The microbial communities inhabiting the fronds of duckweeds have not been investigated in as much detail as those on the roots. We herein examined the microbial communities in three duckweed species using 16S rRNA amplicon sequencing and compared them to those on the roots. The microbial compositions of the fronds were distinct from those of the roots in the three species. Various types of taxonomic bacteria, including rarely cultivated phyla, Acidobacteria , Armatimonadetes , and Verrucomicrobia , were also isolated from the fronds, but at a slightly lower abundance than those from the roots. These results suggest that duckweed fronds are an alternative source for isolating rare and novel microbes, which may otherwise be recalcitrant to cultivation using conventional strategies.

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To obtain novel plant growth-promoting bacteria that inhabit duckweed fronds, 25 bacterial strains isolated from the fronds of three duckweed species were subjected to twolevel screening, whereupon strain SP-2-C10 was selected as the most effective plant growthpromoting bacterium for the common duckweed Lemna minor. As the first-ever plant growth-promoting bacterium isolated from duckweed fronds, strain SP-2-C10 was found to belong to the rarely reported and taxonomically novel genus Aquidulcibacter (Class: Alphaproteobacteria). In an aseptic medium, this strain increased the growth of L. minor by 3.6-fold that of the bacteria-free plant. It even increased the growth of L. minor by 2.1fold in a non-sterile secondary sewage effluent. Strain SP-2-C10 colonized the fronds and roots of L. minor at almost similar cell densities under sterile conditions. By contrast, under non-sterile conditions (sewage effluent), the strain colonized the fronds more strongly than it did the roots, which was likely due to its isolation source-the fronds. This result might be useful information for constructing a functional water purification and/or biomass production system using duckweeds, whose functions were enhanced by inoculating useful microbes such as PGPB.