Agriculture Reference
In-Depth Information
Trade-Offs between Power and Efficiency
Little is known about the selective pressures that shape microbial communities in
soil and, in particular, about fluctuations in environmental conditions that might
trigger the growth of distinct microbes. Ecological strategies of microbes are com-
monly described on a spectrum of alternative responses to nutrient supply. At one
extreme are oligotrophs that are most competitive when organic resources are
scarce. At the other end are copiotrophs that thrive when nutrients are suddenly
abundant. Klappenbach et al. (2000) isolated a collection of bacteria from KBS
LTER soils to develop and test a model positing that these opposing strategies
involve a trade-off between “power” in the rapid response to an influx of resources
and “efficiency” in the use of scarce resources. A copiotroph's capacity for rapid
growth in response to an influx of resources may be the selective pressure driving
the maintenance of multiple copies of rRNA operons in its genome. The cost of
maintaining this capacity for rapid response and growth would be detrimental to
efficiency in nutrient-poor environments, so oligotrophs may be under selective
pressure to minimize the number of their rRNA-encoding genes.
Considerable evidence has accumulated from KBS LTER isolates to support a
conceptual model in which the number of rRNA operons encoded by a bacterium,
which ranges from 1 to 15 (Lee et al. 2009), is indicative of where an organism
lies on a spectrum of ecological strategies between oligotrophy (few rRNA oper-
ons) and copiotrophy (many rRNA operons; Fig. 6.8) (Klappenbach et al. 2000,
(A)
Oligotrophs
Copiotrophs
1
2
4
8
16
rRNA Operon Copy Number
E
cient growth
Rapid response
(B)
promoters
terminator
rrs
rrl
rrf
Gene:
Product:
16S rRNA
23S rRNA
5S rRNA
Figure 6.8
. A) The number of rRNA operons encoded by a bacterium ranges from one to
15 and is proposed to be indicative of where an organism lies on a spectrum of ecological
strategies between oligotrophy (few rRNA operons) and copiotrophy (many rRNA operons).
B) The typical structure of an rRNA operon in bacteria.
