Résumé
Adaptation to local resource availability depends on responses in growth
rate and nutrient acquisition. The growth rate hypothesis (GRH) suggests
that growing fast should impair competitive abilities for phosphorus and
nitrogen due to high demand for biosynthesis. However, in microorganisms,
size influences both growth and uptake rates, which may mask trade-offs
and instead generate a positive relationship between these traits (size
hypothesis, SH). Here, we evolved a gradient of maximum growth rate (μmax)
from a single bacterium ancestor to test the relationship among μmax,
competitive ability for nutrients and cell size, while controlling for
evolutionary history. We found a strong positive correlation between μmax
and competitive ability for phosphorus, associated with a trade-off
between μmax and cell size: strains selected for high μmax were smaller
and better competitors for phosphorus. Our results strongly support the
SH, while the trade-offs expected under GRH were not apparent. Beyond
plasticity, unicellular populations can respond rapidly to selection
pressure through joint evolution of their size and maximum growth rate.
Our study stresses that physiological links between these traits tightly
shape the evolution of competitive strategies.