Lecture notes from the Microbiology lecture 'microbial symbiosis', covering a variety of microbial symbionts and commensals, including methanogenic archaea, lichens, nitrogen fixing bacteria, zooxanthellae and symbiotic gut protozoa
Microbial Symbiosis
- Microbial symbiosis can be defined as any permanent or stable association between a
microbe and at least one other organism
- The ‘other organism’ can be prokaryotic or eukaryotic, and is most often phylogenetically
unrelated
- Both partners can be grown separately from each other in a lab, with the required nutrients
- If the partnership is advantageous, it will remain over time. Common examples of microbial
symbiosis include:
• Symbiosis between microbial organisms – such a methanogenic prokaryotes in the cell
bodies of microbial eukaryotes
• Nitrogen fixing bacteria in the root nodules of plants
• Rumens in the digestive tracts of mammals and insects
• Invertebrates, such as corals and sea anemones
• Partnerships with fungi, such as lichens
Symbiosis
- Symbiosis is a form of metabolic cooperation in some, giving a metabolic advantage, using
each other’s waste products – corals, lichens etc
- These partnerships use environmental resources which would otherwise be unavailable to
the organisms as individuals
- Prokaryotes are very versatile organisms, and are able to form consortia and partnerships
with other organisms
- Symbiosis is crucial in understanding eukaryote evolution
Microbial consortia
- Free living, photosynthetic green-sulfur bacteria surrounding mobile, non-photosynthetic
bacterium (e.g., Chlorochromatium aggregatum – a model for bacterial multicellularity)
- Intracellular examples include endosymbionts (such as methanogens) and Chlorella a green
alga (commonly found in ‘green’ protozoa such as Strombidium viride, Paramecium bursaria,
Stentor polymorphus). Note that care should be taken to distinguish between microbial
ingestion and endosymbiosis
- The freshwater ciliate Cyclidium porcatum contains clusters of methanogen archaea and also
bacteria. The hydrogenosomes of Cyclidium pump out hydrogen, which is taken up by
methanogens, whose waste products are taken up by the bacteria. This partnership is one of
the very few where three domains of life are represented.
- The ciliate Kentrophoros has an ectosymbiotic relationship with the sulfide-oxidising bacteria
Candidatus, which replicate in a V shape and as such remain on the surface of the ciliate.
Kentrophoros has no oral aperture, and ingests the bacteria as its primary food source.
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