HC 8: symbiosis

Cross-feeding

a looser interaction where one microbe consumes the metabolic byproduct of another, but neither organism is strictly dependent on the other for survival or energy production

Syntrophy

a very tight metabolic partnership between microbes where one organism can only efficiently (or sometimes only at all) carry out its metabolism because another organism removes its waste products.

• This keeps the energy-yielding reactions favorable for both partners.

• In syntrophy, electron transfer is central: one microbe produces reduced compounds (like H₂ or formate) that carry electrons, and a partner organism immediately consumes them

External electron transfer

The Black Queen hypothesis: evolution of interdependency

The idea is, the question whether you should be a generalist of a specialist.

• When you’re a generalist, you need the machinery to degrade everything.

• When you’re a specialist, you become very good in the specific thing you do.
  
  

• When you have enough specialism, there is a certain tipping point where its better to be a specialist in one thing, so each organism in the community becomes good at one thing, without having to carry the other baggage.

• So first there is a whole bunch of generalists, than evolve into different very efficient specialist, but now they do be dependent on each other.

Functions of the microbiome

• Metabolism

• Immunity

• Nutrient acquisition

• Hormone adjustment

Mode of microbiome transmission

• Vertical transmission (parent → offspring) usually increases specificity because microbes are passed down together with the host lineage, leading to long-term co evolution and often strong, specialized partnerships.

• Horizontal transmission (from environment or other hosts) tends to reduce specificity because hosts repeatedly acquire microbes from outside, making associations more flexible and less fixed.

The paradox of cooperation; Two problems:

➔ Selfish individuals can exploit mutualism, reaping benefits while paying no costs. So, why cooperate at all?

➔ Costs and benefits of cooperation depend on a suite of external factors, which vary hugely over time and space.

example of how you can study the ecology of cooperation

-      forceren van “cheating”.  Kan door stikstof te vervangen door argon à blokkeert stikstoffixatie

»»  vervolgens kijk je of de plant de “cheaters” straft door de toevoer van zuurstof/suikers te beperken

-      isotopen-tracking: door planten gelabelde suiker (14C) te geven en schimmel gelabeld fosfaat (33P) à aantonen dat planten meer koolstof geven aan schimmels die meer nutriënten terugleveren

the range of types of symbioses involving microbes (and their hosts)

 

1.       Holobiont: de gastheer (plant of dier) + al zijn geassocieerde microben als één eenheid

2.       Mycorrhizae: Schimmels die plantenwortels koloniseren (zowel in als om de wortels)

3.       Korstmossen (Lichens): mutualisme tussen een schimmel (meestal Ascomycete) en een alg/cyanobacterie.

4.       Dierlijke symbiosen: f.e. het menselijke darmmicrobioom dat essentieel is voor spijsvertering en immuniteit.

5.       Endosymbionten bij insecten: f.e. de mealybug, waar zelfs endosymbionten binnenin andere endosymbionten leven