Much recent work has focused on characterising the mutualistic elements of the plant microbiome, often aiming to identify bacterial strains that can increase plant fitness. Although most work has focused on terrestrial plants, Lemna minor, a floating aquatic angiosperm, is increasingly used as a model in host-microbe interactions. Here we assess the fitness and phenotypic consequences of the full microbiome for L. minor by assaying plants from eight natural sites, with and without their microbiomes, over a range of environmental conditions. We find that the microbiome supresses plant fitness, for all genotypes and across all environmental conditions. This decrease in fitness was accompanied by phenotypic changes, with plants forming smaller colonies and producing smaller fronds and shorter roots with the microbiome present. Although the L. minor microbiome clearly includes important symbionts, our findings suggest that we cannot discount the important pathogenic, parasitic, and competitive interactions, whose influence can override that of mutualists.

Duckweeds (Lemnaceae) are increasingly studied for their potential for phytoremediation of heavy-metal polluted water bodies. A prerequisite for metal removal, however, is the tolerance of the organism to the pollutant, e.g., the metal zinc (Zn). Duckweeds have been shown to differ in their tolerances to Zn, however, despite them most commonly co-occurring with other species, there is a lack of research concerning the effect of species interactions on Zn tolerance. Here we tested whether the presence of a second species influenced the growth rate of the three duckweed species Lemna minor, Lemna gibba, and Lemna turionifera. We used four different Zn concentrations in a replicated microcosm experiment under sterile conditions, either growing the species in isolation or in a 2-species mixture. The response to Zn differed between species, but all three species showed a high tolerance to Zn, with low levels of Zn even increasing the growth rates. The growth rates of the individual species were influenced by the identity of the competing species, but this was independent of the Zn concentration. Our results suggest that species interactions should be considered in future research with duckweeds and that several duckweed species have high tolerance to metal pollution, making them candidates for phytoremediation efforts.