lecture 19 (ch 21: symbiosis)

Symbiosis

the most intimate association between organisms of different species (both pos and neg)

obligate symbiosis

constantly in association with each other and cannot survive without the other

facultative symbiosis

associate part of the time

mutualism

pos/pos

commensalism

pos/neutral

amensalism

neg/neutral

parasitism

pos/neg

neutralism

neutral/neutral

How can symbiosis explain why we have yet to culture the vast majority of microbes as isolates in the lab?

many microbes depend on close interactions with other organisms to survive and grow, and these symbiotic relationships are difficult to replicate in the lab

Holobiont

host organism plus all microbes that live or inside it, functioning together as one ecological unit

syntrophy

• metabolic cooperation which requires both partners to complete the metabolism with a ΔG < 0

• Most commonly, one species produces a product that is consumed by the second species. 

  • If this was left to build up in high concentration, it would result in an unfavorable ΔG for its continued production.

How do different symbiotic organisms work metabolically together to allow a termite to live by consuming wood?

• protists (eukaryote) that break down lignin

• bacteria catabolize cellulose.

• Termites themselves cannot digest cellulose, the main component of wood. Instead, they rely on a symbiotic community of microbes living in their guts — including bacteria, archaea, and protists — that work together metabolically to break down wood into usable energy.

rumen

largest chamber of cow’s stomach and its microbial community allows the herbivore to acquire nutrition from complex plant fibers that it cannot otherwise digest (ex. cellulose)

A number of different organisms partially digest food, providing substrates for other organisms.":

• Chytridiomycetes break down complex fibers (fungi).

• Cellulolytic bacteria break down cellulose.

  • Require branched-chain fatty acids, which are produced by amino acid fermenters (symbiosis is everywhere)

How can changing the microbiome of cows can impact global greenhouse gas emissions?

H2 and CO2 frequently produced by ruminal fermentation. this is used by methanogens to produce methane . This methane cannot be used by the cow as food and is released into the atmosphere, contributing significantly to greenhouse gas emissions. By changing the rumen microbiome, it can reduce climate impact

Why is fermentation favored over anaerobic respiration in the human gut?

the human gut is an anaerobic (oxygen-free) environment with few other electron acceptors.

How could microbial metabolism at a hydrothermal vent provide all that is needed by a worm with no digestive systems that lives at this location?

Worms obtain organic carbon from symbiotic autotrophic bacteria that use inorganic chemicals from vents (like H2S, CO2, O2) for chemolithoautotrophy. Sulfur oxidizing symbionts use reduced sulfur to fix CO2 into organic carbon, while sulfate-reducing symbionts recycle host waste and convert sulfate back to sulfide. Together, these microbes create a self-sustaining internal cycle

Give two examples of the ways a world without microbes would mean the end of human life as well.

1. no digestion: without gut microbes, humans or animals can’t properly digest food, leading to severe health problems and world starvation

2. collapse of ecosystem: without soil and aquatic microbes to recycle nutrients, all life forms would die from starvation and oxygen loss 

short-chain fatty acids

produced when gut bacteria ferment complex plant materials. they are absorbed by the body and can provide up to 15% of our daily energy