Soil Microbiology
Soil microorganisms
Fungi
Bacteria
Fungi
Fungi
What are Fungi?
• Soil fungi are eukaryotic organisms, which can be unicellular, but often are multicellular.
• Compared to bacteria, fungi have more complex morphologies and life cycles. Yeasts are single-
celled fungi that reproduce asexually through budding.
• Most other fungi have highly branched filaments with strands 2 to 30 mm in diameter and several
centimeters long.
• Most fungi are aerobic except for yeasts, which can survive in anaerobic environments by
fermenting sugars into alcohol.
Fungi
Examples of Fungi found in Soils
• In many ecosystems, fungi constitute the largest biomass of all the soil
organisms, ranging from 500 to 5000 kg/ha.
• An individual fungus can include many fruiting bodies scattered across a large
area with extensive underground hyphae.
• Among the soil fungi, one can find oomycetes, trichomycetes,
chytridiomycetes, zygomycetes, ascomycetes, basidiomycetes, and imperfect
fungi.
• Fungi found in soil fall mostly within three groups; decomposers, mutualists
(mycorrhizal fungi), and pathogens.
Fungi
Examples of Fungi found in Soils
• Some of the common fungi found in soil include Amanita, Tricholoma,
Torrendia, Descomyces, Thelephora, Verticillium, Phytophthora,
Rhizoctonia, and Pythium.
• Since fungi prefer a rather low soil pH and a wide C/N-ratio, they dominate in
raw humus and mor and mull soils
• Mull - mixture of organic matter and mineral soil of crumbly or compact structure.
• Mor - organic matter practically unmixed with mineral soil, usually more or less matted or
compacted.
• Mycorrhizal fungi are mostly found around plant roots, whereas other groups
of fungi are found distributed throughout the soil.
• Fungi, in general, tend to dominate over bacteria and actinomycetes in acidic
soils as they can tolerate a wider range of pH levels.
Fungi
Positive effects of fungi in Soil
Soil fungi can be classified into three functional groups including:
(1) Biological controllers: regulate diseases, pests, and the growth of other
organisms
(2) Ecosystem regulators: responsible for soil structure formation and
modification of habitats for other organisms by regulating the dynamics of
physiological processes in the soil environment
(3) species participating in organic matter decomposition and compound
transformations
Fungi
Positive effects of fungi in Soil
• Fungi play essential roles in the soil where they help in nutrient cycling and
disease suppression, all of which maintain the health of the soil and increases
crop yield.
• Fungal hyphae bind soil particles together and stabilize soil aggregates.
• Fungi also form beneficial mycorrhizal symbioses with almost all terrestrial plants.
Mycorrhizal fungi enhance the uptake of mineral nutrients (e.g., phosphorus and
zinc) to the plant in exchange for carbon compounds fixed by plant
photosynthesis.
• Fungi are important decomposers of organic matter in the soil here they use
nitrogen in the soil to decompose woody carbon residues.
• The large size of fungi protects plants against pests, diseases, and drought. Fungi
are also producers of antibiotic compounds such as penicillin and cyclosporins.
• Lichens are symbiotic associations of a fungus and an algae or a cyanobacterium.
They are important soil colonizers that play critical roles in soil formation and
stabilization.
Source of antibiotics
Fungi also play a very important and vital in the production of
antibiotics. For example penicillin and grisoflavin, in the production of
vitamins such as riboflavin, and of various important drugs such as
ergotamine and cortisone.
• Fungi as food
Fungi also a great source of food. The mushroom has a great importance as a
food material because it is rich source of protein and vitamins and their excellent
flavour. But some mushroom are deadly poisonous and cause death .
• Source of enzymes, hormones and vitamins
Many species of fungi are good source of enzymes like amylase and invertase.
Similarly fungi are an important source of hormones such as Gibberella
fujikuroi is a plant hormone.
Roots without mycorrhizae Source: Harrison et al 1999
Roots with mycorrhizae
Source: Harrison et al 1999
Ecological plant-microbe Interaction
• Fungi benefit most plants by suppressing plant root diseases and
promote healthier plants by attacking plant pathogens with fungal
enzymes.
• Competition .. succession
• Antagonism
• Supply protective sheath during drought
Ecological plant-microbe Interaction
• Initial colonizers.. “sugar Fungus”
• Absorb simple sugars from plants
• Accumulation of waste… less dominance
• “cellulose degraders”.. Most diverse
• degradation of straw with high C:N ratio
• fungus parasitize or decompose other fungi to obtain
nitrogen for growth and enzyme production.
• Degradation of lignin.. Low N
Fungi
Negative effects of fungi in Soil
• The pathogenic group of fungi in soil results in various plant diseases by penetrating the plant
tissue and creating a weakened, nutrient-deficient plant.
• In addition to the direct effects on the plants, fungi also affect the interactions between plants
which change the competitive balance between two species.
• Mycorrhiza and fungal pathogens also affect seedlings, resulting in harmful consequences for
plant population dynamics.
Soil-Borne Plant Pathogenic Fungi
• Fungal species in the phyla ascomycota and basidiomycota are among the most common soil-
borne pathogens of plants.
• In the absence of a host, these fungi can survive in soil as saprophytes on plant residues or in the
form of spores or resting structures, such as sclerotia, for long periods of time, even under
adverse conditions.
• Roots are often the primary site of infection, although developing seedlings, stem bases, and
crowns may also be the first target of fungal attack.
• Soil-borne plant pathogenic fungi cause a variety of diseases, such as root rot, stem rot, crown
rot, damping-off, and vascular wilts, resulting in significant economic losses in the yield and
quality of agricultural and horticultural crops worldwide.
