Download Chapter 7: Pathways of Elements in the Ecosystem

Chapter 7: Pathways of
Elements in the
Ecosystem
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Background
 Cycling of elements and flux of energy in ecosystems are
fundamentally different:
 chemical elements are reused repeatedly
 energy flows through the system only once
 Many aspects of elemental cycling make sense only when
we understand that chemical transformations and energy
transformations go hand in hand.
(c) 2001 by W. H. Freeman and Company
Assimilatory and Dissimilatory
Processes
 Assimilatory processes:
 incorporate inorganic forms of elements into organic forms,
requiring energy
 example: photosynthesis (reduction of carbon)
 Dissimilatory processes:
 transform organic forms of elements into inorganic forms,
releasing energy
 example: respiration (oxidation of carbon)
(c) 2001 by W. H. Freeman and Company
Energy transformations and element cycling
are linked.
 Organisms play important roles in cycling of elements
when they carry out chemical transformations:
 most biological energy transformations are associated with
biochemical oxidation and reduction of C, O, N, and S
 these assimilatory and dissimilatory processes are often
linked, one providing energy for the other
(c) 2001 by W. H. Freeman and Company
Coupled reactions are the basis of energy flow
in ecosystems.
 A typical coupling of assimilatory/ dissimilatory reactions
might involve:
 oxidation (dissimilation) of carbon in carbohydrate (energy-
yielding), linked to
 reduction (assimilation) of nitrate-N to amino-N (energyrequiring)
 Some processes may involve many steps.
 Energy is lost at each step (inefficiency).
(c) 2001 by W. H. Freeman and Company
Ecosystems may be modeled as linked
compartments.
 An ecosystem may be viewed as a set of compartments
among which elements are cycled at various rates:
 photosynthesis moves carbon from an inorganic
compartment (air or water) to an organic compartment
(plant)
 respiration moves carbon from an organic compartment
(organism) to an inorganic compartment (air or water)
(c) 2001 by W. H. Freeman and Company
Elements move among compartments at
different rates.
 Inorganic carbon released through respiration may
be taken up quickly through photosynthesis. The
organic carbon fixed may be respired again quickly
by plants.
 Organic carbon stored in deposits of coal, oil, or
peat is not readily accessible and may remain in
storage for millions of years.
 Inorganic carbon may also be taken out of
circulation for millions of years by precipitation as
calcium carbonate in aquatic systems.
(c) 2001 by W. H. Freeman and Company
A Physical Model for the Water
Cycle
 The biosphere contains 1,400,000 teratons (TT, 1012
metric tons) of water, 97% of which resides in the
oceans.
 Other water compartments include:
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



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ice caps and glaciers (29,000 TT)
underground aquifers (8,000 TT)
lakes and rivers (100 TT)
soil moisture (100 TT)
water in atmosphere (13 TT)
water in living things (1 TT)
(c) 2001 by W. H. Freeman and Company
The water cycle is solar-powered.
 The water cycle consumes one-fourth of the total solar
energy striking the earth during a year:
 precipitation over land exceeds evaporation by 40
teratons/yr; surplus returns to the ocean in rivers
 evaporation over the oceans exceeds precipitation by 40
teratons/yr; surplus is delivered by winds to the land masses
(c) 2001 by W. H. Freeman and Company
The residence time of water varies by
compartment.
 The atmosphere contains 2.5 cm of moisture at
any time; annual flux into and out of the
atmosphere is 65 cm/yr:
 residence time is compartment size/flux, or 2.5 cm / 65
cm/yr = 0.04 yr, about 2 weeks.
 Soils, rivers, lakes and oceans have same flux
rates as atmosphere, but they contain about
100,000 times as much water, yielding a mean
residence time of 2,800 yr.
(c) 2001 by W. H. Freeman and Company
The carbon cycle is linked to
global energy flux.
 The carbon cycle is the focal point of biological
energy transformations.
