Download Element Cycling in the Ecosystem •Energy flow

Coupled autotrophic and heterotrophic processes cycle matter
between the inorganic and organic domains of the biosphere
Element Cycling in the Ecosystem
CO2
•Energy flow through ecosystems drive matter cycles
•The water cycle and the role of evapotranspiration
•The carbon cycle
•N assumes many oxidation states as it cycles
•The Phosphorus cycle and eutrophication
•The sulfur cycle and the role of microorganisms
Autotrophic
process
(assimilatory)
Nutrients, N, P, etc
Light
Heterotrophic
process
(dissimilatory)
Physical
energy
heat
biomass
O2
Energy flows through the biosphere and drives material cycles within it
We say that energy flows because it enters in a short wave useful form that can be
captured and biochemically used and leaves in a long wave form that cannot be
recaptured—but it does help keep us warm
Living processes sometimes provide the energy source to drive
other material transformations and physical reactions
Photosynthesis
by land plants
CO2
The Global Water Cycle is driven mainly by the sun’s energy
•involves mainly state changes not chemical changes
#’s are teratons (TT),
(=1012T), and TT/yr
Evapotranspiration
Nutrients, N, P, etc
Light
Water from
soil
[P─E sea]
heat
Wind
energy
[P─E land]
biomass
What is the
residence time
of water in the
atmosphere?
O2
Water vapour
The water cycle is mostly
a physical phenomenon,
but plants contribute
toward “making it rain”
through transpiration
Heat energy is captured and stored by the state change from liquid to vapour. This
potential energy is released upon condensation
•an example of a living process enhancing a physical process that is energetically
favourable. In this way terrestrial photosynthesis is an important driver to the
global water cycle
a huge water pump that is driven by the sun—evaporation requires energy,
condensation releases it
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Carbonate precipitation can also be driven by photosynthesis
and sometimes animal metabolism
Photosynthesis
Most of the earth’s C is
contained in sedimentary rocks
CaCO3 (s)
HCO3─
Nutrients, N, P, etc
These massive limestone deposits were
produced by coralline algae and reef
building corals.
H+
Light
heat
biomass
O2
CaCO3 (solid)
OH─
These coralline algae are red
algae that deposit CaCO3 on
their cell walls as a result of
their photosynthesis.
Intracellular pH rises during photosynthesis and helps carbonates ppt
Much of the Carbon in the world is stored in limestone which is being formed by
reef building corals and algae.
The Carbon cycle involves many chemical reactions driven by organisms
#’s are gigatons (GT),
(=109 T), and GT/yr
Plant growth, especially trees, can also facilitate weathering,
which releases dissolved substances from bedrock
Ca++, K+, Mg++ HCO3- ,
SiO3- , PO4-3, clay
minerals etc.
Limestone,
granite, shales
Plant
growth
What is the
residence time
of atmospheric
C?
•the atmospheric
pool is growing
rapidly due to
fossil fuel burning
•Most of the
earth’s C is
stored in rocks,
fossil fuels and
limestone—long
residence time
Deeper rooting
zone
Weathering is the physical and chemical breakdown of rocks (lithosphere), and is the most
important input to the biosphere for many elements.
•P & R plus ocean
exchange rapidly
cycle C through the
atmosphere—short
residence time?
•CO2 in air and carbonates and bicarbonates dissolved in water control pH.
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Atmospheric CO2 levels were historically much higher than today
Land plants evolved near the
end of the Paleozoic era, and
huge forests began to cover a
once bare landscape
Retallack, 1997
Soil clay
content
increased
More C
sequestered
as coal and
ocean CaCO3
Plants
become
larger, more
trees
Driving down
atmospheric
CO2
concentrations
Where did all this C go?
Studies on fossil soils show great
changes as plant evolution progressed
C-transformations in aerobic and anaerobic environments
The Nitrogen cycle is driven by microbes especially N-fixing organisms
4 x 106
Oxidation
state
Where do we find methanogens?
•The residence time
of atmospheric N is
about 16,000 yr
-4
0
•Humans have
increased N-fixation
rates about 3X
through fertilizer
use
+4
•Denitrification
tends to balance
N-fixation on the
global scale
•Under anaerobic conditions organic molecules break down to methane instead of CO2 —This
process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria.
•They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H 20 to build their
biomass.
•The biggest N pool
is the atmosphere
and it is virtually
absent from rock
•N pollution through industry is also significant source of fixed N
3
Many microorganisms can reduce N2 to NH3
Plants such as legumes have N-fixing Rhizobium nodules on their roots
Azolla, an aquatic fern used in rice culture
•The leaves of this aquatic fern
have cavities that harbour
filamentous cyanobacteria
Anabaena azollae
•These nodules maintain an anaerobic environment and are nourished by plant sugars to pay the
energy cost of N-fixation
•Most of the N used by plants in natural ecosystems comes indirectly from N-fixation
(nitrogenase)
•The same reduction reaction N2 + 3H2 → 2NH3 is used to make chemical fertilizers
which supplies most of the N for modern agriculture
•N-fixing crops are often used to restore N content of depleted soils
•Use of N-fertilizers depresses N-fixation rates? Why?
•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.
The Nitrogen cycle involves many different oxidation states, and the
redox processes are facilitated by plants and wide variety of bacteria
Anthropogenic influences on the N-cycle
Chemoheterotrophs (CH)
-3
CH
PA
0
+1
Nitrite
+3
•3-fold increase in N-fixation—agriculture
•Formation of NOx emissions—auto engines, acid rain
•N-losses from clearcut forests
•Eutrophication of waterways through fertilizer & sewage pollution
No detectable impact on atmospheric N2 content
CH
+5
Photoautotrophs (PA)
Chemoautotrophs(CA)
4
The P cycle is involves no redox reactions
•Weathering
liberates large
amounts of PO4-3
from apatite rocks
The S cycle is affected by similar microbially mediated redox reaction
as the N-cycle—no stable gaseous phase
•Most S is found
in the reduced
form in rocks eg
pyrites
•Stored and cycled in
forests, but released in
large amounts by land
clearing—eutrophicati
on of waterways
•Oxidized and
released by volcanic
activity, mining
smelting, fossil fuel
burning (→SO2 ), or
microbial activity
eg hydrothermal
vents, or weathering
•P-fertilizers used
extensively in
agriculture.
•Cycled between
the dissolved and
the particulate
phase (plankton) in
lakes and oceans
•Phosphorus tends to accumulate in sediments except when they become anaerobic
(→SO4 -2)
•A volatile gas DMS is
produced by ocean
plankton—affects cloud
formation
•SO2 is rained out of the
atmosphere as H2SO4
•Taken up by
biota (proteinsSH) and released
as H2S
Chemoheterotrophs (CH)
-2
0
+4
+6
CH
PA
Photoautotrophs (PA) Chemoautotrophs(CA)
Streams draining mine tailings are extremely acidic—the
effect of Thiobacillus oxidizing pyrites and iron.
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Thiobacillus ferrooxidans
oxidizes both the iron,
Fe(+2) to Fe(+3)
and the sulphur in the pyrites,
S(-1) to S(+6) using
molecular oxygen.
Hydrothermal vent fauna: a community founded on
Thiobacillus a chemoautotrophic bacteria
This reaction splits water to
produce a great deal of acid.
How do you suggest that
mine tailings should be
stored?
2 FeS 2 + 7.5O 2 + 4 H 2O ! 4 SO 4 "2 + Fe 2O 3 + 8 H + + Energy
The pogonophoran tube worms harbour nodules of Thiobacillus
inside their body cavities as mutualistic symbionts
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