Chapter 23: Microbial Symbioses with Microbes, Plants, and Animals

Symbiosis can take several forms including mutualism, where both partners benefit, parasitism, where one organism benefits at the expense of the other, and commensalism, where one partner benefits without affecting the other. Microbial symbioses occur even among microorganisms themselves, as illustrated by lichens in which fungi form stable associations with photosynthetic algae or cyanobacteria that supply organic carbon through photosynthesis while the fungal partner provides protection, structural support, and enhanced nutrient uptake. Similar cooperative interactions occur in microbial consortia where metabolically linked species exchange nutrients or electrons, including syntrophic partnerships and direct interspecies electron transfer between microorganisms involved in anaerobic methane oxidation. The chapter then explores plant–microbe symbioses such as the nitrogen fixing association between legumes and rhizobial bacteria, in which specialized root nodules house bacteroids that convert atmospheric nitrogen into ammonia usable by the plant while receiving carbohydrates and a protected environment in return. Another major plant symbiosis involves mycorrhizal fungi that colonize plant roots and greatly enhance nutrient uptake, particularly phosphorus and nitrogen, through extensive fungal hyphal networks that exchange nutrients for plant derived carbon. In contrast, parasitic plant interactions occur when bacteria such as Agrobacterium tumefaciens transfer tumor inducing DNA into plant cells, causing crown gall disease and production of unique nutrients that support bacterial growth. Insect–microbe symbioses are also widespread, with specialized intracellular bacteria such as Buchnera providing essential amino acids to aphids and other insects that feed on nutritionally limited diets. Defensive symbioses allow microbes to produce chemical compounds that protect insect hosts from predators or pathogens, while termites rely on complex gut microbial communities to degrade lignocellulosic plant material through anaerobic fermentation processes. Marine invertebrates likewise depend on microbial symbionts for survival, including the bioluminescent bacterium Aliivibrio fischeri that produces light for camouflage in the Hawaiian bobtail squid and sulfur oxidizing bacteria that supply organic carbon to deep sea hydrothermal vent animals such as Riftia tube worms through chemosynthetic metabolism. Coral reef ecosystems depend on mutualistic interactions between corals and photosynthetic dinoflagellates that provide organic carbon to the host through photosynthesis. Finally, herbivorous mammals rely on dense microbial communities within specialized digestive systems, such as the rumen in ruminant animals, where diverse bacteria, archaea, fungi, and protists ferment plant polysaccharides to produce volatile fatty acids that serve as the host’s primary energy source. These examples illustrate the fundamental role of microbial symbioses in shaping biological evolution, ecosystem function, and host physiology across diverse environments.