Lecture 5 - Exam 2: Occurrence and Distribution of Soil Microorganisms

Question 1

Discuss the disintegration of the supercontinent.

Answer 1

Microbial life was on the supercontinent and the disintegration of the supercontinent carries microbes with them.

Question 2

What are the dispersal mechanisms of microbes?

Answer 2

Air/wind, water, animal and human transport.

Question 3

Are there regional differences when it comes to the types of microorganisms?

Answer 3

Yes, the type of environmental conditions determine what kind of microbes are there. “Everything is everywhere but the environment selects.”

Question 4

Microorganisms are found at different depths of soils. Discuss that.

Answer 4

In the subsurface soil we typically find more prokaryotes than eukaryotes. Don’t find larger organisms usually in the subsurface soil. In the lower soil profile, you would find less microbes due to food availability.

Question 5

What is the rhizosphere? How unique and how immediate is it?

Answer 5

It is the soil region under immediate influence of plant roots. It is unique and is 1-2mm immediate.

Question 6

The rhizosphere is the ______ for living organisms.

Answer 6

Hot spot (because of food availability)

Question 7

Away from the rhizosphere we have ____ soil, not under the immediate influence of plant growth.

Answer 7

bulk

Question 8

________ consumption comes from the rhizosphere, about ___%-____%.

Answer 8

Oxygen ; 33-66

Question 9

What is mucigel?

Answer 9

Mucigel keeps plant roots intact with soil particles. It is a gooey material that covers the root surface. A gelatinous material that covers the root.

Question 10

What are the root exudates? Where is the location of excretion? What is the amount of fixed carbon excreted?

Answer 10

Types of chemicals, like sugars, amino acids, organic acids, vitamins, insoluble polymers, and biocidal compounds. Location of excretion: root caps and tips
10-30% fixed carbon

Question 11

What microorganisms are in the rhizosphere?

Answer 11

Bacteria, Fungi, and fauna

Question 12

What type of bacteria is generally found in the rhizosphere? How big are they? What are their storage materials? What is the surface coverage?

Answer 12

Usually Gram-negative bacteria in the rhizosphere. Bigger microbes here because there is more food and they will grow faster. Their storage materials are PHB, glycogen, and polyphosphate. They cover 5-10% of root surface area.
Note: Anaerobes can also live in the rhizosphere because when the aerobic microbes are consuming a lot of food and are active, they use up a lot of oxygen, so you can get anaerobes there.

Question 13

____%-____% of microbial population in the rhizosphere is fungi. They occupy about ___% of root. What kind of fungi do we find more of here?

Answer 13

10%-20% ; 3%
Find more symbiotic and pathogenic fungi in the rhizosphere.

Question 14

Are there more fauna in the rhizosphere, too?

Answer 14

Yes!

Question 15

Between the rhizosphere and the bulk soil, which is more diverse?

Answer 15

The bulk soil, but we are not sure why. Chemical heterogeneity and food source could be a factor, along with the rhizosphere being a more uniform environment.

Question 16

Winogradsky’s grouping and related terms: Zymogenous vs Autochthonous.

Answer 16

Zymogenous and autochthonous have different responses to input of simple carbon and readily compostable carbon.
Autochthonous organisms prefer low nutrient conditions and use complex carbon sources. Don’t use simple and readily decomposable carbon sources.

Question 17

What are the carbon sources for zymogenous and autochthonous organisms? What about their growth rates and population sizes?

Answer 17

Z: carbon source - simple
A: carbon source - complex
Z: growth rate - fast
A: growth rate - slow
Z: pop size - fluctuates
A: pop size - high and constant

Question 18

Copiotrophs vs oligotrophs

Answer 18

Copiotrophs are the organisms with a lot of rich nutrients. Oligotrophs prefer low nutrients and can’t handle a lot of carbon source & can’t handle the reactive oxygen species that are produced.

Question 19

R-selected vs K-selected organisms.

Answer 19

R: growth rate and reproduction - rapid
K: growth rate and reproduction - slow
R: handling stress & starvation - poorly
K: handling stress & starvation - well

Question 20

Describe soil aggregates.

Answer 20

Silt, clay, and sand particles exist and form secondary structures and that is soil aggregates. Soil aggregation is very important for controlling microbial activity and organic matter turnover.

Question 21

What are the components involved in aggregate formation?

Answer 21

Particles, adhesives, and physical forces

Question 22

What are the particles involved in aggregate formation?

Answer 22

Nanometer range: fine clays, organic molecules
Micrometer range: Microbes and silt particles
Millimeter range: Small rootlets, small fauna and microaggregates

Question 23

What are the adhesives involved in aggregate formation?

Answer 23

Polysaccharides, glycoproteins, and clays

Question 24

What are some physical forces in aggregate formation?

Answer 24

Wetting and drying is a natural force, root growth, driving machineries or walking over fields

Question 25

Discuss larger aggregates being composed of smaller aggregates.

Answer 25

Primary particles group together by adhesive compounds and plant debris and that will compose submicroaggregates and those come together and make microaggregates, making up macroaggregates.

Question 26

What are the aggregate sizes?

Answer 26

Microaggregates: <250um
Macroaggregates: >250mm
Different pore sizes in these aggregates. Microaggregate pore sizes: 0.2-6um
Macroaggregates pore sizes: 25-100um

Question 27

What are open pores?

Answer 27

Both the pore size and pore neck matter. Pore size determines who can live there and pore neck size determines who can get in. Pores provide shelter for microbes. A smaller pore neck size can protect microbes from predators and there is usually better moisture content & usually higher nutrient content as well.

Question 28

Describe O2 concentrations within aggregates form a cultivated soil.

Answer 28

Oxygen concentration is usually lower than outside.
Typically we have anaerobes living inside the center of aggregates and have aerobes living on the outside. Higher nutrients inside the aggregates.

Question 29

What are the survival strategies of soil bacteria?

Answer 29

Form small colonies, attach to surfaces (because that is where the organic carbon is, stay where the food is), colonize soil pores (to protect against predators and gain access to higher nutrient content inside the aggregates & prevent desiccation), reduce cell size (reduce growth rate when there is not food around. They use up their storage compounds when there is no external food source), use up storage compounds, have the ability to grow at low substrate concentrations, reorganize metabolic pathways to reduce the requirement for deficient nutrients, use alternative substrates, nonobligate bacterial predators, and become temporarily inactive.

Question 30

What type of bacterial cell has teichoic acids in their cell wall? What are teichoic acids rich in?

Answer 30

Gram +
Rich in phosphorous