Introduction
Duckweeds are small, floating aquatic plants with a simplistic morphology. Individual duckweeds consist of single fronds with zero to multiple roots attached to the bottom surface. Duckweeds can flower; however, they rarely do so (Hicks, 1932). Their main reproduction strategy consists of asexual budding, where several daughter fronds bud and then detach from the mother frond (Laird & Barks, 2018). An individual frond can produce up to a couple dozen of daughter fronds over its life, which is only a few weeks short. Duckweeds are found around the world in slow flowing freshwater systems, when suitable anchoring possibilities are present (Landolt, 1986). Their fast reproduction cycle and their short life span make duckweeds useful model organisms for research in ecology and evolution (Laird & Barks, 2018). Duckweeds can tolerate high levels of nitrogen, phosphorus and heavy metals and different species can have different responses to temperature, light, nutrients and toxicants (Landolt, 1996). In nature, duckweed species frequently co-exist (Landolt, 1986).
Zn is an essential trace element for plant growth, but elevated concentrations inhibit growth and can lead to chlorosis. Therefore, elevated Zn levels are phytotoxic (Rout & Das, 2009). Zn is a commonly used building material and through run-off from roofs, galvanized items, and pipes it finds its way into (underground) waters, leading to Zn pollution (AWEL, 2006). In Switzerland, between 2006 and 2014, Zn concentration measurements exceeded the indicator value of 5 µg/l at 15 measuring stations, more than any other trace element measured (Bundesamt für Umwelt BAFU, 2019).
In the face of pollution of water systems, Lemnaceae are studied as potential organisms for phytoremediation (Liu et al., 2021). For example, one species of duckweed, Lemna minor L. (common duckweed), has shown to be a good accumulator of heavy metals such as Cadmium, Selenium and Copper (Zayed et al., 1998). Several studies have shown metal accumulation in different duckweed species (Lahive, O’Callaghan, et al., 2011), which depended both on the species (Cardwell et al., 2002) and on the metal (Gaur et al., 1994). A prerequisite for metal accumulation, however, is the tolerance of a species to elevated levels of heavy metals. Duckweed species differ in their tolerance to Zn: L. minor was shown to tolerate Zn concentrations above 100 mg/L but the gibbous duckweed Lemna gibba only tolerated concentrations up to 10 mg/L (Lahive, O’ Halloran, et al., 2011). Zn tolerance of other duckweed species such as Lemna turionifera(red duckweed) was, to our knowledge, never investigated.
Additionally, there is a lack of research concerning the influence of species interaction on duckweed resistance to metal pollution. Previous studies suggest that species interactions in duckweeds can influence growth rates (Clatworthy & Harper, 1962; Gopal & Goel, 1993; Peeters et al., 2016). Here, we hypothesized that the presence of a second species in a mixed setting could increase heavy metal tolerance because of facilitation. The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from competition to facilitation as environmental stress increases (Callaway, 1995). At high Zn concentrations, if the more tolerant duckweed species accumulates Zn present in the medium, this could facilitate the persistence or even growth of the co-occurring, less heavy-metal tolerant species. However, at low levels of Zn concentration, competition for nutrients could override any facilitative mechanisms, leading to a negative effect of the presence of a second species.
To test our hypothesis, we grew three Lemnaceae species in isolation and in two-species pairings along a zinc sulfate (ZnSO4) concentration gradient (0, 0.45, 1.82, 11.35 mg/L Zn). We measured the Zn tolerance of three duckweeds speciesL. minor, Lemna gibba, and L. turionifera over 17 days in replicated microcosms under sterile and controlled conditions.