An Introduction to Microbial Ecology

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An Introduction to Microbial Ecology

• Microbes dont just live on petri-dishes!
• They dont just make us sick!
• They are an important part of our ecosytem and we
  need to understand their ecology, life-history
  strategy and interactions.

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Community an intergrated assemblage of
microbial populations occurring and interacting
within a given location
ECOSYSTEM COMMUNITY GUILD POPULATION INDIVIDUA
L
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Life History Strategiesr and K Adaptations

• r-strategists - high rates of reproduction
• r- selected micro-organisms early successionals,
  opportunists, uncrowded conditions in unstable
  environment. Often form abundant and resistant
  spores.
• e.g. Saccharomyces on sugar-rich plant exudates
• Archaea in thermal vents
• K-strategists - optimal utilisation/conservation
  of resources
• K-selected micro-organisms resource-limited
  environments, highly adapted to sequester
  resources in competitive env.
• e.g. Soil Streptomycetes that grow slowly on
  complex soil organic matter. Desmids in
  oligotrophic lakes.
• Organisms EITHER optimise reproductive output OR
  conservation of resources

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- An ecological niche is the role that an
organism plays (or the job it does) in a
community- Whilst filling its own niche an
organism will interact with other organisms.

• Interaction Population A Population B
• Neutralism 0 0
• Commensalism 0
• Synergism
• Competition - -
• Mutualism
• Ammensalism 0/ -
• Predation -
• Parasitism -

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Microbe-Plant Interactions

• 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 in or from the plant
  root.
• Rhizobacteria and Mycorrhizal fungi can exist as
  symbionts inside plant roots where they play in
  important role in nutrient uptake.

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Microbe-Animal Interactions

• Microbes are ingested by animals (especially in
  aquatic environments) as a food source.
• Also ingested to act as commensals ...
• help digestion
• synthesise growth factors and vitamins.
• MAINLY act as a barrier against pathogens
• or mutualists .
• When animals derive all or most nutrients from
  compounds that are hard to digest (e.g.
  cellulose)- intestinal symbionts become more
  specific and mutualistic. (e.g termites,
  leaf-cutters).

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Leaf Cutter Ants

• Mutualistic relationship with fungi.
• Recipe cut up fresh leaves, mix with saliva and
  faecal extract and innoculate with Leucocoprinus
  fungi
• Maintenance eat the decomposing plant material
  and nibble the fungal hyphae to prune it a keep
  it growing well.
• Fungus gardening by ants is approx. 50myr old.
• Some current fungal lineages have apparently been
  propagated by the same ant species as long as 23
  myrs.

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Symbiotic Light Production

• Some marine invertebrates and fish have
  mutualistic relationship with luminescent
  bacteria.
• Bacteria are contained in special organs, that
  have pores to let bacteria in and out.
• Luminescence is used by fish to communicate, for
  recognition and to lure prey. Mainly found in
  deep sea species. Difficult to study in the lab.

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Succession

• Non-seasonal, directional continuous pattern of
  colonisation and extinction on a site by
  populations.
• Early Successional species that first appears in
  new habitat (usually r-selected organism)
• Facilitation influence of early species
  facilitate the establishment of later ones by
  changing the conditions encountered.
• Inhibition tendency of early successionals to
  resist invasion by later species.
• Tolerance modification by early successionals
  has little effect on survival of later
  successionals
• remember biofilms and winogradsky columns!

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Diversity Indices

• How many species are present?
• What is a species? Can we really count them
  all?
• Species richness (d) S-1 S no species
• log N
  N no individuals
• Shannon (H) ? Pi ln Pi pi proportion
  that each species contributes to the
  total sample
• Evenness (J) H H ? Pi ln Pi
• H max lnS

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Collection Curves

• Collection curves are a good way of looking at
  community diversity and evenness. A community
  that is dominated by one species will have a more
  shallow curve than a highly diverse and even
  community.
• Demo Looking at diversity in fruit
  communities.
• We have two different communities of fruit. Look
  at the collection curves you obtain.
• Close your eyes and randomly take a piece of
  fruit from your community and then fill in the
  table on the board. Take a total of ten pieces
  of fruit. Then plot a graph of the number of
  pieces of each type of fruit.