Lecture 13: Microbiology of Soil Flashcards
What are the 2 groups soil can be divided into?
Mineral Soils: derived from rock weathering & other inorganic materials
Organic soils: derived from sedimentation in bogs and marshes
What kind of microorganisms are present in the highest amount in soil?
Proteobacteria
What is in the O layer of soil?
Layer of undercomposed plant materials.
What is in the A layer of soil?
Surface Soil, high in organic matter, dark in colour, high microbial acivity, tilled for agriculture
What is in the B layer of soil?
Subsoil, Minerals, humus leeched from soil surface accumulate here, little organic matter, less microbial activity than A
What is in the C layer of soil?
Soil base, develops directly from underlying bedrock, low microbial activity
What is humus?
Dead plant material resistant to decomposition, keeps H2O and nutrient in soil=good for fertility
What are prokaryotes largely responsible for?
- production of humus
- cycling of nutrients
- release of minerals from soil particles (production of acids from organic compounds solubilize the minerals
- N fixation
Which layer of soil contains the most microorganisms?
Top layer of soil
What is the rhizosphere?
Soil that surrounds plant roots and receive plant secretions-> also rich in microorganisms
What is the largest source of CO2?
Atmosphere
What is the largest reservoir of C?
Rocks and sediments
What kind of N fixers are azotobacter and cyanobacteria?
Free living aerobic
What kind of N fixers are Clostradium and purple and green bact.?
Anaerobic Free living
What are the symbiotic N fixers?
Rhizobium, Frankia
What catalyses the fixing of N2?
Nitrogenase complex (Metal cofactors)
What is the final product of the nitrogen cycle?
NH3
How many electrons are needed for the nitrogen cycle?
8, from pyruvate, 2 lost as H2 in the process
What is dinitrogenase reductase inhibited by?
O2, (O2 inhibits N fixation)
Where are free living N fixers found?
Soil, esp for inorganic matter
What is azotobacter?
Strict aerobe, N fixer, enzyme protected by high O2 consumption keeps intracellular environment anaerobic
What is colostridium?
Strict anaerobe, free living N fixer
What is cyanobacteria?
Major N fixing organisms in nature, only some species can fix N2. Produce energy via oxygenic photosynthesis
What is a heterocyst? (Cyanobacteria)
Specialized anaerobic cells in cyanobacteria. Lack PSII so can’t produce O2. Regular cells provide carbs, cell wall is extra thicc to slow O2 diffusion.
How are nodules formed?
- Recognition and attachment of bacterium to root hair
- Excretion of nod factors by bacterium=root hair curls
- Bacterial invasion of root hair
- Travel to main root via infection thread (Cellulose tube)
- Formation of bacteroid state within plant cell
- Continued plant and bacterial division, forming mature root nodule
How are O2 levels controlled in nodules?
O2 binding protein leghemoglobin, produced by plant cells
What is a bacteriod?
Swollen and misshapen bacteria, fix N2 while being surrounded by plant cytoplasm. Terminal structure, cannot shed in environment.
How does the Sierra mixe maize fix nitrogen?
In aerial roots, through mucilage associated microbiota
What is mucilage?
Viscous sugar rich fluid, with N fixers
What promotes sulfur reduction?
Anaerobic. Produces H2S which is toxic to plants.
Which is more soluble, ammonium or nitrate?
Nitrate, more readily available for plants but they can use both.
Where do most plants get their N?
Free living N fixers
What is a source of nitrate?
Nitrifying bacteria (Competes with plants for NH3)
What promotes denitrification?
Anaerobic. Bad for plant growth.
