Microorganisms in terrestrial ecosystems Flashcards
Soils as a Microbial Habitat
- Diverse soil types
- Inorganic particles enmeshed with complex biological communities
- Rainfall can:
–generate saturated soils
–wash nutrients out of soil- waterlog soils -> anoxic (No O2)
- Soil structure:
- A ‘well structured’ soil has plenty of pore
spaces for the movement of water, gases,
nutrients, roots and a vast array of
organisms
- A ‘well structured’ soil has plenty of pore
Major Factors Affecting Soil Microorganisms
- Soil moisture
- Soil temperature
- Soil aeration
- Soil pH
- Available nutrients
- Microbial intercations
Effects of soil moisture
–saturated soils stimulate anaerobic microbes
–dry soils limit all microbial activity
Effects of Soil Temperature
–a major environmental factor
–tropical soils are relatively stable
–mid-latitude (temperate) soils are very variable
(summer-winter)
–soil microbial activity is limited in winter
Effects of soil aeration
–essential for gas exchange
–saturated and clay soils have restricted gas exchange
Effects of soil pH
–optimum varies among microbial groups
–pH 5-8 for most bacteria, cyanobacteria
and protozoans
–pH 4.5- 6.5 for fungi
Effects of available nutrients
–organic matter provides carbon and energy
–inorganic nutrients
–‘mineral soils’ <20% organic C
–‘organic soils’ >20% organic C
Effects of microbial interactions
–positive and negative (Symbiosis)
Soil Organic Matter
Supplies the nutrients for microbial growth:
*Retains nutrients
*Contributes to soil structure
*Holds water for plant use
Three general pools:
1. living biomass (roots, animals, microorganisms)
2. decomposing residues of living things and added organic wastes
3. humus
Microbial diversity in soils
Is very complex and contains a large variety of habitats
–as a result, microbial diversity in soils is greater than aquatic environments
–this diversity has been a challenge to our understanding of soil microbial ecology
bacteria, archaea, protists (protozoans and microalgae), fungi and viruses all grow
only a small portion has been cultured, not even close to knowing everything
metagenomics and soil
the entirity of genomes in a sample
- allows for the identification of organisms in a sample e.g. soil using their genomes
The organisms are referred to as OTUs
OPERATIONAL TAXONOMIC UNITS
The more OTUs the more diverse the sample
Important activities of microbes in soil
- Decompose (‘remineralise’) soil organic matter
- Major roles in biogeochemical cycling
–C, N, S, Fe and Mn cycles - Support the soil food web
–i.e. a source of food for soil protists and
animals - Contribute to soil structure
- Form associations with plants that
determine plant survival
Microorganisms contributing to soil structure
- Soil stabilisation by microbes
- production of hyphae, matrix formed helps bind soils together
- bacterial capsules, bacterial secretion helps bind soils together - Binding of soil particles to bacteria due to electrostatic charges
- Humus
* Sponge-like material
* Retains H2O, organic & inorganic chemicals in the soil
* Important for soil fertility
Humus as a soil stabiliser
Formed from easily degraded SOM when microbes break it down
Complex structural carbohydrates are degraded
- Humus is the highly stable and complex organic material left after these extensive
decomposition processes
Mycorrhizal (‘fungus-root’) associations
- Mutualistic
- In most cases, the fungus enters the root
tissue and cells (endomycorrhizal) can also be ectomycorrhizae - Involves mycorrhizal fungi that:
1. Colonize plant roots
2. Obtain photosynthetically-derived carbohydrates from the host plant (i.e. NOT saprophytic)
3. Provide a variety of benefits to host plants
Ectomycorrhizae (ECM)
*Formed by ascomycete and basidiomycete fungi
* Colonize almost all trees in cooler climates
*Transfer P and N to the roots
*Aggregated fungal hyphae produce rhizomorphs that extend from the plant root and a meshwork of extracellular hyphae within the root (Hartig net)
* Nutrients taken up by rhizomorphs pass through hyphal sheath then into Hartig net filaments which have numerous contacts with root cells
Arbuscular Mycorrhizae (AM)
EXAMPLE OF ENDO
*The most common type of mycorrhizal
association
*Typically associated with crop and
tropical plants
* Characterized by tree-like hyphal
networks called arbuscules that
develop intracellularly in host cells
between the cell wall and the plasma
membrane (intracellular)
Mycorrhizal (‘fungus-root’) associations benefits to plants
- Enhance uptake of essential nutrients e.g. P and N
- Provide plant host with protection from disease, drought, nematodes, and other pests
- In arid environments, they aid in water uptake
Nitrogen Fixation
- Mutualistic
a symbiotic relationship between N-fixing bacteria and plants
–N-fixing bacteria (“rhizobia”) invade plant roots, forming “nodules” inside which rhizobia enzymatically convert gaseous nitrogen (N2) to ammonium (NH4
+) – incorporated into amino acids
Process of nodules forming in nitrogen fixation
- plant flavonoids secreted into soil activate rhizobial nod genes
- the resulting “nod factors” induce
plant root cell division (curling) and
“uptake” of rhizobia by the plant - rhizobia enter the root via an
“infection thread” and are released intracellarily into plant root cells - the rhizobia are enclosed in host cell
membrane to form a symbiosome - the bacterial cells change morphology, forming N-fixing bacteroids in the symbiosome
- the assembly of many bacteroids
causes root enlargement forming
nodules in the root, where N fixation
occurs - Depends on a key enzyme -
nitrogenase only produced by bacterias
Nitrogenase in Nitrogen fixation
– Nitrogenase is very sensitive to O2
– The O2 concentration is regulated by
the Fe-containing compound leghaemoglobin
– Leghaemaglobin ‘regulates’ the O2
concentration in the nodules
