Strain variability in growth and copper tolerance based on
mono-clonal observations
Copper, nickel and lead concentrations were elevated in the sediment
deposited at the mining inlet during the past century (see Supplemental
information, and Table S2). Across both populations, the copper
tolerance (EC50) of mono-clonal strains was close to eight µM copper,
and the means did not differ significantly between the two populations
(Fig. 1A, Welch t -test, p = 0.4. However, the variation
was larger in the mining inlet population (F -test, p =
0.04). This was driven by three very tolerant strains (VG1-2_81,
VG1-2_89, and VG1-2_105) with EC50 above nine µM copper, and
VG1-2_67, which was a particularly sensitive strain (Fig. 1A). The
comparatively low variability in copper tolerance in the reference inlet
population made it difficult to distinguish between the strains’ acute
72-hrs dose-responses to copper (Fig. 1B), while amongst the mining
inlet strains, more copper tolerance differences could be clearly
resolved (Fig. 1C).
Each strain’s maximum growth rate was used as a proxy for fitness in a
non-toxic environment (Fig S2A). Several strains in both populations
experienced sudden cessation, or even periods of negative growth, during
the initial growth cycle (Fig. S2B and C). Overall strains from the
mining inlet grew faster than the reference (average 1.5±0.28 versus
1.3±0.24 day-1, Welch t -test, p = 0.02).
Unlike the EC50, which followed a Gaussian distribution (Fig. 2A), the
growth rates leveled at an apparent maximum value for each population
(Fig. S2A). In the mining site population, three strains stood out with
rapid and statistically indistinguishable growth rates around 1.9
day-1 (VG1-2_74, VG1-2_78, and VG1-2_103), while
the growth rate of more than half (15) of the fastest growing strains
from the reference inlet ranged between 1.4-1.6 day-1(Fig. S2A).