L30: Microbial Symbiosis

Question 1

Range of possible physical associations in symbiotic interactions

Answer 1

Consortia: complex communities of diverse interacting populations. E.g. in sewage bioreactors or aquatic ecosystems

Ectosymbiosis: symbiont is associated with host surfaces

Endosymbiosis: symbiont is internal to host

Can be permanent, intermittent or cyclical associations

Question 2

Mutualism

Answer 2

Obligate relationship for survival in habitat

Involves reciprocal benefit to both partners

Often based on protection and nutrient/waste cycling and exchange— partners are commonly metabolically interdependent

Occur in nutrient-poor waters: zooxanthellae live in coral animal tissues

Basis: mutual nutrient cycling, shelter (for zooxanthellae)

Obligate

Question 3

Cooperation

Answer 3

Loose relationship; sometimes referred to as synergism

Pop capable of independent survival in habitat are mutually advantaged by interaction (beneficial, not obligatory)

Often based on satisfaction of nutritional requirement (syntrophism)

Example: Chlorobium/Desulfovibrio nutrient exchange in aquatic sediments. Products of each organism are substrates required by other

Question 4

Commensalism

Answer 4

One organism benefits while other is unaffected

Diverse basis- shelter, resources, habitat modification

Example: bacterial interactions in cycling of nitrogen

a) N2 is converged to organic N by nitrogen-fixing bacteria
b) heterotrophic bacteria use organic N and produce NH4+
c) nitrifying bacteria oxidise NH4+ to NO2- then NO3- (gaining energy)
d) denitrifying bacteria use NO3- in anaerobic respiration releasing N2

Question 5

Predation

Answer 5

‘Predator’ engulfs smaller ‘prey’ organism

Microbes are ecologically important prey species. Some are predators

Marine food webs and fisheries depend on predation by predator protists on phytoplankton and bacteria

Bacteria predating other bacteria (Bdellovibrio, Vampirococcus, Daptobacteria)

Nematode-trapping fungi (Arthrobotyrs)

Question 6

Parasitism

Answer 6

Interaction involving co-existence, usually at cellular level

Parasite benefits and host is usually harmed

Can lead to genomic reduction and parasite dependence on host: Mycobacterium leprae lost about 2000 genes and is known to survive outside hose cells

Benefits of parasitism: pop control (density-dependent)

Example: bacterial viruses (bacteriophages)- lytic and lysogenic states

Question 7

Amensalism

Answer 7

Unidirectional interaction involving compounds of one organism inhibiting another

Basis for many pharmaceutical and industrial applications: antibiotics (e.g. penicillin from Penicillium notatum), alcohol formation by yeasts (used in wine production), acid formation (used in yoghurt)

Atta (attini) ants maintain ‘fungal gardens’ as a food source for colony (a mutualistic association). Gardens are subject to infection and destruction by parasitic fungus. Ants co-cultivate Pseudonocardia bacteria, which produce inhibitors of parasite, and bu amensalism control its abundance to low levels

Question 8

Competition

Answer 8

Interaction of different pop requiring a limited resource

Likely outcomes:

1. Reduced growth: coexistence but neither achieves max potential growth (-/-)
2. Competitive exclusion: one is eventually eliminated from habitat (+/-)
3. Niche separation: the competitors occupy different niches in habitat (0/0)
4. Temporal resource sharing: the ‘competitors’ occupy the habitat at different times (e.g. phytoplankton pop over season cycles) (+/-) and (-/+)

Question 9

Competitive exclusion

Answer 9

Pops having the higher growth rate ultimately excludes competitors from niche

E.g. Paramecium aurelia and P. caudatum

In mixed culture P. Caudatum is eventually excluded from habitat

Growth of P. Aurelka ultimately recovers

Question 10

Symbiosis in complex systems and evolution

Answer 10

Symbiotic interactions do not occur in isolation in nature

Interactions and feedback responses impact broader communities and the surrounding environment

Natural communities reflect diversity and complexity of +ve, -ve and neutral pop interactions that are occurring at any instant

Question 11

-ve interactions

Answer 11

Limit pop density

Act to prevent individual pop dominance

Drive increased genetic diversity

Question 12

+ve interactions

Answer 12

Promote more efficient use available resources

Enable organisms to colonise habitats they could not colonise alone

Enhance growth and survival rate among benefitting pop

Question 13

Symbiosis

Answer 13

The living together of unlike organisms

Question 14

Symbiotic interactions

Answer 14

Range from antagonistic to mutually beneficial

Can involve varying degrees of physical association between host and symbiont, including intercellular and intracellular interactions

Basis for formation of many products useful in industrial and pharmaceutical applications

Important drivers of evolutionary processes