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 to plants 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 water environments, as well as in three different concentrations of Zn. To investigate whether 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. Finally, we measured total nitrogen and total organic carbon to describe the abiotic environment in more detail. The four Lemna ecotypes exhibited significantly different growth rates across the water treatments. This difference in fitness was matched with DNA sequencing revealing genetic differentiation between the four ecotypes. However, the effect of the water and zinc treatment 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.

Summary

The data are based on a glasshouse common garden experiment in which four Lemna minor ecotypes from Switzerland were matched to their home water and received the water from the three other ecotypes (away). Additionally, we crossed this design with the application of Zn (in the form of ZnSO₄). We used three Zn treatment levels: no Zn (control), low Zn and high Zn. Each treatment combination was replicated four times, resulting in a total of 192 experimental units (four ecotypes x four water environments x three Zn treatments x four replicates. The experiment ran for 22 days in a glasshouse.

The data submission includes two data files:

1) Data_fronds_fluoroprobe.xlsx contains the data on the number of Lemna fronds counted during the experiment, as well as the Fluoroprobe data measured at the end of the experiment. A second sheet also includes the fluoroprobe data from field samples. Metadata are included.

2) TOC_TN.xslx contains the data on the final nutrient concentration in the experiment. It also contains the nutrient data from the field samples. Metadata are included.

The data was then processed using R. R code is uploaded separately.

Methods

On 7 and 8 August, 2021, we collected four Lemna minor populations from four geographic regions of Switzerland: Koblenz: 47°36’03.3” N, 8°13’32.0” E, Yverdon: 46°47'50.8"N, 6°37'59.6"E, Motto: 46°25’43.6” N, 8°58’03.4” E, Ramosch: 46°50'01.4"N, 10°24'04.0"E.

To create the Zn treatments for each site, we mixed ZnSO₄•7H₂O (Alfa Aesar) with the filtered source water at a concentration of 3.4 mg [Zn]/L for the low treatment and 11.36 mg [Zn]/L for the high treatment.

To initiate the experiment, each cup received 100 mL of filter source water and 30 L. minor individuals. No nutrients were added to the cups. All 192 cups were spread onto four different tables, one table per replicate. The experiment ran for 22 days. Fronds were counted based on photographs taken of the cups from above using Image J (https://imagej.nih.gov/ij/index.html).

At the end of the experiment (day 22), an unfiltered 50-mL water samples from each cup was analyzed for chlorophyll-a concentration fluorometrically through a Fluoroprobe (bbe Moldaenke, Germany).

In addition, a 30-mL water sample was analyzed for its inorganic carbon, total organic carbon (TOC) and total nitrogen (TN) concentrations (Skalar Formacs HT – I TOC/TN Analyzer). Since the samples contained more inorganic carbon than organic carbon, we were not able to analyze TOC via subtraction (i.e., TOC = TC – IC). Thus, we measured non-purgeable organic carbon (NPOC). NPOC is the carbon that remains in a solution after the sample has been acidified and purged with aid of a gas flow. It is often reported as TOC since most samples contain a negligible amount of POC (NPOC = TOC – POC). Part of each sample (7 mL) was acidified with 100 μL of 10% HCl and purged for two minutes with N2 gas prior to the analyzer measurement. TN was measured simultaneously in a parallel compartment of the analyzer. 

We used two metrics to evaluate population fitness. The first metric was initial population growth rate calculated as ln(N₂/N₁)/(t₁- t₂) where N is the number of fronds, and t₁=1 and t₂= 8 represent the first and eighth days of the experiment. The second metric was total population growth rate where t₁=1 and t₂=22, which represented the first and final days of the experiment.

Data analyses were done in R. The R scripts can be found on Zenodo.