Prescott’s Microbiology, 9th Edition
28
Biogeochemical Cycling and Global Climate
Change
CHAPTER OVERVIEW
This chapter considers nutrient cycling and the roles of microorganisms in global element relationships. The
chapter also includes discussions of the carbon, nitrogen, sulfur, and other biogeochemical cycles. A discussion
of global climate change and its consequences concludes the chapter.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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explain the term redox potential and how it influences the flux of elements found in specific habitats
sketch generalized carbon, nitrogen, phosphorous, sulfur, iron, and manganese cycles
identify, by metabolic type, the microorganisms responsible for carbon and nitrogen fixation, and the
mineralization of organic carbon and nitrogen under oxic and anoxic conditions
list environments in which methanogenesis and methane oxidation occur
differentiate between the terms “global warming” and “global climate change” and identify greenhouse
gases
explain the link between greenhouse gas accumulation and microbial cycling of carbon and nitrogen
discuss the origin of the greenhouse gases CO2, methane, and nitrogen oxide species
CHAPTER OUTLINE
I.
Biogeochemical Cycling
A. Biogeochemical cycling (nutrient cycling) involves both biological and chemical processes
1. Oxidation-reduction (redox) reactions change the chemical and physical properties of nutrients
2. Environments with high redox potentials are more likely to accept electrons, while those with
low redox potentials are more likely to donate electrons; reduction and oxidation reactions are
always coupled
B. Carbon cycle
1. Carbon can be interconverted among methane, complex organic matter, and carbon dioxide;
conversions involve reductants (sources of electrons) and oxidants (sinks for electrons)
a. Organic compounds are produced primarily by carbon fixation carried out by the
activities of plants, cyanobacteria, green algae, photosynthetic bacteria, and
chemolithoautotrophs
b. Organic carbon is returned to the atmosphere by oxidation to carbon dioxide (inorganic
carbon) through respiration and fermentation
c. Inorganic and organic carbon can be reduced anaerobically to methane by archaea in
anoxic zones (e.g., marshes, rice paddies, ruminant guts, and termite guts); the methane
produced moves into aerobic areas where it is oxidized by methylotrophic bacteria,
although methane oxidation is also seen in anoxic environments
2. Carbon dioxide is produced by the degradation of organic matter through the process of
mineralization
a. Organic matter varies in terms of elemental composition, structure of basic repeating
units, linkages between repeating units, and physical and chemical characteristics
b. Degradation of organic matter is influenced by redox potential, nutrients availability,
abiotic conditions (pH, O2, osmotic conditions), and the microbial community present
1
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Prescott’s Microbiology, 9th Edition
c.
C.
D.
E.
F.
Microbial degradation of complex organic material occurs when microbes use these
molecules for growth
1) Chitin, protein, and nucleic acids contain large amounts of nitrogen; when these are
degraded, any excess nitrogen is released by mineralization
2) Molecules containing only hydrogen, carbon, and oxygen cannot support the growth
of microbes; the inability to assimilate sufficient levels of nutrients limits growth
3) Immobilization occurs when nutrients held in biomass are unavailable for nutrient
cycling
d. Most organic substrates can be degraded in the presence or absence of oxygen; however,
degradation of hydrocarbons and lignin usually occurs aerobically
1) Hydrocarbon degradation usually requires oxygen because the first step adds
molecular oxygen to the molecule; slow anaerobic digestion in the presence of
sulfate or nitrate has been observed
2) Filamentous fungi are major lignin degraders and they require oxygen; anaerobic
degradation is slower and thus lignified materials accumulate in peat bogs
e. The presence or absence of oxygen affects the final products that accumulate when
organic substances are degraded
1) Aerobic conditions—oxidized products are made (carbon dioxide, nitrate, sulfate)
2) Anaerobic conditions—reduced end products are formed (ammonium, sulfide,
methane)
Nitrogen cycle
1. Nitrogen is cycled among organic nitrogen, ammonia, nitrogen gas, nitrite, nitrous oxide, and
nitrate
2. Nitrogen fixation
a. Sequential reduction of gaseous nitrogen to ammonia; it requires an expenditure of
energy; the ammonia is immediately incorporated into organic matter as an amine
b. Can be carried out by prokaryotic aerobes or anaerobes; the actual reduction process
must be done anaerobically, even by aerobic microorganisms; physical barriers, O2scavenging molecules, and high rates of metabolic activity are used to maintain the
anaerobic conditions required for nitrogen fixation; cyanobacteria form specialized cells
