Culturing duckweed for food web research and educational outreach

Background/Question/Methods: Duckweed is a group of small aquatic plants from the family Lemnaceae ubiquitous in still water ecosystems. Duckweed's rapid clonal reproduction has the potential for easy propagation in the laboratory. It has already been exploited to test water quality in ecotoxicology, and, with its multiple species and palatability to insect herbivores, duckweed may also be an effective research and educational tool for the study of population and food web dynamics. To explore this ecological application, we collected three duckweed species (Lemna minor, Spirodela polyrrhiza, and Landoltia punctata) and two insect herbivores, the water lily aphid (Rhopalosiphum nymphaeae) and a leaf-mining fly (Lemnaphila scotlandae) to maintain as laboratory cultures. Initially, the use of duckweed in long-running research experiments was hindered by the high levels of algal and bacterial contamination, and educational outreach was inhibited by the logistics of transporting large quantities of growth media from the lab. In order to enhance the use of this system, we designed experiments with two main objectives: 1) to develop a methodology for creating and maintaining axenic duckweed and insect populations and 2) to formulate a duckweed growth medium from easily available household chemicals and tap water. Results/Conclusions: We initially removed contamination from duckweed cultures by bleaching fronds for enough time to kill naturally co-occurring bacteria and algae cells while sparing enough duckweed cells to allow regeneration. This method was successful at eliminating algae, but bacteria were later contaminating the cultures. In the parthenogenetically reproducing water lily aphid, we created algae-free populations by intercepting juvenile aphids from their mother before they contacted contaminated duckweed fronds or media. Bacterial growth in all cultures was difficult to eliminate completely, so we sought to limit its growth by introducing low levels of cupric sulfate. We found that the duckweed was nearly as sensitive to copper as bacteria, so this method caused too much stress on the plants to be successful. To fulfill our second objective of creating classroom-friendly growth media, we added varying concentrations of miracle grow to tap water to find the solution at which duckweed growth rates were highest. We will use our optimal solution for an educational outreach at a local public school to ground test its effectiveness.