Amensalism - (antagonism)• one population produces antimicrobialsubstances active against a second– e.g. pH (acids), antibiotics, toxic metabolites• alteration in [inorganics]–pH• oxidation of S by Thiobacillus thiooxidansproduces H 2 SO 4 and greatly reduces pH• acid mine drainage precludes most microbialgrowth

Production of `organic toxins'• antibiotics– substance produced by one M.O. that in lowconc. inhibits growth of another M.O.– biostatic vs. biocidal• large conc. produced in nutrient-richmedia, but role in natural habitat littleunderstood– probably important in zones of high nutritionsuch as plant residue

Parasitism• food relationship– one organism uses another for its food base– constant association• contrast to predation• usually used in reference to pathogen– i.e. causing harm, but this is not necessary– necrotrophic parasite• host tissue killed in advance of infection– biotrophic parasite• host tissue kept alive

• viral parasites– lytic phage attack bacterial populations– specific phages demonstrated for Gram -• bacterial parasites– Bdellovibrio parasitic on Gram (-) bacteria• penetrates the wall and lysis cell– Myxobacterium Gram + and Gram - bacteria• fungal parasites– chytrids on algae, protozoa and other fungi• hyperparasites - parasites feeding on parasites• e.g. Trichoderma feeding on root-infecting fungi suchas Sclerotium sp.

Bdellovibrio life cycle

Predation• one organism engulfs or digests another– interaction of short duration• protozoan or nematode populations feeding onbacteria– non-discriminatory consumption (grazing)• slime molds ingest bacteria• Biofilms protect cells from predation• Microcolony formation in response to predation:– Quorum sensing in planktonic bacteria.



What is the primary factor thatstructures soil communities??

• Mutualism?• Predation?• Amensalism?• Environmental variation?• Competition?• Spatial Separation?

RhizospheresChapter 8

Rhizosphere (plant /microbe)• Hiltner (1904)– region of the soil in which roots generallyinduce proliferation of M.O.– metabolic activities of rhizosphere microbesare vital to plant growth– can be several mm thick

ASA Slide Set

• R/S ratiosRhizosphere/Soil Ratio– 10 to 50 for bacteria– 5 to 10 for fungi• greatest stimulation of short, Gram (-) rods• Characteristic of soil, plant.• Generation times– Pseudomonads 5 h in rhizo vs 77 h in bulk– Bacillus 39 h in rhizo vs 100 h in bulk

Rhizodeposition• Organic compounds excreted by roots–amino acids– organic acids– carbohydrates - mostly simple sugars– nucleic acids– vitamins - biotin, inositol, thiamine– enzymes - amylase invertase, phosphatase, protease– other - hormones (auxin, ethylene), lipids, peptides• serve as source of energy, carbon, andnitrogen for microorganisms

Mucigel - plant and microbialcarbohydrates• gelatinous material on the surface of roots• maintains root - soil contact• plant origin– polysaccharides produced by Golgi apparatusof root cap cells• Extensive quantities produced. e.g. for wheatmore than 7 tons / ha (more than grain yield)•many COO - groups which help cation exchange– lysates - autolysis of older epidermal cells

– other root excretions -• numerous low M.W. compounds released– e.g. organic acids– Enzymes– Amino acids–Vitamins– Sugars– Pyrimidines and puridines• microbial origin– bacterial polysaccharides (e.g. capsules)– lysed microbial cells

• RhizoplaneAdditional Terms– bare root surface composed of epidermis,sloughed cells and mucigel• intergrades with the rhizosphere• Spermosphere -– carbohydrates which leach from seed andinfluence microbial populations• can support many of the same populations asrhizosphere, but to a lesser extent.

• Mycorrhizosphere -– zone of enhanced microbial activitysurrounding mycorrhiza• Rhizosheaths– persistent nodal roots of corn and somedesert and dune grasses have `soil sheaths'• the sheath is permeated by extracellular mucilagewhich similar to that produced by the root cap– sheath may provide barrier to gas diffusion• increase activity of free-living N 2 fixing bacteria

Rhizosphere modified by:• input of nutrients from the root• microbial activity– changes in pH, reduced O 2 , increased CO 2 ,reduced redox potential, nutrientimmobilization• cultural practice– e.g., effect of NH 4 + (H + ) and NO 3- (OH - ) on pH• cell must maintain ionic balance!

Rhizobacteria• bacteria that co-evolved with plants andaggressively colonize the root surface– can be beneficial, deleterious, or neutral– beneficial ones may displace pathogens in therhizosphere• characteristics of beneficial bacteria :– mostly Pseudomonas putida or P. fluorescens– about 2-5% specifically benefit root growth• plant growth promoting rhizobacteria (PGPRB)

– the beneficials, when applied to seed, aresubsequently found in the root system• rhizosphere competent– most efficacious strains produce broadspectrumantibiotics and siderophores in vitrobut not necessarily correlated to field activity– benefits may not be seen under gnotobioticconditions• not really PGPR but rather pathogen suppressive– act against subclinical pathogens» no obvious symptoms, but reduce plant growth

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