(heterocysts) for nitrogen fixation
3. Nitrification—aerobic oxidation of ammonium ion to nitrite and ultimately to nitrate; it is
carried out by chemolithoautotrophs and by some heterotrophs (heterotrophic nitrification)
4. Nitrogen assimilation
a. Assimilatory reduction—reduction of nitrate and incorporation into new microbial
biomass
b. Dissimilatory reduction—reduction of nitrate to nitrite, nitrous oxide, and gaseous N2;
the sequence of reactions is known as denitrification
5. Anoxic ammonia oxidation (anammox reaction)—oxidation of ammonia is coupled with the
reduction of nitrite to nitrogen gas by chemolithotrophs
Phosphorus cycle
1. This cycle has no gaseous component; found in soils in inorganic and organic forms as
phosphates or polyphosphates; it is always in valence state +5 (not redox active)
2. Inorganic phosphorus compounds are relatively insoluble and not bioavailable, often limiting
microbial growth in the environment
Sulfur cycle
1. Sulfur can be interconverted among elemental sulfur, sulfide, and sulfate by the actions of
various microorganisms; under certain conditions, transformations of the sulfur cycle can
occur in the absence of microorganisms
2. Dissimilatory sulfate reduction (anaerobic respiration) produces sulfide, which accumulates in
the environment; assimilatory sulfate reduction results in the reduction of sulfate for use in
amino acid biosynthesis
3. Dissimilatory elemental sulfur reduction is known and sulfite reduction is seen in a variety of
microbes
Iron cycle
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
1.
II.
Iron can be interconverted among ferric iron, ferrous iron, and magnetite; iron in aerobic
environments above pH 4 is mainly insoluble, so microbes use siderophore molecules to
capture Fe+3 (ferric) which is reduced to Fe+2 (ferrous) for assimilation
2. Iron reduction from Fe+3 to Fe+2 occurs under anaerobic conditions and is carried out by
bacteria that use Fe+3 as a final electron acceptor, often in a crystalline form outside of the cell;
this metabolism is widespread and may have ancient origins as evidenced by banded iron
formations
3. Magnetotactic bacteria reduce iron to magnetite, which is used to construct intracellular
magnetic compasses to migrate toward oxygen
4. Iron oxidation from Fe+2 to Fe+3 is carried out by a number of genera under aerobic conditions;
can occur under anaerobic conditions using nitrate as the electron acceptor
G. Manganese and mercury cycles
1. Microbial transformation of manganous ion (Mn+2) to MnO2 (with Mn+4); occurs in
hydrothermal vents and bogs
2. Mercury and other metals can be methylated in anaerobic sediments becoming volatile and
lipid soluble; methylated metals can be concentrated in the food chain (a process known as
biomagnification) and have severe health consequences
H. Interaction between elemental cycles—numerous links exist between biogeochemical cycles through
such processes as carbon fixation using electrons from ammonium or iron and anaerobic respiration
oxidizing organic carbon compounds using inorganic electron acceptors
Global Climate Change
A. Over the last century, the rate at which greenhouse gases (CO2, CH4, nitrogen oxides) have entered
the atmosphere has outpaced the rate at which biogeochemical cycles can recycle them
B. The burning of fossil fuels, deforestation, and agricultural practices have led to the accumulation of
greenhouse gases and the upset of global nitrogen cycles
C. Global warming has been attributed to the accumulation of greenhouse gases; ecological
ramifications of climate change include infectious disease outbreaks and animal extinctions
CRITICAL THINKING
1.
Explain why wood structures can exist for longer periods of time in swamps and bogs with little or no
structural degradation. What would happen if the water table fell or the swamp was drained?
2.
The release of greenhouse gases and the production of fixed nitrogen compounds has increased over the
last century. What are the consequences of disrupting the carbon and nitrogen cycles? What measures
might you suggest to limit negative consequences and how might this affect modern life?
3.
What are the advantages of crop rotation between non-symbiotic plants that cannot fix nitrogen and those
that are symbiotic with bacteria such as the Rhizobia?
4.
Consider the oxygen necessary to decompose of biomass. Why do algae blooms often result in low
oxygen conditions once the algae has used up a limiting nutrient? What are the consequences for the
aquatic ecosystem under such conditions?
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
CONCEPT MAPPING CHALLENGE
Construct a concept map using the words listed below and using your own linking words.
C cycle S cycle
N cycle Dissimilatory reduction Sulfur respiration N2 fixation
Aerobic
Anaerobic
Anammox
Assimilatory reduction
Methanogenesis Denitrification Fermentation
Nitrification
symbiosis nitrogenase nodules
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.