Symbioses Between Microbes and Their Hosts — Mutualism, Parasitism, and Coevolution Explained | Chapter 23 from Brock Biology of Microorganisms

How do microbes interact with other organisms—and with each other—to shape life on Earth? Chapter 23 of Brock Biology of Microorganisms dives deep into the diverse symbiotic relationships between microbes and their hosts, from lichens and plant roots to insects, marine animals, and mammals. These interactions underpin nutrient cycling, defense, energy flow, and even evolutionary processes across ecosystems.

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Microbial–Microbial Symbioses

Some of the earliest and most fascinating symbioses occur between different microbes. Lichens are a classic example—mutualistic associations between a fungus and a photobiont (green alga or cyanobacterium). The photosynthetic partner provides organic carbon, while the fungus offers protection, structure, and access to water.

Other examples include consortia like Chlorochromatium aggregatum, where green sulfur bacteria pair with motile heterotrophs to coordinate metabolism, and direct interspecies electron transfer (DIET) in anaerobic environments, such as those involving ANME archaea and sulfate-reducing bacteria.

Plant–Microbe Symbioses

Microbes form essential partnerships with plants:

• Rhizobia infect legume roots, creating nodules that fix atmospheric nitrogen (N₂ → NH₃), providing a critical nutrient for plant growth. This involves specialized molecules (Nod factors), root hair curling, and leghemoglobin to regulate oxygen.
• Mycorrhizae are fungal symbionts that enhance plant nutrient uptake:
  • Ectomycorrhizae: Form a sheath around roots.
  • Endomycorrhizae: Penetrate root cells, forming arbuscules for exchange.
• Agrobacterium tumefaciens is an example of a parasitic relationship, transferring T-DNA into plant cells to form crown gall tumors and produce opines that the bacteria use as nutrients.

Insect–Microbe Symbioses

Many insects depend on microbes for nutrition and protection:

• Primary symbionts are essential, vertically transmitted microbes housed in specialized cells (bacteriocytes). For example, Buchnera in aphids provide essential amino acids missing from their diet.
• Secondary symbionts confer benefits like defense against pathogens or parasites.
• Termite gut microbiomes allow termites to digest cellulose. Lower termites rely on protists, while higher termites utilize bacteria in an anoxic hindgut to generate acetate and fix nitrogen.
• Defensive symbioses are seen in some beetles, where bacteria like Pseudomonas produce toxins to deter predators.

Other Invertebrate Symbioses

Symbiotic relationships are also common among marine and terrestrial invertebrates:

• Bioluminescent bacteria (e.g., Aliivibrio fischeri in bobtail squid) use quorum sensing to coordinate light production, providing camouflage for the host.
• Hydrothermal vent tube worms (Riftia pachyptila) harbor sulfur-oxidizing bacteria in a specialized organ called the trophosome, enabling them to thrive in extreme environments.
• Entomopathogenic nematodes carry bacteria like Photorhabdus or Xenorhabdus to infect and kill insect hosts, then use the cadaver as food.
• Corals and dinoflagellates (Symbiodinium): Algal symbionts photosynthesize and provide organic carbon to corals, while corals supply nutrients and shelter. Loss of Symbiodinium under stress leads to coral bleaching.

Mammalian Gut Symbioses

Microbes are critical for animal nutrition and health:

• Herbivores depend on gut microbes to break down cellulose and generate volatile fatty acids (VFAs), a major energy source.
• Foregut fermenters (e.g., cows, sheep) have a rumen—a warm, anaerobic chamber with bacteria, archaea, fungi, and protozoa—where fermentation occurs before the stomach. Methanogenic archaea convert hydrogen and CO₂ into methane.
• Hindgut fermenters (e.g., horses) perform fermentation in the colon or cecum.
• Gut microbes also produce essential vitamins, amino acids, and help detoxify plant compounds.

Glossary: Key Terms from Chapter 23

• Nod factors / Myc factors: Molecules that initiate plant–microbe symbiosis.
• Leghemoglobin: Oxygen-binding protein in root nodules.
• Trophosome: Organ housing symbiotic bacteria in vent animals.
• Bacteriocyte / Bacteriome: Specialized insect cells and tissues for symbionts.
• Arbuscule: Fungal structure inside plant root cells (endomycorrhiza).
• Autoinduction: Quorum sensing-based gene regulation.
• VFAs: Volatile fatty acids—main fermentation products in animal guts.
• Ti plasmid / T-DNA / vir genes: Components of Agrobacterium’s DNA transfer system.

Conclusion: The Power of Microbial Symbiosis

Chapter 23 reveals the vast importance of microbial symbioses—from ecosystem productivity and agriculture to animal health and biotechnology. Recognizing these complex partnerships is key to understanding evolution, adaptation, and the health of all living systems.

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