BIOL 360 Lecture Notes - Lecture 18: Anabaena, Intraspecific Competition, Heterotroph
Microbial Ecology – Microbial Interactions Notes
Note: These lecture notes were not written, and should not be used, as a substitute for the
lecture. Notes are presented here only as an aid to students who missed lecture, or whose
notes may be incomplete in certain particulars.
Contents:
Microbial Population Interactions 1
Interactions within a population 1
Positive interactions 1
Negative interactions 2
Interactions between populations 2
Neutralism 2
Commensalism 2
Synergism/Protocooperation 3
Mutualism/Symbiosis: 3
Competition 5
Predation 4
Amensalism/Antagonism 5
Parasitism 6
Microbial Population Interactions
Quote of the Day:
If there are any marks at all of special design in creation, one of the things most evidently
designed is that a large proportion of all organisms should pass their existence in
tormenting and devouring other animals.
- John Stuart Mill (1874)
Interactions within a population
Positive interactions
Positive, or cooperative interactions usually dominate at low population densities,
whether this refers to interactions within a population (intraspecific), or between
populations of different species (interspecific).
Any positive interaction will increase a population’s growth rate, negative interactions will
have the opposite effect. For any population in a given habitat, there is an optimal
population density at which the population growth rate will be maximized.
Colony formation and higher level interactions within a population are based upon
cooperation. It is believed that colony formation provided the basis for the emergence of
multicellular organisms possessing specialized tissues. An example of such cooperative
interaction involves the cellular slime mold, Dictyostelium.
Cooperative interactions are particularly important when: 1) substrates are difficult to
digest (e.g., lignin & cellulose) and/or 2) when organisms occur in a hostile environment.
Cooperation can also involve genetic exchange, for example through transformation,
transduction, conjugation or sexual spore formation.
Negative interactions
Most negative interactions can be lumped under the heading of competition. These
competitive interactions are believed to be a major driving force for evolution by natural
selection. You have all heard of the concept of “survival of the fittest,” and this concept
applies no less in the micro- as in the macro- world.
As with positive interactions, negative interactions can be either direct or indirect. In
addition to the direct competition for available resources like nutrients, there can be actual
competition for space to grow. Take any environmental factor that can regulate microbial
growth and we can imagine a scenario in which there is inter- or intraspecific competition
based on that factor. Microbes have a variety of biochemical and genetically-based
strategies for “winning" the struggle for existence.
Interactions between populations
Neutralism
A population of species A has absolutely no impact on a population of species B and
species B has absolutely no impact on species A. OK. In theory I suppose it could happen.
Commensalism
One population benefits, while the other is unaffected (+,0). Some examples are based
upon cometabolism, for example during nitrification where nitrate reducers form nitrite
which can then be reduced further by other species. In some examples of cometabolism
certain microbes cometabolically oxidize a second substrate, while growing on a favored
substrate, to form a product(s) that can be degraded by a different microbe. According to
a strict definition of cometabolism, the second substrate is not assimilated by the primary
organism, but the oxidation products are available for use by at least one other microbial
population. In lab I gave you the example of Mycobacterium vaccae, which uses an
Document Summary
Note: these lecture notes were not written, and should not be used, as a substitute for the lecture. Notes are presented here only as an aid to students who missed lecture, or whose notes may be incomplete in certain particulars. If there are any marks at all of special design in creation, one of the things most evidently designed is that a large proportion of all organisms should pass their existence in tormenting and devouring other animals. Positive, or cooperative interactions usually dominate at low population densities, whether this refers to interactions within a population (intraspecific), or between populations of different species (interspecific). Any positive interaction will increase a population"s growth rate, negative interactions will have the opposite effect. For any population in a given habitat, there is an optimal population density at which the population growth rate will be maximized. Colony formation and higher level interactions within a population are based upon cooperation.
