ANSWERS TO QUESTIONS
Chapter 4
Earth Environments
Raina M. Maier and Ian L. Pepper
1A.
Calculate the volume of bacterial biomass in one gram of soil, given the following
assumptions: soil bacteria are small spheres of diameter 1 μm, the total number of
bacteria is 108/g, and the bulk density of soil is 1.4 g/ml.
The volume of 1 cell is 4/3πr3 where the radius of the cell is ½ the diameter = 0.5 um
4/3 π (0.5 um)3 = 0.523 um3/bacterial cell
There are 108 bacteria/g soil
Therefore the volume of 108 bacteria in a gram of soil is:
(108 bacterial cells/g soil)(0.523 um3/bacterial cells) = 5.23 x 107 um3/g soil
B.
If the porosity of the soil is 50%, what percentage of the pore space is occupied by
bacteria?
The bulk density of the soil is 1.4 g/ml. Therefore 1 g of soil occupies a volume of:
1 g soil
= 0.71 ml
1.4 g soil/ml
50% of this volume is pore space therefore the volume of pore space in 1 g of soil is:
0.5 ( 0.71 ml) = 0.36 ml
From part A, the volume of 108 bacteria in a g of soil is 5.23 x 107 um3
Now convert um3 to ml: 1 um3 = 1 x 10-12 ml
5.23 x 107 um3 (1 x 10-12 ml) = 5.23 x 10-5 ml is the volume occupied by 108 bacteria
um3
5.23 x 10-5 ml (100%) = 0.014% of the pore space in this g of soil is occupied by bacteria
0.36 ml
2.
Is soil a favorable environment for microorganisms?
This is a great discussion question because the answer is yes and no. The following answer can
be used to begin this discussion. The soil is a very competitive environment, one that often has
low amounts of substrate and essential nutrients available. In addition, there are often abiotic
stresses imposed by changes in water content, temperature, pH, and oxygen availability. So for
certain microbial populations that can competitively adapt to these conditions, soil is a favorable
environment. On the other hand, for microbial populations that are added to the soil through
either natural processes or intentionally, establishing a niche in a soil can be very difficult
because it will require displacing an already established population. In this case soil is not a
favorable environment.
3.
Define the terms surface soil, vadose zone, and saturated zone.
A surface soil is that found at the earth’s surface and reaching down through the plant rooting
zone where the action of climate and living organisms (including plants, animals, insects, and
microorganisms) has weathered soil parent material over time.
The vadose zone is the unsaturated region that lies between the surface soil and the saturated
zone containing mostly unweathered parent materials and very low organic carbon content. The
pores in vadose zone materials are not saturated with water.
The saturated zone lies directly beneath the vadose zone and is composed of unweathered parent
materials where the pores are saturated with water. These regions are also known as aquifers.
In terms of an earth environment for microorganisms, compare and contrast levels of
microbial activity in: a) surface soils; b) the vadose zone; and c) the saturated zone.
In general microbial activity is highest in surface soils because organic carbon and nutrient levels
are much higher than in the vadose and saturated zones due primarily to the presence of plants
and plant root exudates. The level of microbial activity in shallow vadose and saturated zones
tends to be higher than for deep vadose and saturated zones. Again, this is because organic
carbon and nutrients and leach from surface soils into shallow vadose and saturated zones more
easily than deep vadose and saturated zones.
4.
a.
Give at least two examples of aerobic heterotrophic bacteria commonly
found in surface soils.
Rhizobium, an important genus of Gram-negative nitrogen fixing bacteria found in soil.
Bacillus, a commonly cultivated Gram-positive soil genus that is known for spore formation and
general aerobic degradation of carbon substrates.
b.
Give at least two examples of anaerobic heterotrophic bacteria commonly
found in surface soils.
Clostridium is an obligately anaerobic fermentative Gram-positive spore-forming genus found in
soil. They are considered opportunistic pathogens and many species produce toxins, for example,
Clostridium botulinum is known for production of the neurotoxin botulin.
Desulfovibrio is a Gram-negative sulfate-reducing genus that utilizes small molecular weight
carbon compounds such as lactate or ethanol as electron donors. It is found in anaerobic
environments such as saturated surface soils that contain sulfate.
c.
Give at least two examples of aerobic autotrophic bacteria commonly found
in surface soils.
Nitrosomonas, a chemoautotrophic Gram-negative genus that is important in nitrification
(conversion of ammonium to nitrite).
Acidothiobacillus, a chemoautotrophic Gram-negative genus that thrives in low pH environments
and participates in the formation of acid mine drainage through the oxidation of sulfur and iron.
d.
Give at least two examples of anaerobic autotrophic bacteria commonly
found in surface soils.
Green sulfur bacteria are a photoautotrophic family of bacteria that oxidize sulfide to elemental
sulfur. They are found in soil and perhaps more commonly in water environments.
Desulfovibrio, which was also mentioned in the answer to part b, is a Gram-negative sulfatereducing genus that can utilize H2 as an electron donor instead of small molecular weight carbon
compounds. In this case it is acting as a chemoautotroph.
e.
Describe the contribution of each of the microbes you have listed to the
ecology of the soil environment.
See answers above.
5.
How does soil moisture affect the activity of aerobic and anaerobic soil microbes?
The availability of water is necessary for all microbial activity. For aerobic microorganisms,
typically optimal microbial activity occurs at -0.1 atm which is the transition between capillary
water and free water in a soil. This maximizes both water and oxygen availability. In contrast,
saturated conditions are more conducive to anaerobic activity. In this case because the solubility
of oxygen in water is low, oxygen is quickly used up creating anaerobic conditions and
enhancing anaerobic activity.
6.
Identify 5 major functions or roles of soil microbes that beneficially impact human
health and welfare.
1) Carbon and nutrient cycling
2) Aid in plant establishment and growth
3) Are an important component of soil formation
4) Nitrogen fixation
5) Produce natural products used in medicine (e.g., antibiotics) and industry (e.g., enzymes)
6) Water purification
7.
How does cation exchange capacity affect microbial activity in a soil?
Soils generally have an average CEC of 15-20 meq per 100g. Soils that have low CEC (sandy
and low organic matter content) often have low water holding capacity and limited nutrient
content because they cannot hold cations tightly and therefore these nutrients are leached during
precipitation events. Thus, soils with low CEC generally have lower microbial activity.
8.
What are viable but nonculturable microbes? What is the “normal” percent of
viable-but non-culturable bacteria in a healthy soil? What is likely to happen to this
viable but non culturable population if the soil is adversely impacted by addition of
a toxic amendment?
As discussed in the text, perhaps a better term for viable but nonculturable is viable but difficult
to culture. Normally 90 to 99% of the bacteria in a healthy soil are not observed on a typical
medium used to culture bacteria from soil such as R2A. However, scientists are developing
specialized media and culture techniques to recover higher percentages of bacteria from soil.
When a toxic substance is added to soil, the difficult to culture percentage increases.