Chapter 14
Biogeochemical Cycling
Raina M. Maier
1. Give an example of
a. a small actively cycled reservoir
b. a large actively cycled reservoir
c. a large inactively cycled reservoir
A great example of a small actively cycled reservoir is the atmospheric CO2 reservoir. Human
activity has dramatically affected this reservoir in the last 160 years. The CO2 concentration in
the atmosphere has increased from 280 ppm in 1850 to 387 ppm in 2009 (a 38% increase)
primarily due to use of fossil fuels.
2. Describe how the ocean has reduced the expected rate of increase of CO 2 in the
atmosphere since industrialization began.
The reservoir of carbonate found in the ocean acts as a buffer between the atmospheric and
sediment carbon reservoirs through the equilibrium equation shown below:
H2CO3

HCO3-

CO2
Thus, some of the excess carbon dioxide that has been released has been absorbed by the oceans.
This is not without consequences. The increase in carbon dioxide in the oceans has caused an
accompanying slight decline in pH. One repercussions of a decrease in ocean pH is that
organism with calcium carbonate (CaCO3) shells will not be able to deposit calcium normally
(e.g., corals) resulting in weakened shells and structures.
3. What is the concept behind fertilization of the ocean with iron?
It has been hypothesized that iron is the limiting nutrient for photosynthesis in portions of the
ocean where nutrients levels are relatively high but chlorophyll levels are low. Thus, it has
been suggested that fertilizing the oceans (which are responsible for 50% of the
photosynthesis on earth) with iron could increase photosynthesis which would in turn
increase CO2 uptake from the atmosphere (see Information Box 14.5).
4. What strategy is used by microbes to initiate degradation of large plant polymers such
as cellulose?
Large polymers like cellulose, hemicelluloses, and lignin are so poorly water soluble that cells
cannot take them up. Therefore, cells produce and release enzymes (extracellular enzymes)
that can initiate the degradation process producing smaller organic compounds that can be
taken up and metabolized by the cell. The best described example of this is for cellulose.
Cells produce three different extracellular enzymes, a β-1,4-endoglucanse that can cleave the
interior of cellulose molecule randomly, a β-1,4-exoglucanse that cleaves the last two
residues from the end of the cellulose molecule forming cellobiose, and a β-1,4-glucosidase
that cleaves cellobiose into glucose which can be taken up by the cell.
5. Define what is meant by a greenhouse gas and give 2 examples. For each example
describe how microorganisms mediate generation of the gas, and then describe how
human activity influences generation of the gas.
A greenhouse gas is one that can absorb infrared radiation (heat) from the sun that is being
reflected from the earth’s surface back into space. Trapping of this radiation or heat causes
warming of the earth’s atmosphere. Two examples of greenhouse gases are CO2 and CH4.
Microbes mediate generation of CO2 through general respiration of organic matter where the
organic matter is oxidized to CO2 to generate energy for metabolism and creation of new
cells. Human activity has increased emissions of CO2 to the atmosphere through use of fossil
fuels. Basically, we burn fossil fuels (oxidize them to CO2) for energy to drive cars and heat
our homes.
Microbes mediate generation of CH4 through general anaerobic respiration of organic matter.
The use of CO2 as a terminal electron acceptor results in generation of CH4. Human activity
has enhanced some important reduced environments that produce large quantities of CH4
through anaerobic microbial activities. These include rice paddies, landfills, and the rumen
of cattle, all environments that have added substantially to the CH4 released to the
atmosphere.
6. Both autotrophic and heterotrophic activities are important in element cycling. For
each cycle discussed in this chapter (carbon, nitrogen, sulfur, and iron), name the most
important heterotrophic and autotrophic activity. Justify your answer.
For the carbon cycle, the most important heterotrophic activity is respiration and the most
important autotrophic activity is photosynthesis. These two activities are the basis for the
carbon cycle which moves carbon between its fixed organic forms and its inorganic form
(CO2).
For the nitrogen cycle, arguably the most important heterotrophic activity is denitrification or the
use of nitrate as a terminal electron acceptor and the most important autotrophic activity is
nitrification where the energy obtained from the oxidation of ammonium is used to fix CO 2.
These activities cycle nitrogen between its most oxidized form (nitrate) and its most reduced
form (ammonium) and make it available to cells and higher organisms (nitrogen makes up
approx. 14% of the dry weight of a cell). Nitrogen fixation is also an important activity but it
is not confined solely to heterotrophs or autotrophs.
For the sulfur cycle, the most important heterotrophic activity is sulfate reduction where sulfate
serves as the terminal electron acceptor in anaerobic respiration. The most important
autotrophic activity depends on the environment. In aquatic environments, where anaerobic
photosynthetic bacteria are more abundant, anaerobic photosynthesis results in the oxidation
of sulfide. In soil environments, chemoautotrophic aerobic oxidation of sulfide is more
common.
For the iron cycle, the most important heterotrophic activity is iron (III) reduction where iron
serves as the terminal electron acceptor in anaerobic respiration. This is a common activity
in anaerobic soils since iron is abundant in the earth’s crust. The most important autotrophic
activity in soils is the aerobic chemoautotrophic oxidation of iron (II) which is used to fix
CO2. This is usually found in acidic soils or hot springs.
7. What would happen if microbial nitrogen fixation suddenly ceased?
Microorganisms are the only naturally occurring source of fixed nitrogen. Thus, if nitrogen
fixation ceased, the amount of fixed nitrogen available for microorganisms, plants, and animals
would decline and eventually limit their growth. Humans can also fix nitrogen using fossil fuels
but it is an energy intensive and, thus, expensive process.