Module 6 - Microbes in the Environment

Question 1

Define autotrophic respiration

Answer 1

CO2 flux (rate) that is strongly influenced by recently photosynthesised carbon from plants (e.g., root respiration, root-associated microbial respiration)

Question 2

Define heterotrophic respiration

Answer 2

CO2 flux (rate) that is strongly influenced by decomposition of older carbon such as organic matter (e.g., many free-living organisms)

Question 3

What are the two main pathways of carbon into soils (and which of these involves more labile carbon)?

Answer 3

Litter (e.g, dead shoots, roots, symbionts)
EXUDATES (active carbon via live roots and mycorrhizal fungi) -> more labile carbon, faster pathway

Question 4

What have large-scale forest girdling experiments shown about photosynthesis and soil respiration?

Answer 4

That Current Photosynthesis largely drives soil respiration (~50% less CO2 released from soil when trees have been girdled)

Question 5

Define Priming

Answer 5

An increase in degradation of native (often recalcitrant) carbon, because of inputs of labile carbon

Question 6

What is the difference between the degradation caused by White Rot fungi vs Brown Rot?

Answer 6

Brown Rot: degrades cellulose -> lignin
White Rot: degrades lignin -> cellulose

Question 7

Name 3 examples from the suite of enzymes required to break down complex molecules such as cellulose

Answer 7

Endoglucanase, Exoglucanase, Cellubiase

Question 8

Name 3 monomers that make up lignin

Answer 8

p-coumaryl alcohol; coniferyl alcohol; sinapyl alcohol

Question 9

Name the two enzymes found in White Rot fungi which can transfer electrons from H2O2 to lignin

Answer 9

Lignin peroxidase or manganese peroxidase

Question 10

Name the non-peroxidase enzyme which is found in White Rot fungi, and what it does

Answer 10

Laccase - directly oxidises lignin (demethylation)

Question 11

What happens to increase the rate of lignin breakdown when White Rot fungi start to degrade it?

Answer 11

Uncontrolled chain reaction splits open ring structures

Question 12

What are the three (mentioned) examples of important symbioses?

Answer 12

Lichens, Nitrogen-Fixing Bacteria and Mycorrhizal Fungi

Question 13

What are the two components of lichen?

Answer 13

Mycobiont (fungus) and Photobiont (algae or cyanobacteria)

Question 14

What do the two components of lichen each contribute?

Answer 14

Alga provides sugars via photosynthesis; fungus protects alga from dessication

Question 15

What is noticeable about where lichen grow (2 things)?

Answer 15

They are tolerant of extremes (pioneer colonisers on rock faces etc) and SOME are sensitive to air pollution - bioindicators

Question 16

What is the name of the family of plants that can fix nitrogen?

Answer 16

Fabaceae

Question 17

How does nitrogen fixation occur in Fabaceae?

Answer 17

Due to mutualistic bacteria (alpha- and beta-proteobacteria) that colonise Fabaceae roots; catalysed by the enzyme nitrogenase

Question 18

How do plants prevent “cheating” by some bacteria, when multiple species colonise a plant?

Answer 18

They control oxygen supply to nodules to “reward” co-operative bacteria

Question 19

What are Mycorrhizas?

Answer 19

A mutualistic association between plant roots and soil fungi [based on transfer of soil derived nutrients, N + P, to the plant, and reciprocal transfer of photosynthate (carbon) to the fungus]

Question 20

What is the defining feature of Arbuscular Mycorrhizal Fungi?

Answer 20

Arbuscules - the site of nutrient exchange (between plant and fungus) within the root

Question 21

What is the other feature of arbuscular mycorhizzas besides arbuscules?

Answer 21

The Extra-Radical (or external/extramatrical mycelium) -> grows beyond roots into soils and forms extensive networks to facilitate nutrient and water uptake

Question 22

Which types of plants are the most common ectomycorrhizal host plants?

Answer 22

Woody, perennial plants

Question 23

What are the effects of increased Arbuscular Mycorrhizal diversity on plants?

Answer 23

Increased shoot biomass, root biomass, hyphal length and phosphorus content of plants

Question 24

What “dogma” of inorganic nutrient use by plants, has been challenged recently?

Answer 24

That plants can only take up inorganic forms, and rely on free living microbes to convert organic forms with enzymes;
It is possible that plants, aided by Mycorrhizal Fungi, could also mineralise or take up simple organic forms directly

Question 25

Name the species of parasitic fungus that mainly infects ants?

Answer 25

Cordyceps sp.

Question 26

What three types of relationship are technically considered to be “symbiosis”?

Answer 26

Mutualism, Commensalism and Parasitism

Question 27

What does Cordyceps sp. do once it has infected an insect?

Answer 27

It causes the host to move to an area where conditions are optimal for fungal spore dispersal, then the host dies.

Question 28

Describe what is meant by “fungal gardeners”

Answer 28

Some ants “harvest” particular genotypes of fungi so they can use the substrates that the fungus breaks down
Some also harvest bacteria that produce antibiotics to reduce competition for the “farmed” fungus

Question 29

Name a species of nematophagous fungi

Answer 29

Drechslerella sp.

Question 30

In what kind of soil are nematophagous fungi usually found?

Answer 30

Nitrogen-deficient

Question 31

What animal species do some Ectomycorhizzal Fungi (EMF) predate?

Answer 31

Springtails (Collembola)

Question 32

What effect did the presence/absence of Collembola have on CO2 release from soils with AMF?

Answer 32

CO2 release was LOWER when Collembola were present - “Collembola reduce flux of recent plant assimilate from Mycorrhizal Fungal networks”

Question 33

What is meant by PSF?

Answer 33

Plant soil feedback - plants have an effect on both their biotic and abiotic community, which can effect subsequent generations of plants

Question 34

What is the common plant response to aphid attacks?

Answer 34

Plants release Volatile Oxygen Compounds (VOCs) which are repellant to aphids and attractive to aphid predators (e.g., wasps)

Question 35

Is the increased attractiveness to aphids when plant roots are colonised by AMF due to increased Phosphorus?

Answer 35

NO - adding AMF increased aphid attractiveness, but adding P did NOT. “Must be something else”

Question 36

What is believed to be the reason that AMF presence affects plant attractiveness to aphids?

Answer 36

Hyphal network signalling

Question 37

What is the benefit to the fungus of hyphal network signalling to repel aphids?

Answer 37

The fungus benefits from a good carbon source from all the plants in the network

Question 38

Name the three main families of plants (mentioned) that are involved in ectomycorrhizae

Answer 38

Pinaceae, Dipterocarpaceae and Fagaceae