The population density or average distance between microbial cells determines the type of interaction (Fig. 7.1).
Our general view is that when the population density is low, organisms have neither positive, nor negative, interactions. When the population density is medium, organisms compete among themselves for the availability of resources, by rate or efficiency of growth, and by production of metabolites, which negatively affect the growth of competitors. When the population density is high, cells usually aggregate and cooperate between themselves. Therefore, cooperation, not competition, can be expected between microorganisms isolated from aerobically grown microbial granules.
Both competition and cooperation are carried out mainly due to the changes of chemical factors of environment such as concentration of nutrients, pH, and redox potential of the medium, excretion of antibiotics, extracellular digestive enzymes, or heavy metals binding exopolysaccha-rides, simultaneous biodegradation of substances.
Hypothetically, cooperation between microorganisms in the granules might be aimed for enhancement of the following functions:
(1) cell aggregation;
(2) formation of flexible mechanical frame of the granule by filamentous microorganisms;
(3) sequential utilization of carbon source, especially xenobiotics;
(4) formation of the intragranular storages of carbon (extracellular and intracellular storage of polysaccharides), phosphate (intracellular polyphosphate), iron, and other nutrients;
(5) formation of sheath or envelope of granule, protecting cells in the interior from toxic substances or unfavorable environmental conditions.
For example, there might be following mechanisms of commensalisms or mutualism between the microorganisms in the granule:
(1) facultative anaerobes use oxygen and create the conditions for the growth of obligate anaerobes inside granule; this interaction is important in the formation of anaerobic layer of granule grown under aerobic conditions;
(2) one strain produces a growth factor essential for another strain; this interaction is especially important in granules grown in simple medium with sole source of carbon;
(3) biodegradation of xenobiotic is performed as a sequence biochemical reactions by different microbial strains; in this case total rate of biodegradation will depend on the activities of different microbial groups in the granule.
A microbial aggregate can be considered as a multicellular organism if its parts have different coordinated or synchronized physiological functions, i.e. growth, motility, sexual interactions, assimilation of atmospheric nitrogen, production of extracellular polysaccharides, transport and distribution of nutrients, and reduction of oxygen.
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