Screening and biodiversity analysis of cultivable inorganic phosphate–solubilizing bacteria in the rhizosphere of Hydrilla verticillata

As can be seen from Table 2, the pH of the culture solution of the strains with high phos- phate-solubilizing ability is lower, while the pH of the culture solution of the strains with low phosphate-solubilizing ability is higher. Fewer strains show special characteristics, indicating that these strains could secrete some acidic substances during the culture process, resulting in a decrease in pH and the dissolution of insoluble phosphorus. According to the relationship between the concentration of dissolved inorganic phosphorus and pH of 28 IPB strains, the phosphate-solubilizing ability of IPB is significantly negatively correlated with the pH of the solution (R = -0.925, N = 28, P < 0.01). The lower the pH value of the culture solution, the higher the concentration of dissolved inorganic phosphorus is generally observed. The results show that all 28 strains have phosphate-solubilizing characteristic, among which 25 strains, such as strains SWIP-1, SWIP-3 and SWIP-5, have phosphate-solubilizing efficiency of more than 20%. Among these, the strain SWIH-9 has the highest phosphate-solu- bilizing efficiency of about 71% and DIP of about 55.74 mg/L, which is significantly higher than other bacteria (P < 0.05). Seven strains, such as SWIP-1, SWIP-10, SWIV-2, SWIV-3, SWIV-12, SWIH-5 and SWIH-8, have phosphate-solubilizing efficiency between 36% and 42%, and the DIP is about 28.48–33.37 mg/L. Seven strains, such as SWIP-6, SWIP-9, SWIP- 11, SWIP-13, SWIV-7, SWIV-10 and SWIH-7, have phosphate-solubilizing efficiency between 25% and 33%, and the DIP is about 19.84–25.83 mg/L.

This research isolated 28 strains of IPB of two phyla and nine genera from Hydrilla verticil- lata. Chen et al. [24] obtained 25 strains of endophytic bacteria of two phyla and eight genera from Nymphaea tetragona in the Guan Hall Reservoir in Beijing. The two phyla were Gamma- proteobacteria and Bacilli, and the eight genera were Pseudomonas, Enterobacter, Aeromonas, Klebsiella, Pantoea, Bacillus, Paenibacillus and Lactococcus. Compared with the results of Chen et al., the diversity of IPB isolated in this research is richer. Wang Xiaodan et al. [25] isolated two strains from the rhizosphere soil of Pennisetum alopecuroides, inoculated the strains in the NBRIP liquid medium, and measured the phosphate-solubilizing ability of strains WXD 2–1 and WXD 2–2 to be 26.99 mg/L and 31.99 mg/L, respectively. Compared with the results of Wang et al., the phosphate-solubilizing ability of the strain SWIH-9 obtained in this research is 55.74 mg/L, which indicates that the IPB strains obtained in this research have higher phos- phate-solubilizing activity. Gong Ruihong et al. [26] isolated 42 strains of phosphate-solubiliz- ing bacteria from the rhizosphere soil of four dominant plant species in the desert steppe of Siziwang Banner, and isolated phosphate-solubilizing bacteria using NBRIP medium with 10 g/L Ca3(PO4)2. The concentration of dissolved inorganic phosphorus of the phosphate-solubi- lizing bacteria was 0.95–77.57 mg/L. The phosphate-solubilizing ability of IPB isolated in this research ranges from 2.19 mg/L to 55.74 mg/L. Compared with the research of Gong Ruihong

et al., some IPB strains obtained in this research have better phosphate-solubilizing activity. In addition, Man Jing et al. [27] isolated and screened 26 strains of IPB from the rhizo- sphere of Leymus chinensis in Lanxi, Xilinhot and Hulun Buir. The strains were inoculated in a medium with Ca3(PO4)2 as the sole source of phosphorus and cultured. The concentration of dissolved inorganic phosphorus of these strains was measured to be 7.08–82.71 mg/L. Com- pared with the research results of Man Jing et al., the phosphate-solubilizing activity of IPB obtained in this research is weaker. Man Jing et al. studied IPB in soil patches containing the rhizosphere of Leymus chinensis. In this research, IPB in the rhizosphere of Hydrilla verticillata in high-organic matter urban landscape water is studied, and potential new species are found, which enrich the source of phosphate-solubilizing microorganisms. In the future, these IPB strains may be more adaptable to the high-organic matter environment and play a greater role in the treatment of high-organic matter polluted water. Efficient phosphate-solubilizing strains obtained by laboratory isolation decrease in phos- phate-solubilizing efficiency after several generations of transmission [28]. In contrast, in this PLOS ONE Screening and biodiversity analysis of cultivable inorganic phosphate–solubilizing bacteria PLOS ONE | https://doi.org/10.1371/journal.pone.0297047 January 19, 2024 8 / 11