Symbiosis Lecture Notes

Symbiosis is a special topic with ongoing research for many years.
Recommendation: Ed Young's book "I Contain Multitudes" about how microbes make animals.

Symbiotic interactions are common, with virtually everything interacting with everything else.
Symbiosis comes in various forms: temporary or lasting millions of years.

Symbionts provide various benefits to their hosts, influencing animal and plant phenotypes.
Focus on animal-bacteria interrelationships.
Consideration of the implications for understanding what it means to be human.

Symbiosis can occur between various combinations of organisms: animals, plants, fungi, algae, microeukaryotes, bacteria, and archaea.
Archaea are the least symbiotic, but still engage in symbiosis.

Plants interact with lots of bacteria.
Plants commonly have bacteria covering their roots.
Tighter symbiosis: root nodules in clover plants housing nitrogen-fixing bacteria.
Legumes provide ammonia to the plant.
Parasitic plant symbiosis: mistletoe growing on trees, draining resources from the host plant.
Plant-animal symbiosis: Acacia trees with ant houses (domatia) developed by the plant to encourage ant habitation for defense.

Animals and bacteria: Animals have bacteria in their guts (e.g., rumen in cows).
Animals and fungi: Leafcutter ants using fungi to digest leaves.
Animals and plants: Acacia trees providing housing for ants.
Animal-animal symbiosis: Ants farming mealybugs for their juices.
Animals and algae: Corals relying on algae for sucrose; salamander eggs containing algae for photosynthesis.

External digestion: Fungus gardens of attine ants as external guts.
Cellulose digestion in cows mediated by bacteria in the rumen.
Termites rely on diverse communities of protists and bacteria to break down lignin and cellulose.
Microbes break down complex polysaccharides into short-chain fatty acids, which animals use for nutrition.

Plants produce toxins (e.g., creosote) to deter consumption.
Chemical formula for creosote: $C{14}H{14}O$ . It contains a phenolic compound, where a hydroxyl group (-OH) is bonded directly to an aromatic hydrocarbon group.
Rodents eating creosote bushes have gut microbes that break down the toxins.
Experiment: Mice fed a creosote diet with antibiotics die; those with their natural bacteria thrive.

Essential amino acids cannot be produced endogenously and must be obtained from the diet.
Aphids eating plant sap rely on bacterial symbionts to produce essential amino acids like tryptophan.
Weevils rely on bacteria for production of tolzien.
Tolzien is the basic pulp up, foundation of melanin, which forms the insect exoskeleton and is made by bacteria.
Blood-feeding tsetse flies rely on bacteria to synthesize B vitamins.

Protective symbionts aid in avoiding predation, parasitism, and pathogens.
Bobtail squid use bioluminescent bacteria for counter-illumination, reducing their shadow and avoiding predators.
Gopher beetles use bacteria to produce poederin, a toxic compound for defense.
Chemical formula for Pederin: $C{25}H{45}NO_9$ . Is a vesicant with a complex molecular structure.
Fungi in locoweed produce lysergic acid (precursor to LSD) to deter herbivores.
Lysergic acid chemical structure: $C{16}H{20}N2O2$ . It is a complex organic compound.
Ants protect acacia trees from predators and competing plants.
Beewolves use bacteria to produce antibiotics that protect their pupae from fungal and bacterial infections.
Insects utilize bacterial antibiotics in their cuticle for defense against other bacteria.
Some microbes protect hosts from viral attacks (e.g., Wolbachia in flies).
Bacteria like Spiroplasma protect insects from parasites like worms and wasps by secreting toxins.

Microbes are used for attack purposes by some predators or parasites.
Nematodes use bacteria to kill insect larvae by releasing toxins and suppressing the insect's immune system.

The holobiont concept emphasizes the individual and its associated microbes.
Physiological functions of organisms cannot be understood without considering their microbes.
Multicellular life depends on microbes for survival and well-being.
Example: Software development in an acidic world without bacteria takes about 15 days instead of 8 because it is so unhappy.
The integration of bacteria into the organism helps to understand organismal physiology.

A human is composed of both human and bacterial cells.
Bacteria are slightly more numerous than human cells in the human body; about half bacteria.
The human gut has a diverse community of bacteria.
Bacterial metabolism in the colon digests complex polysaccharides and produces short-chain fatty acids.
The cow bacteria do most of the enzymatic action, rather than the cow itself, referring to the digestive symbiosis
15% of calories for omnivores and 40% of calories for vegans come from bacterial metabolism.
The gut microbiome defends against pathogens.
Bacterial molecules and short-chain fatty acids control satiation and sugar homeostasis.
The absence of bacteria leads to gut dysfunction, impaired organismal function, and reduced pathogen resistance.

Most eukaryotes engage in symbiosis with microbes or other eukaryotes.
Symbiosis is diverse in terms of formation, localization, integration, and impact.
Microbes provide a wide variety of services to their hosts.
Aspects of host biology are driven by microorganisms and their genomes.
Humans are no exception to symbiotic relationships.