Soil Ecology Module 4: Soil Biotic INteractions

Mutalism

Symbiotic relationship where both partners benefit

Examples of mutalism

Legume/rhizobium, bobtail squid sheltering luminescent vibrio fisheri, gamma proteobacteria symbiont in tubeworm,

Competition

Both partners are harmed

Predation, Parasitism, and Pathogenesis

One organism is benefit, one organism is harmed

Co-Occurance

Nobody is helped or harmed

Commensalism

One partner benefits and the other is not affected

Changes is symbiotic relationship happen because of

environmental factors like resource depletion and temperature fluctuation

Example of antagonist competition between microbes

fungi (penicillin) or bacteria (ex. actinobacteria) producing antibiotics

Example of resource competition between microbes

extracellular enzymes degrading complex humic polymers

Example of mutualism between microbes

arbuscular microrhizal fungi and soil bacteria

Example of predation between microbes

lytic infection by viruses

Example of mutualism between bacteria

Sippewisset Pink Berries

Sippewisset Pink Berries

Outside: purple sulfur bacteria oxidize sulfide; Inside: sulfate-reducing bacteria

Arbuscular Mycorrhizal Fungi

hyphae successfully penetrate the epidermal cell via hyphopodium, then penetrates the cortex cell, forming highly branched tree-like structures inside the root (arbuscules)

Arbuscules

cauliflower shape maximizes SA for nut exchange; good for delivering phosphorus

Ectomycorrhizal Fungi

hyphae grow between root cortical cells but do not penetrate cell wall, forming intricate intracellular network (Hartig net)

Hartig net

Ectomycorrhizal fungus communicates with its plant host and exchanges nitrogen for fixed carbon

Mantle

Dense hyphal sheath around outer surface of root, allowing fungus to interact dynamically with rhizosphere microorganisms and shielding roots from soil pathogens (ecto)

Essential for water and nutrient uptake due to mantle around root surface

Ectomycorrhizal fungi

Colonized ecto roots

fungal mantle envelops root apex; tips swell into fork-shaped structures

Uncolonized roots

thin, small, unbranched

Glomeromycota

Most arbuscular species belong to this phylum; no fruiting bodies

Ectomycorrhizal Fungi Species

Independent evolutionary origin; most belong to ascomycota or basidiomycota

Fungi get 20% of plant’s carbon; arbuscular is 100% reliant on this

Benefits of nutrient exchange to fungi

Benefits of nutrient exchange to fungi

Boost drought/stress resistance via improved soil structure; prevents pathogens via antimicrobial suppression and physical obstruction

Ectorhizosphere

Rhizosphere soil: Narrow zone of soil directly influenced by root exudates (a few mm out from rhizoplane)

Rhizoplane

Epidermal surface of root: includes stick mucilage layers (produced by roots)

Endorhizosphere

All internal root tissues: includes intercellular spaces between cortical cells that microbes can inhabit

Global rhizosphere

Higher microbial activity when compared to bulk soil, even with smaller population size, due to higher energy availability

Plant influence (spatial gradient)

strongest at root surface, tapers into soil matrix via rhizodeposition

Rhizodeposition (spatial gradient)

Releases organic carbon into soil (20% of produced by plants)

Microbial activity (spatial gradient)

Peaks at root surface and decreases proportionally w/ plant influence

Diversity (spatial gradient)

Increases with distance to root

Plant Growth Promoting Rhizobacteria (PGPR)

Recycles nutrients via decomposition, protects plant from pathogens, improves soil structure through EPS

How legume roots allow Rhizobium entry

Roots excrete flavonoids which stimulate nod factors on roots. Nod factors cause root hair curling which allows Rhizobia to enter the plant.

Bacterioids

Legume divides cortical cells into nodule structure, which holds bacteria and allows them to differentiate into large, non-dividing bacterioids optimized for nitrogen fixation.