The existence of cyanobactericidal bacteria and growth-inhibiting bacteria on water plants in Lake Ohnuma, Japan

Microorganisms such as bacteria are considered to be control agents against toxic cyanobacterial blooms such as Microcystis aeruginosa. We investigated cyanobactericidal bacteria and growth-inhibiting bacteria against M. aeruginosa in the biofilm on the surface of the water plants Trapa japonica, Myriophyllum verticallatum and Utricularia vulgaris at three sites in Ohnuma Quasi-National Park during the period from July to October 2012. Bacteria were isolated using nutrient agar plates, and killing abilities and growth-inhibiting abilities of isolated bacteria were examined for M. aeruginosa with co-culture experiments. Effective bacteria (sums of cyanobactericidal bacteria and growth-inhibiting bacteria) were confirmed from the leaves and water roots of T. japonica with densities of 3.6 × 10 ⁵  CFU g ⁻¹ wet weight—1.2 × 10 ⁷  CFU g ⁻¹ wet weight and 1.5 × 10 ⁶  CFU g ⁻¹ wet weight—1.4 × 10 ⁸  CFU g ⁻¹ wet weight, respectively. M. verticillatum and U. vulgaris harbored effective bacteria with densities of 2.5 × 10 ⁶  CFU g ⁻¹ wet weight—1.1 × 10 ⁷  CFU g ⁻¹ wet weight and 2.3 × 10 ⁶  CFU g ⁻¹ wet weight—9.2 × 10 ⁶  CFU g ⁻¹ wet weight, respectively. Effective bacteria were also detected from water samples collected from all three sites and the most numerous values were detected at Sansui with abundant water plants such as T. japonica and M. verticillatum. Densities of M. aeruginosa tended to be fewer at Sansui and were more abundant at Ohnuma Park with almost no water plants. The results of PCA suggested that the absence and/or lower densities of M. aeruginosa was closely related to the abundant presence of effective bacteria detected from the water and biofilms of water plants such as T. japonica at Sansui. The present findings provide new insights on the ecology of cyanobactericidal bacteria and growth-inhibiting bacteria, and suggest that water plants provide an environment that influences the abundance of cyanobacterial blooms in freshwater ecosystems.