The Role of Microbes, Trace Gases and Agriculture in Environmental Control

1
The Role of Microbes, Trace Gases and Agriculture
in Environmental Control
Microbial Communities and Global Change
2
The 3-Domain System
Based on ribosomal RNA gene sequences
Almost all life is microbial! The diversity of
microorganisms is vast
crown group of Eukaryotes, includes animals,
plants and multicellular algae
3
Microbial Communities
Fantastically diverse Thousands of bacterial
species are present in a gram of soil. Most are
never isolated in a laboratory. They are not well
understood.
Control Global (and local) Biogeochemistry Most
steps in the C, N, S cycles are performed
exclusively by prokaryotes (including trace gas
production). Decomposition is dominated by
microorganisms (bacteria and fungi). Photosynthesi
s half of Earths primary production of carbon
is by cyanobacteria and algae.
Species-specific interactions with
plants Mutualisms (mycorrhizae, N fixers),
Diseases.
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Possible Microbial feedbacks in global change
Plant community change
?
Nutrient mineralization
Plant growth
_

_
Warming
Microbial trace gas production


CO2 increase
Microbial Respiration
Red positive feedback (destabilizing) Green
negative feedback (stabilizing) Purple uncertain
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Nitrous oxide (N2O)
About 300 ppb in atmosphere Strong greenhouse
gas 200X worse than CO2. lifetime150 years.
Contributes to stratospheric ozone depletion
(after conversion to NO, nitric oxide)
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Methane (CH4)
About 1.7 ppm in atmosphere Strong greenhouse
gas Important in ozone chemistry
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Elevated atmospheric CO2
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Global C cycle
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NOx in fossil fuel emissions
Clean air act
However, N2O concentrations still increase by
0.3/year.
10
Atmospheric methane is increasing in the
industrial age
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But why?
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(aerobic)
(anaerobic)
CO2
respiration
C fixation
Organic C
methanogenesis
Methanotrophy (methane oxidation)
CH4 (methane)
- primary production, i.e. photosynthesis,
chemoautotrophy
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Methanogens (Archaea)
Methanopyrus sp.
Methanococcus jannaschii
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Trichonympha, symbiotic protist in termite gut,
with its own symbiotic methanogens
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More symbiotic termite gut protists (Dynenympha
and Microjoenia) that contain their own symbiotic
methanogens, Plus termite gut epithelium with
symbiotic methanogens (E)
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Global N cycle
(Units are 1012g/year)
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Simplified N cycle
anaerobic
N fixation
N2
NH4
(by-products of nitrification)
N2O
(N2O, NO)
Organic N
denitrification
ammonia oxidation (nitrification)
NO2-
NO3-
nitrite oxidation (nitrification)
aerobic
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nitrifiers
Nitrosococcus
Nitrosolobus
Nitrospina
Nitrosospira
Nitrosomonas
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Agriculture and Methane production
Rice paddies Projected to increase by 70 in
next 25 years Anaerobic, rich in organic C
leads to methane production Some oxidiation
occurs due to O2 conducted by rice plants into
rhizosphere
Effect of N fertilization Stimulate plant and
methanogen growth Inhibit methane oxidation (in
most studies of upland rice and other
ecosystems)
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Competitive inhibition of methane oxidation by
ammonium
H
H
C
H
N
H
H
H
H
H
Ammonium
Methane
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In rice paddy soils, ammonium additions
stimulated CH4 oxidation methanotrophs were N
limited
Bodelier et al. 2000
Many other studies show the opposite effect. Why?
Depends on CH4 and N availability lots of CH4
in rice paddy, overcomes competitive inhibition
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Agriculture and Nitrous oxide
N2O
NO3-
NH3
N2O
leaky pipe model
More N fertilization leads to more NOx emissions
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Hall and Matson 1999
N additions stimulate NOx emissions in P-limited
tropical forest
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Eutrophication
Nutrients lead to bloom, algae decompose, use up
oxygen
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Effect of fertilizer runoff on denitrification in
coastal areas
Off coast of India during monsoon season N in
runoff causes eutrophication of coastal
waters Lower oxygen leads to increased
denitrification
(Naqvi et al. 2000)
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Hypothesized denitrification control over global
climate after last glacial maximum (22,000 ya)
High denitrification rates in ocean
Lower NO3- in ocean
Lower production rates in ocean
Slower CO2 removal by ocean
Climate warms