Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 834

One should note that symbiosis and mutualism are not the same. Symbiosis implies a relation-
ship when two organisms live together. A symbiotic relationship may be mutualistic, but it
may also be neutralistic, parasitic, commensalistic, and so on.
Commensalism is an interaction in which one population is positively affected by the pres-
ence of the other. However, the second population is not affected by the presence of the first
population. Various mechanisms may yield a commensal interaction. Two common mecha-
nisms are the following:

(1) The second population produces a required nutrient or growth factor for the first
population.
(2) The second population removes a substance from the medium that is toxic to the first
population.

An example of the first type of commensal interaction is the production of H 2 S by Desul-
fovibrio (through the reduction of SO 2 4 ), which is used as an energy source by sulfur

bacteria.

Sulfur bacteria
ð aerobic Þ

SO 2 4 H 2 S !

Desulfovibrio
ð anaerobic Þ

An example of the second type of commensal interaction is the removal of lactic acid by
the fungus Geotrichium candidum, which allows the growth of Streptococcus lactis. This inter-
action is utilized in cheese making using S. lactis. Lactic acid produced by S. lactis inhibits the
growth of the bacteria. The fungus metabolizes lactic acid and improves the growth condi-
tions for S. lactis.
Amensalism is the opposite of commensalism. In amensalism, population A is negatively
affected by the presence of the other population (B). However, population B is not affected
by the presence of population A. Various amensal interaction mechanisms are possible.
Two common mechanisms are the following:

(1) Population B produces a toxic substance that inhibits the growth of population A.
(2) Population B removes essential nutrients from the media, thus negatively affecting the
growth of population A.

One example of the first type of amensal interaction is the production of antibiotics by
certain molds to inhibit the growth of others. Some microbes excrete enzymes that decom-
pose cell-wall polymers. Such organisms destroy their competitors and also utilize the nutri-
ents released by the lysed cells. The microbial synthesis of organic acids reduces pH and
inhibits the growth of other organisms.
Predation and parasitism are interactions in which one population benefits at the expense of
the other. These two interactions are distinguished by the relative size of organisms and the
mechanisms involved. Predation involves the ingestion of prey by the predator organism. A
good example of prey e predator interaction is the ingestion of bacteria by protozoa. This
interaction is common in aerobic waste-treatment reactors such as activated sludge units.
In parasitism, the host, which is usually the larger organism, is damaged by the parasite.
The parasite benefits from utilization of nutrients from the host.