 Principal classes of carbon-cycling processes:
 assimilatory/dissimilatory processes (mainly
photosynthesis and respiration)
 exchange of CO2 between atmosphere and oceans
 sedimentation of carbonates
(c) 2001 by W. H. Freeman and Company
Photosynthesis and Respiration
 Approximately 85 GT of carbon enter into balanced
assimilatory/dissimilatory transformations each year.
 Total global carbon in organic matter is about 2,650 GT
(living organisms plus organic detritus and sediments).
 Residence time for carbon in biological molecules = 2,650
GT / 85 GT/yr = 31 yr.
(c) 2001 by W. H. Freeman and Company
Ocean-Atmosphere Exchange
 Exchange of carbon across the atmosphere-ocean
interface links carbon cycles of terrestrial and aquatic
ecosystems.
 Dissolved carbon pool is 30,000 GT, nearly 50 X that of
atmosphere (640 GT).
 Net atmospheric flux (assimilation/ dissimilation and
exchange with oceans) is 119 GT/yr for mean
atmospheric residence time (640 GT / 119 GT/yr) of
about 5 years.
(c) 2001 by W. H. Freeman and Company
Precipitation of Carbonates
 Precipitation (and dissolution) of carbonates occurs in
aquatic systems:
 precipitation (as calcium and magnesium carbonates) leads
to formation of limestone and dolomite rock
 turnover of these sediments is far slower than those associated
with assimilation/dissimilation or ocean-atmosphere exchange
 carbonate sediments represent the single largest compartment
of carbon on planet (18,000,000 GT)
(c) 2001 by W. H. Freeman and Company
Precipitation of Calcium and Carbon Through
the Ages
 CO2 dissolves in water to form carbonic acid, which
dissociates into hydrogen, bicarbonate, and
carbonate ions:
CO2 + H2O  H2CO3
H2CO3  H+ + HCO3-  2H+ + CO32-
 Calcium ions combine with carbonate ions to form
slightly insoluble calcium carbonate, which
precipitates:
Ca2+ + CO32-  CaCO3
(c) 2001 by W. H. Freeman and Company
Slow Release of Sedimentary Calcium and
Carbon
 Calcium removed from the water column in the oceans is
replaced by calcium dissolved from limestone sediments
on land by slightly acidic water of rivers and streams.
 Carbon is also slowly released from oceanic sediments as
limestone is subducted beneath continental plates, and
CO2 is outgassed in volcanic eruptions.
(c) 2001 by W. H. Freeman and Company
Reef-Builders extract carbon
from water.
 In neutral conditions of marine ecosystems, extraction of
CO2 from water column drives precipitation of CaCO3:
CaCO3 + H2O + CO2  Ca2+ + 2HCO3 Reef-building algae and coralline algae incorporate
calcium carbonate into their hard structures, forming
reefs.
(c) 2001 by W. H. Freeman and Company
Changes in the Carbon Cycle
Over Time
 Atmospheric CO2 concentrations have varied
considerably over earth’s history:
 during the early Paleozoic era (550-400 Mya),
concentrations were 15-20 X those at present
 concentrations declined to ca. present level by 300 Mya,
then increased again to 5 X present level through the
early Mesozoic era (250-150 Mya) and have declined
gradually since
 early Paleozoic and early Mesozoic eras were extreme
greenhouse times, unlikely to be equaled by effects of
current human enhancement of atmospheric CO2
(c) 2001 by W. H. Freeman and Company
Nitrogen - A Most Versatile
Element!
 Ultimate source (largest reservoir) of this
essential element is molecular N2 gas in the
atmosphere, which can also dissolve in water to
some extent.
 Nitrogen is absent from native rock.
 Nitrogen enters biological pathways through
nitrogen fixation:
 these pathways are more complicated than those of the
carbon cycle because nitrogen has more oxidized and
(c) 2001 by W. H. Freeman and Company
reduced forms than carbon
Ammonification
 Plants assimilate inorganic nitrogen into proteins, which
may be passed through various trophic levels.
