the role of microbes in nutrient cycling Flashcards
1
Q
the terrestrial carbon cycle
A
- higher plants fix carbon into sucrose, most sucrose is converted into plant cell walls (lignocellulose)
- fungi (dominant decomposers, responsible for most respiration) in the soil decompose lignocellulose and respire, releasing CO2
2
Q
the aquatic carbon cycle
A
- phytoplankton leak organic carbon
- oligotrophic bacteria (survive in low nutrient conditions and grow very slowly) survive off the very dilute solution of sugar and amino acids and respire
3
Q
overall carbon cycle
A
- photosynthesis = respiration
- photosynthesis is from >50% microbial/marine photoautotrophs (don’t need organic carbon for growth)
- respiration is from mainly microbial chemoheterotrophs (need to ingest organic carbon)
4
Q
organic carbon
A
- compounds that have carbon-carbon bonds
- CO2 has inorganic carbon (mineral)
5
Q
soil
A
- progressive decomposition of plant litter downwards with fresh material added on top
- subsoil = lighter, less organic matter
6
Q
MOR soil
A
- acidic moorland soil
- distinct layers due to absence of earthworms
7
Q
rate of decomposition
A
- depends on resource quality (C:N ratio) and environmental factors
- time to 95% decomposition is 6 months in rainforest and 100 years in rainforest
8
Q
measuring decomposition
A
- measure dry weight loss from litter bags (7mm mesh to keep earthworm access)
- similar curve to radioactive decay curve
- can measure half life and time to 95% decomposition, useful because majority of compounds easy to break down but it takes a relatively long time to break down the 5% left
9
Q
cotton strips and CO2 measurement
A
- cotton sheet (cellulose) inserted into soil
- measure how fast strip decays, top vs bottom
- using tensionmeter, loss of tensile strength if more degraded
10
Q
IRGA. CO2 measurement
A
- infra red gas analyser
- measures soil respiration directly
- oxygen double bonds in CO2 has a distinctive infrared absorbance
- easier to measure when plants are absent as plants may fix some of carbon
11
Q
effect of biocides on litter
A
in order of most impact on litter respiration rate
1. benomyl, kills fungi
2. DDT, kills soil animals
3. streptomyocin, kills bacteria
12
Q
interactions with soil animals
A
- detritivores (earthworms, nematodes, springtails, woodlice)
- can facilitate and promote action of fungi and bacteria
- low biomass, high numbers
- <10% soil respiration
- main effect is physical, comminution (chewing up) of litter increases surface area for microbial attack
e.g. earthworms can break oak leaf into 1bn fragments
13
Q
nutrients
A
- nitrogen, major nutrient required for proteins, often limiting
- phosphorous, needed for making ATP and ribosomes, insoluble so low availability to plants but microbes can secrete acids to solublise it
- K, S, Mg, Fe and micronutrients needed as cofactors for enzymes
14
Q
resource quality and decomposition
A
= ratio of energy to nutrients (C:N)
- increases during decomposition
- C lost as CO2 (respiration), nutrients conserved
- C:N (200:1) ratio of wood becomes more favourable as it is broken down
15
Q
fungal nutrition
A
- penetration of substrate by hyphae by using turgor pressure and tissue softening hyphae
- can secrete enzymes inside tissue:
- cellulases (C)
- ligninases (C)
- proteases (N)
- lipases (C)
- phosphatases (P)
- nucleases (N,P)
16
Q
composition of plant litter
A
- cellulose (20-45%)
- hemicellulose, more branched (10-30%)
(sugar polymers for energy) - lignin (5-30%)
(aromatic polymers, recalcitrant)
17
Q
decomposition from white rots
A
- can decompose cellulose, hemicellulose and lignin
- uses ligninases and cellulases
- ligninases cause the bleaching of wood (lignin is brown)
- ligninase is manganese peroxidase
18
Q
decomposition from brown rots
A
- evolved 4 times from white rot ancestor (convergent evolution, more efficient)
- can decompose cellulose, hemicellulose but not lignin
- uses non-enzymatic demethylation to partially break down lignin
- e.g. dry rot
- lignin not removed, wood stays brown
19
Q
decomposition from soft rots
A
- decomposition of cellulose and hemicellulose, not lignin
- uses only cellulase
- water required to swell fibres
20
Q
non-self recognition of rot fungi, zone lines
A
- somatic incompatability
- different species or individuals of the same species come into contact, hyphae fuse/anastomose and die if non-self
- protective melanised zone lines formed around each colony, protecting against disease transmission
21
Q
white rots, breakdown of lingin
A
- manganese peroxidase enzyme (manganese is cofactor)
- converts hydrogen peroxide into hydroxyl free radicals (*HO)
- Mn2+ + H2O2 = 2Mn3+ + 2*HO
- hydroxyl free radicals are highly reactive, so degrades lignin
- also secrete laccases, oxidises phenolics to quinones which stabilises broken lignin bond, stopping it from reforming
- lignin is catabolised so the fungi can more readily access cellulose (not likely to be direct energy gain from catabolism)
22
Q
evolution of lingin and manganese peroxide
A
- lignin evolved 400mya, novel compound that could not be broken down
- created carboniferous ‘coal swamps’
- decomposition halted increasing O2 concentration in the atmosphere
- evolution of giant dragonflies (more efficient gas exchange)
- fungi evolved MnP enzymes 300mya (ear fungus ancestor), decreasing O2 concentration
23
Q
brown rot fungi, non-enzymatic demethylation of lignin
A
- lost ability to make MnP (costly enzyme to produce)
- harnessed the Fenton reaction to create hydroxyl free radicals (fenton reaction causes aging and cell damage in humans_
- Fe2+ + H2O2 = Fe+ + OH- + *HO
- non-enzymatic so cheaper and quicker, H2O2 penetrated wood tissues better than enzymes
- lignin is demethylated, not fully broken down (fungi doesn’t produce laccases to prevent bond reformation)
- lignin is weakened, forming a brown powder, fungus is able to access cellulose fully
- cellulose is fully degraded
