Environmental Engineering Reference
In-Depth Information
A
0.8
0.7
0.6
Species 1
Species 2
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
100
120
SiO
4
2-
(
μ
g L
-1
)
0.7
B
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
100
120
PO
4
3-
(
g L
-1
)
μ
FIGURE 18.12
Competition of two species for silicon (A) and phosphorus (B). Species 1
has a relatively higher affinity for low concentrations of silicate but is outcompeted at higher
concentrations. The situation is reversed at higher concentrations.
communities (Tilman
et al.,
1982; Kilham
et al.,
1996), but care must be
used when extrapolating from results of laboratory experiments to the ef-
fect of nutrient ratios on natural populations (Sommer, 1999).
Interference competition is more difficult to establish for microbes. In-
terference competition can occur through production of toxic compounds
that inhibit a potential competitor. Organic compounds that alter growth
rates of other organisms are called
allelochemicals
and they are produced
in planktonic (Keating, 1977; 1978) and benthic communities. Chemically
mediated interactions (allelopathy) can drive successional sequences in
phytoplankton (Rice, 1984). Given evolutionary and ecological constraints,
such chemicals would be excreted expressly to act as allelochemicals only
under certain conditions. The benefits to the organism must exceed the cost
of synthesizing the chemicals. In phytoplankton communities allelochemi-
cals that are synthesized specifically to lower competition are excreted only
when cell densities are sufficient to bring total concentration in the water
up to effective levels (Lewis, 1986). Such may be the case during algal
blooms. However, some chemicals are released into the water for other rea-
sons (e.g., cell lysis and excretion of exoenzymes) and such chemicals may
serve as environmental cues for planktonic species, causing them to in-
crease or decrease growth rates (Keating, 1977; 1978).
In biofilms, in which cells are always in proximity to the same com-
peting cells and molecular diffusion dominates (e.g., diffusion is relatively
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