 Ammonification (dissimilation of N) is carried out by all
organisms:
 initial step is breakdown of proteins into constituent amino
acids by hydrolysis
 carbon (not nitrogen) in amino acids is then oxidized,
releasing ammonia (NH3)
(c) 2001 by W. H. Freeman and Company
Nitrification
 Nitrification is oxidation of ammonia:
 first step is oxidation of ammonia to nitrite (NO2-), carried
out by Nitrosomonas in soil and Nitrosococcus in oceans
 nitrite is then oxidized to nitrate (NO3-) by Nitrobacter in soil
and Nitrococcus in oceans
 nitrification is an aerobic process; the nitrifying organisms
involved are chemoautotrophic bacteria
(c) 2001 by W. H. Freeman and Company
Denitrification
 Denitrification is the reduction of nitrate to nitric
oxide (NO), which escapes as a gas:
 occurs in waterlogged, anaerobic soils, oxygen-
depleted sediments, and bottom waters in aquatic
ecosystems
 carried out by heterotrophic bacteria such as
Pseudomonas denitrificans
 further N-reductions may lead to production of nitrous
oxide (N2O) and molecular nitrogen (N2), both gases
 denitrification may be one of the principal causes of low
availability of nitrogen in marine systems
(c) 2001 by W. H. Freeman and Company
Nitrogen Fixation
 Loss of nitrogen to atmosphere by denitrification
is offset by nitrogen fixation:
 fixation is carried out by:
 free-living bacteria such as Azotobacter
 symbiotic bacteria such as Rhizobium, living in root nodules of
legumes and other plants
 cyanobacteria
 N-fixation is an energy-requiring process, with energy
supplied by oxidation of organic detritus (free-living
bacteria), sugars supplied by plants (bacterial
symbionts), or photosynthesis (cyanobacteria)
(c) 2001 by W. H. Freeman and Company
Significance of Nitrogen
Fixation
 Nitrogen fixation balances denitrification on a global
basis:
 these fluxes amount to about 2% of total cycling of nitrogen
through ecosystems
 Nitrogen fixation is often very important on a local scale:
 N-fixers dominate early colonizers on nitrogen-poor
substrates, such as lava flows or areas left bare by receding
glaciers
(c) 2001 by W. H. Freeman and Company
The Phosphorus Cycle
 Phosphorous is an essential element, constituent of
nucleic acids, cell membranes, energy transfer systems,
bones, and teeth.
 Phosphorus may limit productivity:
 in aquatic systems, sediments act as a phosphorus sink
unless oxygen-depleted
 in soils, phosphorus is only readily available between pH of 6
and 7
(c) 2001 by W. H. Freeman and Company
Phosphorus Transformations
 Phosphorus undergoes relatively few
transformations:
 plants assimilate P as phosphate (PO43-) and incorporate
this into organic compounds
 animals and phosphatizing bacteria break down organic
forms of phosphorus and release the phosphorus as
phosphate
 phosphorus does not:
 undergo oxidation-reduction reactions in the ecosystem
 circulate through the atmosphere, except as dust
(c) 2001 by W. H. Freeman and Company
The Sulfur Cycle 1
 Sulfur is an essential element and, like nitrogen,
has many oxidation states and follows complex
chemical pathways.
 Sulfur reduction reactions include:
 assimilatory sulfate reduction to organic forms and
dissimilatory oxidation back to sulfate by many
organisms
 reduction of sulfate when used as an oxidizer for
respiration by heterotrophic bacteria in anaerobic
environments
(c) 2001 by W. H. Freeman and Company
The Sulfur Cycle 2
 Sulfur oxidation reactions include:
 oxidation of reduced sulfur when used as an electron donor
(in place of oxygen in water) by photosynthetic bacteria
 oxidation of sulfur by chemoautotrophic bacteria that use
the energy thus obtained for assimilation of CO2
(c) 2001 by W. H. Freeman and Company
Sulfur in Coal and Oil Deposits
 Iron sulfide (FeS) commonly associated with coal and oil
deposits can result in environmental problems:
 oxidation of sulfides in mine wastes to sulfate, which
combines with water to form sulfuric acid, associated with
acid mine drainage
 oxidation of sulfides in coal and oil releases sulfates into
atmosphere, which then form sulfuric acid, a component of
acid rain
(c) 2001 by W. H. Freeman and Company
Microorganisms assume diverse roles in
element cycles.
