High tolerance to zinc but no evidence for local adaptation in the aquatic plant Lemna minor

Sofia Vamos, Cheng Li, Aboubakr Moradi, Sofia Julia van Moorsel 

Duckweeds are a widely distributed and economically important aquatic plant family that have high potential for phytoremediation of polluted water bodies. We collected four ecotypes of the common duckweed (Lemna minor) from the four corners of Switzerland and assessed how their home vs. away environments influenced their growth. Additionally, we investigated their response to a metal pollutant (Zn) in both their home and away environments. Zn is found in freshwater systems and can become harmful at elevated concentrations. We hypothesized that growing in their home environment would help the plants buffer the negative effect of the metal pollutant. To test this, we measured Lemna growth in a common garden experiment in a glasshouse where the four ecotypes were grown in each of the environments, as well as in three different concentrations of Zn. To investigate whether facilitative or competitive interactions between Lemna and their microbial community can enhance or reduce tolerance to heavy metal pollution, we sampled chlorophyll-a as a proxy for algal biomass and measured total nitrogen and total organic carbon.
The four Lemna ecotypes exhibited significantly different growth rates across environments. This difference in fitness was matched with DNA sequencing revealing genetic differentiation between the four ecotypes. However, the effect of the environment on Lemna growth was the same for all ecotypes. We did not find evidence for local adaptation; instead, we observed strong plastic responses. Lemna growth rates were higher under higher Zn concentrations. This positive effect of Zn on Lemna growth could be in part due to reduced competition with algae. We conclude that L. minor ecotypes may exhibit large differences in growth rate, but that the species overall have a high Zn tolerance and strong plastic adaptive potential in novel environments.