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Lecture 21

Biology 2483A Lecture Notes - Lecture 21: Decomposer, Chlorophyll, Nitrogenase

Course Code
BIOL 2483A
Hugh Henry

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Unit 6: Ecosystems Lesson 21: Nutrient Supply and Cycling (Nov. 23rd 2017)
Chemical breakdown of minerals in rocks or through fixation of atmospheric gases.
All organisms have similar nutrient requirements, but amounts and specific nutrients needed
A orgais’s utriet requirements are related to its physiologymode of energy acquisition
(autotrophic or heterotrophic), mobility, thermal physiology (endotherm or ectotherm).
Variation in the composition of organisms.
o Carbon, oxygen, hydrogen, nitrogen.
o Phosphorous, silicon, iron and sodium are actually
very important to some animals.
All nutrients are ultimately derived from abiotic sources:
o Gases in the atmosphere:
Nitrogen (78%), oxygen (21%), carbon,
Cycle instead of coming and going.
Oxygen appeared around 2-4 bya, from the
activity of cyanobacteria.
Beginning of chloroplasts, changed
the atmosphere, added oxygen.
Great oxygenation event.
Our atmosphere is made by organisms, like a pump, gives us our essential gases.
Precipitation falls over rocks, extracts nutrients, one being phosphorous.
Detritus is a very important pool of nutrients.
o Minerals in rocks:
Elements are released from rock minerals by weathering.
Mechanical weathering physical breakdown of rocks.
Rain, uplifting, volcanos.
Freezethaw and dryingrewetting cycles break rocks into smaller
Rocks break down into big boulders gravel sand silt clay.
Vernal pools hold water due to having a clay base holding onto water
Water drains through sand and silt easily.
For a river, detrital sources from the edges of the river give energy.
Plant roots and gravity.
Chemical weathering chemical reactions release soluble forms of the mineral
Soil properties influence nutrient availability. Soils with a high proportion of
sand have large spaces between the particles and do not hold water well.
Texture influences soil water holding capacity.
Parent material rock or mineral material that was broken down by weathering
to form a soil.
Influences abundance, growth, and diversity of plants in an ecosystem.
Parent material may be bedrock or sediment deposited by glaciers (till), wind
(loess), or water.
Soils derived from limestone have high levels of Ca2+, K+, and Mg2+.
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Soils derived from granite have lower levels of these elements and lower pH.
Lower pH reduces availability of N and P to plants.
Low pH lower diversity in animals and plants in that area.
Important in lake systems, animals need to be able to tolerate low pH
Tropical rainforests:
Does not have a large rich top soil area.
Rich in phytoplankton.
High rates of weathering and leaching for a long time and are nutrient-
Leaching causes nutrients to fall deep into the soil, becoming
unreachable for plant or animal use.
Most nutrients in the living tree biomass.
Climate influences rate of soil development, fastest in warm, wet
Lush forest with a very thin layer of nutrient rich soil of detritus.
All the nutrients in a tropical forest are held in the trees.
When tropical forests are clear cut or burned for agriculture, the layer
of rich top soil disappears.
At higher latitudes, soils have slower leaching rates and are usually
richer in mineral nutrients.
Organisms, especially plants, bacteria, and fungi, contribute organic matter to
Organic matter is a reservoir of nutrients such as N and P.
These nutrients must be transformed or fixed by organisms.
N2 has a triple bond that needs to be broken (hard to do)
Primary fixation is done by bacteria in the roots of legumes.
Takes 25% of photosynthetic energy that is provided by a plant.
Trade off allocation of energy to N-fixation rather than growth
reduces ability to compete for other resources.
The atmosphere is the ultimate source of carbon and nitrogen for ecosystems.
Carbon is taken up as CO2 by autotrophs through photosynthesis
The atmosphere is 78% N2. and fixed into organic compounds.
Break the triple bond.
Nitrogen fixation
N2 into a biologically useful form.
Legumes form nitrogen fixing nodules:
o Biological N fixation uses the enzyme nitrogenase, which only occurs in certain bacteria.
o Some N-fixing bacteria are free-living; others are symbionts.
o Symbiotic relationships include legume plants and bacteria in the family Rhizobiaceae.
o Legume plants provide the bacteria with a habitat in special root structures, called
nodules, and supply them with carbon compounds as an energy source.
o The plants get fixed nitrogen in return.
Other nitrogen fixing symbioses:
o Alders and Frankia.
o Water fern Azolla and cyanobacteria.
o Lichens that include fungal and N-fixing cyanobacterial symbionts.
o Termites with N-fixing bacteria in their guts.
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