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lecture note 23 for BGYB50


Biological Sciences
Course Code
Herbert Kronzucker

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LECTURE 23/24:
- Unlike competition or predation, mutualism is a species-species interaction where both
species benefit from the interaction
- Important examples include: squids and Vibrio bacteria (bacteria are attracted to live
inside a specialized “light organ” of the squids, protecting the bacteria from predation,
and giving the squid the ability to change colour on account of the bacteria’s ability to
produce bioluminescence); N-fixation symbioses; mycorrhizae (the close physical
association between fungi and the roots of plants; the fungal partner enhances the nutrient
acquisition capacity of the plant, esp. for phosphorus and nitrogen, while the plant
supplies the fungus with carbohydrates; 80% of higher plant species are mycorrhizal, not
just trees), lichens (composite organisms consisting of a fungus, known as the mycobiont,
and an alga or cyanobacterium, known as the phycobiont; the mycobiont is very efficient
at obtaining nutrient ions from the environment, while the phycobiont supplies
photosynthate; lichens are critical components of primary succession, where they aid in
the weathering of rocks and the production of soil), or intestinal bacteria (e.g. bacteria in
the guts of termites enable termites to digest lignin from woody substrate; humans also
have numerous symbiotic gut bacteria that aid in the digestion process; all these bacteria
get shelter and nutrients from the “host in return)
- Mutualisms can also be non-symbiotic (such as between hummingbirds and the flowers
they feast on), and may be either obligatory (i.e. the mutualistic partners would die
without the mutualism; e.g. lichens) or facultative (i.e. the mutualism isoptional, e.g.
N-fixation symbiosis)
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