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).