Microbial Symbioses Lecture Notes

Microbial Symbioses

Introduction to Symbioses

Microbes engage in long-term relationships with macroorganisms and other microorganisms known as symbioses.

• Mutualism: Interaction beneficial to both organisms. Most mutualistic relationships arise from coevolution, occurring over millions of years.

Lichens

• Definition and Structure:

  • Lichens are a mutualistic association primarily between a fungus and an alga (or cyanobacterium).

  • They can be found on various surfaces, such as rocks, tree trunks, roofs, and bare soils.

  • Components include:

    • Alga: A photosynthetic partner that produces organic compounds, can fix nitrogen, and provides a food source for the fungus.

    • Fungus: Provides structural integrity, protects the algal cells from erosion, and supplies essential nutrients.

    • Additionally, complex relationships may involve other microorganisms such as bacteria and archaea, contributing to nutrient cycling and resilience.

Examples and Importance

• Lichens are considered more complex than previously thought due to interactions with bacteria and archaea, which enhance their adaptability and survival in extreme environments.

Phototrophic Bacterial Consortia

• Chlorochromatium Aggregatum:

  • Found in freshwater, these are mutualistic consortia formed by green sulfur bacteria and flagellated bacteria.

  • They thrive in stratified sulfidic lakes, representing a significant portion of bacterial biomass.

  • Green sulfur bacteria are anaerobic phototrophs that play a critical role in their unique ecosystems, utilizing light energy to fix carbon and sustain microbial communities.

Methanotrophic Consortia

• Overview:

  • In anoxic marine sediments, methanotrophic consortia couple the activities of anaerobic microbes to oxidize methane, significantly impacting the global carbon cycle.

  • They involve methane-oxidizing archaea working alongside sulfate-reducing bacteria to facilitate this process effectively.

Legume-Root Nodule Symbiosis

• Importance:

  • This symbiosis between legumes (e.g., soybeans, clover) and nitrogen-fixing bacteria (e.g., Rhizobia) is crucial for enhancing soil nitrogen levels, thus benefiting agricultural productivity.

• Mechanism:

  • Steps in nodule formation include:

    • Recognition and attachment of the bacterium to root hairs.

    • Excretion of Nod factors by bacteria, signaling plant roots.

    • Bacterial invasion via infection threads, leading to the formation of specialized structures called bacteroids, which are essential for nitrogen fixation.

  • Leghemoglobin inside the nodules protects nitrogenases by sequestering oxygen, creating an optimal environment for nitrogen fixation, which is vital for plant growth and soil fertility.

Mycorrhizae

• Overview and Types:

  • Mycorrhizae are mutualistic associations between fungi and plant roots, enhancing nutrient uptake.

  • Types include:

    • Ectomycorrhizae: Fungi form a protective sheath around roots without penetrating the cells.

    • Endomycorrhizae: Fungi penetrate root tissues, forming arbuscules that enlarge the surface area for nutrient exchange; this is more common and beneficial for over 80% of terrestrial plants.

• Function:

  • Mycorrhizae enhance the absorption of essential nutrients, such as phosphorus and micronutrients, while receiving carbohydrates in return from the plant, fostering a symbiotic relationship that is critical for plant health and growth.

Agrobacterium and Crown Gall Disease

• Mechanism and Impact:

  • Agrobacterium tumefaciens induces crown gall disease on plants through a parasitic mechanism that exploits the plant's genetic machinery.

  • The Ti plasmid triggers tumor formation by integrating into the plant's genome, which not only causes disease but is also harnessed in genetic engineering to create genetically modified crops with desirable traits.

Insect-Microbe Symbiosis

• Types of Symbionts:

  • Heritable symbionts can be:

    • Primary: Essential for the reproduction and survival of the host.

    • Secondary: Provide additional benefits, including enhanced nutrition and protection from pathogens.

• Examples:

  • Rickettsia-infected whiteflies demonstrate that such symbionts confer reproductive advantages over uninfected counterparts, leading to greater population success.

• Evolution and Adaptation:

  • Symbionts in insects exhibit significant gene reduction, retaining only those genes necessary for host fitness, allowing them to adapt to their specific relationships effectively.

Defensive Symbioses

• Role and Impact:

  • Many insects produce antimicrobial compounds influenced by their microbial associates, which protect them against pathogens or predators.

  • For example, the Paederus beetle uses pederin, a potent chemical produced by the endosymbiotic Pseudomonas, to defend its nest and deter predators.

Leafcutter Ants

• Example:

  • Leafcutter ants example of symbiosis where they partner with cultivated fungi, utilizing leaf fragments as a substrate for fungal growth.

  • This relationship leads to mutual benefits: ants receive nutrients from the fungus, and the fungus gets a steady supply of organic matter to thrive, also providing chemical defenses for the ant colony.

Coral-Symbiont Relationships

• Overview:

  • Corals possess phototrophic symbionts, primarily dinoflagellates, which are crucial for their survival and energy production through photosynthesis.

• Coral Bleaching:

  • Occurs due to environmental stressors, leading to the loss of these vital symbionts and resulting in detrimental effects on coral health, highlighting the fragility of coral reef ecosystems.

Ruminant Gut Microbiota

• Structure and Function:

  • Ruminants such as cows rely on complex gut structures (rumen) for cellulose digestion, facilitated by microbial fermentation.

  • Microbes in the rumen synthesize essential amino acids and vitamins, maintaining a symbiotic relationship that supports the ruminant's nutritional needs.

• Impact of Diet Changes:

  • Abrupt dietary changes can lead to significant alterations in rumen flora, resulting in acidosis, inflammation, and nutritional issues in cattle and other ruminants.

Concluding Notes

Understanding these complex microbial symbioses provides insights into ecological interactions, agricultural practices, and potential biotechnological applications. The varied relationships between microbes and their hosts underline the significant roles these interactions play in nutrient cycling, agricultural productivity, and maintaining ecosystem balance.