • Deciduous leaves vary but are relatively rapid
       • Conifer needles decompose relatively slowly
       • Lignins are phenolic polymers which strengthen wood
         • Can be broken down only by "white rot" fungi
       • Content of nitrogen, phosphorus and other nutrients required by bacteria and fungi influence decomposition rate
     • Unique role of fungi
       • Most fungi consist of a mycelium composed of hyphae
       • Secrete enzymes into the substrate and absorb digested sugars and amino acids
       • Can digest cellulose with enzymes which split the cellulose molecule into simple sugars
       • Mycorrhizae
         • Association of a soil fungus with the root of a plant
         • Facilitates nutrient uptake for the plant
           • Accesses a greater volume of soil than the roots can
           • Increases the surface area for nutrient assimilation
           • Enzyme and acid secretion makes them more effective at extracting certain nutrients, e.g., phosphorus
           • May produce antibiotics that protect against pathogens
           • Plant supplies a carbon source for the fungus, which accounts for a significant fraction of net production
   • Tropical and temperate forests
     • Difference in effects of climate on weathering, soil properties, and decomposition of detritus
     • Tropical soils
       • Deeply weathered, low clay content, poor ability to retain nutrients (low cation exchange capacity)
       • High productivity depends on rapid regeneration of nutrients
       • Cleared areas quickly turn into laterite and lose fertility
       • Fertility is dependent on the presence of vegetative cover
       • Living biomass is the main nutrient reservoir, soils are nutrient poor
       • Some eutrophic forests have fertile soils
       • Dense roots close to the surface provide efficient nutrient uptake
     • Temperate systems
       • Clear cutting revealed the importance of vegetative cover in the Hubbard Brook Experimental Forest
         • Stream flow increased from lack of transpiration
         • Loss of cations increased 3-20 times
         • Forest gained nitrogen but loss of nitrate from clear-cut soared
         • Forest floor litter is an important nutrient reservoir

3. Aquatic ecosystems
   • Nutrients accumulate in bottom sediments
     • Sediments are often not located close to photosynthetic zones
     • Decomposition in anoxic sediments is slow
   • Currents, especially in areas of upwelling, bring nutrients to photic areas
   • Regeneration of nutrients takes place in the water column before sedimentation
     • For some elements regeneration rates match production rates
   • Heterogeneous distribution of nutrients leads to luxury consumption
   • Vertical mixing and stratification
     • Temperature and salinity influence water density
     • Waters of different densities resist mixing and become stratified: surface waters deplete nutrients, deep waters regenerate nutrients
     • Winds produce turbulent mixing and upwelling currents
     • Where nutrient-laden water mixes with warm surface waters, primary productivity is high
   • Nutrient limitation of production in the oceans
     • Production of marine systems is closely related to nutrient supply, especially nitrogen in surface waters
       • Highest production occurs in shallow seas and areas of upwelling
       • In areas of abundant nitrogen and phosphorus, other elements (e.g., iron, silicon) may limit productivity and have low densities of phytoplankton: high-nutrient low-chlorophyll (HNLC) areas
         • Experiment of iron addition off the Pacific Coast of South America
           • Concentration of iron increased 100 times, concentration of chlorophyll increased 3 times, zooplankton populations increased
   • Regeneration of nutrients in deep, oxygen depleted waters
     • Stratification results when the thermocline separates epilimnion from hypolimnion
     • Bacterial respiration in the hypolimnion depletes oxygen
       • Sulfate and nitrate are used as oxidizers leading to a buildup of reduced sulfur and ammonia
       • Insoluble iron-phosphate complexes become solubilized
     • When the thermocline breaks up and oxygen returns, water chemistry reverses
   • Phosphorus and eutrophication
     • Low levels of phosphorus limit production of freshwater systems
       • Experiment: additions of phosphorus, nitrogen, and carbon
     • Relative importance of external loading and internal loading vary with the size of lakes
     • Nutrient inputs convert oligotrophic to eutrophic lakes
       • Rich lakes from natural or pollution sources have a high demand for oxygen used in microbial decomposition (BOD = biochemical oxygen demand)
       • Eutrophication can be reversed by eliminating high nutrient input

4. Estuaries and marshes
   • High production results from rapid and local regeneration of nutrients as well as from external loading from rivers
   • Net export of organic matter to adjoining marine ecosystem: Georgia salt marsh exported 10% of its net primary production as organisms, particulate detritus, and dissolved organic material carried out by the tides
   • Important nursery areas for larvae and juvenile fishes and invertebrates
   • Dynamic balance between nutrient inputs and outputs
   • High microbial activity under both oxic and anoxic conditions

When you have completed this lesson, go on to Review Questions