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 two-level screening, whereupon strain SP－2－C10 was selected as the most effective plant growth-promoting 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 2.1-fold 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.

The purpose of this study was to evaluate the effect of inhibiting the growth of Microcystis spp. by water purification system （coagulating sedimentation and air flotation） applied to an urban park pond where cyanobacterial blooms occurs. By using enclosures installed in an urban park pond, the water quality of test area to which the purification system applied and that of control area were compared for 2.5 years. We removed suspended solid and improved to full transparency （1 m） by operating the water treatment system for three and a half days before Microcystis blooms occurred. The following results were obtained: in control area, Microcystis blooms were occurred in the summer for three years. On the other hand, in test area, the total density of Microcystis spp. in the first summer （2 colonies/mL） was significantly low compared to that in control area （13,700 colonies/mL）. Furthermore, in test area, the total density of Microcystis spp. in the second summer （2,890 colonies/mL） was relatively low compared to that in control area （6,170 colonies/mL）. However, in test area, the total density of Microcystis spp. in the third summer （4,800 colonies/mL） was higher than that in control area （3,120 colonies/mL）.