 Decomposition in anaerobic organic sediments is
dependent on certain specialized microbes, the
denitrifiers:
 these heterotrophic organisms use oxidized forms of N, S, and
Fe as electron acceptors (oxidizers) in the absence of oxygen
 for example, some anaerobic bacteria utilize nitrate as an
alternative electron acceptor for the oxidation of glucose:
glucose + NO3-  CO2 + H2O + OH- + N2 + energy
(c) 2001 by W. H. Freeman and Company
Biological Nitrogen Fixation
 Biological nitrogen fixation (by bacteria and
cyanobacteria) is essential to counterbalancing N
losses associated with denitrification.
 Nitrogen is often implicated as a limiting nutrient
in terrestrial and aquatic systems.
 Nitrogen fixation is critical to ecosystem
development in primary succession.
 Continued nitrogen input is essential for long-
term health of natural ecosystems.
(c) 2001 by W. H. Freeman and Company
Autotrophic Diversity
 All autotrophs are capable of assimilating (reducing)
carbon in CO2 into organic forms (initially glucose):
 photoautotrophs accomplish this by capturing energy from
sun through photosynthesis:
 green plants, algae, and cyanobacteria use water as an electron
donor (reducing agent) and are aerobic
 purple and green bacteria use H2S or organic compounds as
electron donors and are anaerobic
(c) 2001 by W. H. Freeman and Company
Chemoautotrophs
 Chemoautrophs are not photosynthetic,
reducing inorganic carbon (from CO2), but
using energy obtained from aerobic oxidation
of inorganic substrates:
 methane - Methanosomonas, Methylomonas
 hydrogen - Hydrogenomonas, Micrococcus
 ammonia - nitrifying bacteria Nitrosomonas,
Nitrococcus
 nitrite - nitrifying bacteria Nitrobacter, Nitrococcus
 hydrogen sulfide, sulfur, sulfate - Thiobacillus
 ferrous iron salts - Ferrobacillus, Gallionella
(c) 2001 by W. H. Freeman and Company
Deep-Sea Vent Ecosystems
 Deep-sea vent ecosystems are far below the
penetration of any light, dependent on
chemoautotrophic production:
 hot water coming from vents is charged with hydrogen
sulfide, H2S
 chemoautrophic bacteria use oxygen from seawater to
oxidize H2S, then use the energy thus obtained for
assimilatory carbon reduction
 other members of vent communities (clams, worms,
crabs, fish) ultimately depend on primary production of
these bacteria
(c) 2001 by W. H. Freeman and Company
Living things are intimately involved in
elemental cycles.
 Elements are cycled through ecosystems primarily
because metabolic activities result in chemical
transformations.
 Each type of habitat presents a different chemical
environment, especially with respect to:
 presence/absence of oxygen
 possible sources of energy
 Numerous adaptations have arisen to meet these
challenges.
(c) 2001 by W. H. Freeman and Company
Summary 1
 Unlike energy, nutrients are retained in ecosystems and
may cycle indefinitely.
 The movements of energy and elements, especially
carbon, parallel one another in ecosystems.
 Energy transformations result from the coupled oxidation
and reduction reactions of various elements.
(c) 2001 by W. H. Freeman and Company
Summary 2
 The water cycle is a physical analogy for element
cycling in ecosystems; many elements are also
transported by the water cycle.
 The carbon cycle involves both biological and
nonbiological processes fundamental to
functioning of the biosphere.
 The nitrogen cycle involves many transformations
and oxidation states. Microorganisms play
essential roles throughout.
(c) 2001 by W. H. Freeman and Company