IB Biology Topic C4.1.12: Mutualism as an Interspecific Relationship That Benefits Both Species Notes

Mutualism is an interspecific relationship where both species benefit

1. In mutualism, each partner gains something it could not easily get alone, so both improve their survival, growth, or reproduction.
2. The three classic IB examples are legumes with Rhizobium, orchids with mycorrhizal fungi, and corals with zooxanthellae.

Legumes and Rhizobium trade nitrogen for sugars

1. Most plants cannot use atmospheric nitrogen (N₂) directly.
2. Legumes such as peas, beans, and clover solve this by partnering with nitrogen-fixing bacteria called Rhizobium.

1. Root nodules form
   1. The legume grows root nodules that house the Rhizobium bacteria.
2. Nitrogen is fixed
   1. Inside the nodules, Rhizobium converts N₂ into ammonium (NH₄⁺), which the plant can absorb.
3. Both partners benefit
   1. The legume gains usable nitrogen, letting it grow well even in nitrogen-poor soil.
   2. The bacteria gain a protected home in the nodules plus sugars from the plant for energy.

Orchids and mycorrhizal fungi trade sugars for nutrients

1. Orchid seeds are tiny and carry almost no food store, so seedlings cannot survive on their own.
2. They depend on a mycorrhizal relationship between fungi and their roots, especially in nutrient-poor soil.
3. The fungus colonises the roots
   1. Fungal threads grow into and around the orchid's root cells, forming a network.
4. Nutrients and sugars are exchanged
   1. The fungus draws water and minerals like nitrogen and phosphorus from the soil and passes them to the orchid.
   2. Once the orchid photosynthesises, it supplies the fungus with sugars in return.

Corals and zooxanthellae trade shelter for food

1. Hard corals build reefs, but they cannot survive without zooxanthellae, photosynthetic algae that live inside their cells.
2. The algae live inside the coral
   1. Zooxanthellae sit within the coral's tissue and photosynthesise there.
3. Food and shelter are exchanged
   1. The algae supply the coral with glucose and oxygen from photosynthesis.
   2. The coral gives the algae shelter plus carbon dioxide and nutrients from its waste.
4. Both partners benefit
   1. The coral gets up to 90% of its energy from the algae and uses it to build its calcium carbonate skeleton.
   2. The algae gain a protected place to live and a steady supply of nutrients.

Mutualism supports ecosystems and human industries

1. Agriculture
   1. Legumes cut the need for nitrogen fertiliser, supporting sustainable farming.
2. Forestry
   1. Mycorrhizal fungi boost tree growth, helping reforestation.
3. Marine ecosystems
   1. Coral reefs, built on mutualism, provide habitat for a huge variety of marine life.