Conclusions
We have shown that copper tolerance has evolved in a small subset of
strains in a local mining-exposed population of S. marinoi , and
that the resting stage population have retained these strains even
though the mining activity and pollution level has declined. Our
artificial evolution experiment highlights that selection from such
standing genetic variation in phytoplankton enables populations to
respond to environmental stress much faster than through de-novomutations. It also suggests that extensive species dynamics might be due
to evolutionarily important strains that sometimes occur in low
frequency but can be swept to high frequencies by directional selection
under specific conditions. Importantly, our experimental approach
exemplifies how strain-specific metabarcoding can be employed to track
selection and quantify strain fitness during co-cultivation, enabling
incorporation of higher amounts of strains than other approaches permit.
With careful experimental design, future strain-specific metabarcoding
experiments should be able to track selection processes in diverse
populations inhabiting dynamic and complex environments, more similar to
the challenges that phytoplankton face in their natural habitat.
Although further development and evaluations of the analytical
approaches in mixed DNA samples of unknown allele composition are
warranted, it should in theory be possible to track selection processes
in natural populations based in situ sampling of environmental
DNA from monitoring programs or targeted sampling efforts. Collectively
such approaches will yield important new insight into how the
intraspecific diversity enables phytoplankton to adapt to life in a
dynamic environment.