| Summary: | Duckweeds are small, fast growing, and starch- and protein-rich aquatic plants expected to be a next generation energy crop and an excellent biomaterial for phytoremediation. Despite such an importance, very little is known about duckweed–microbe interactions that would be a key biological factor for efficient industrial utilization of duckweeds. Here we first report the duckweed growth promoting ability of bacterial strains belonging to the phylum <i>Acidobacteria</i>, the members of which are known to inhabit soils and terrestrial plants, but their ecological roles and plant–microbe interactions remain largely unclear. Two novel <i>Acidobacteria</i> strains, F-183 and TBR-22, were successfully isolated from wild duckweeds and phylogenetically affiliated with subdivision 3 and 6 of the phylum, respectively, based on 16S rRNA gene sequence analysis. In the co-culture experiments with aseptic host plants, the F-183 and TBR-22 strains visibly enhanced growth (frond number) of six duckweed species (subfamily <i>Lemnoideae</i>) up to 1.8–5.1 times and 1.6–3.9 times, respectively, compared with uninoculated controls. Intriguingly, both strains also increased the chlorophyll content of the duckweed (<i>Lemna aequinoctialis</i>) up to 2.4–2.5 times. Under SEM observation, the F-183 and TBR-22 strains were epiphytic and attached to the surface of duckweed. Taken together, our findings suggest that indigenous plant associated <i>Acidobacteria</i> contribute to a healthy growth of their host aquatic plants.
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