Ecological Interactions

1
Ecological Interactions
Every Organism has a niche - a job that it does
- the place and the way in which it lives.
Whilst occupying that niche the organism
interacts with other organisms. How many other
organisms are YOU interacting with RIGHT now?
2
A BCommensalism

• One population beneficially affects a second
  population, but the second has NO effect on the
  first.
• mensa table (living off table scraps)
• Rarely obligatory
• examples
• A modifies habitat - B benefits (facultative
  aerobe uses O2)
• A excretes growth factor that is used by B
  (Flavobacteria brevis
  Legionella pneumophila)
• Human Commensals - term used for non-pathogens
  that live in/on our bodies - not necessarily
  commensal relationship.

3
A BSynergism

• Where the product of the interaction is greater
  than the sum of the parts.
• Not an obligatory relationship
• Synthrophism - supplying each others nutritional
  needs
• Arginine E. coli Agmatine (dead-end
• metabolite)
• E. faecalis
• Ornithine E. coli Putrescine (important
  for Ef and Ec)

4
A BMutualism

• OBLIGATORY relationship between 2 populations
  that benefits both parties.
• Highly Specific.
• Requires close proximity between parties.
• Both parties couldnt survive without mutualism
  in that particular niche.
• Sometimes called Symbiosis
• examples lichens (phycobiont mycobiont)
• Nitrogen fixing bacteria

5
Symbiosis/Endosymbiosis

• Symbiosis - living together
• it is a form of mutualism in which the organisms
  involved live together.
• If one organism lives inside the cells of the
  other organism it is called Endosymbiosis.
• e.g. Rhizobacterium living inside fungal hyphae.
• Photosynthetic algae living in the first
  eukaryotes (which became chloroplasts)

6
A BCompetition

• Interaction where the birth/growth rates are
  depressed and/or death rates increased by the
  presence of the other organism.
• Occurs when more than organism requires access to
  the same resource.
• Exploitation Competition (using the same resource
  until it is used up)
• Interference Competition (excreting chemicals to
  reduce competitors ability to use resource)

7
Commensal Mutualistic Intestinal Symbionts

• 1g faeces - 10 x 1011 microorganisms 400 spp.
• Commensal Gut Microbes
• May help digestion, but often compete with
  host for digested nutrients.
• Can synthesise growth factors and vitamins.
• MAINLY act as a barrier against pathogens.
• Mutualistic Gut Microbes
• When animals derive all or most nutrients from
  compounds that are hard to digest - intestinal
  symbionts become more specific and mutualistic.

8
Rhizosphere

• Narrow zone of soil that is subjected to the
  influence of a living root.
• No. micro-organisms in rhizosphere can be 10 -
  100 fold greater than in bulk soil
• Bacteria and fungi in the rhizosphere feed on
  organic matter released from the plant root.
• Competition for these resources is high - leading
  to complex interactions between the
  micro-organisms and with the plant itself.

9
PGPRPlant Growth-Promoting Rhizobacteria

• e.g. rhizobia, pseudomonads, Azospirillum
• influence plant growth directly/indirectly by
  releasing compounds (mineral nutrients
  phytohormones antimicrobials)
• Rhizobacteria can live in rhizosphere,
  endosymbiotically inside the plant, or free
  living.
• Free living rhizobia are internalised by
  leguminous plants, where they fix atmospheric
  nitrogen.

10
Bacterial symbionts of AM fungi

• Electron microscopy showed bacteria-like
  organisms inside the cytoplasm of AM fungi.
• PCR of AM spores using eubacteria specific
  primers yielded single DNA fragment. Sequencing
  of fragment shows it to be the bacteria
  Burkholderia
• Bacteria are also found on outside of AM hyphae -
  MHB - Mycorrhization Helper Bacteria

11
Succession

• Non-seasonal, directional continuous pattern of
  colonisation and extinction on a site by
  populations.

• Autogenic Succession microorganisms modify the
  habitat that permits new populations to develop
• Allogenic Succession habitat is altered by the
  environment (such as seasonal changes)
• Heterotrophic Succession where R gt P. e.g. a
  rotting log.
• Autotrophic Succession where P gt R . e.g. on a
  newly exposed rock