Periphytic biofilm and rice rhizosphere phosphate-solubilizing bacteria and fungi: A possible use for activating occluded P in periphytic biofilms in paddy fields

Abstract

Periphytic biofilm is a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that is attached to submerged surfaces in most aquatic ecosystems. They have been shown to assimilate and store bio-available phosphorus (P) in paddy fields. However, it is necessary to make the occluded P available to the rice plant in the final stage of the plant growth. It has been reported that phosphate-solubilizing bacteria and fungi have a role in activating the occluded P in the periphytic biofilms. However, the ability of the bacteria and fungi isolated from the periphytic biofilms to solubilize sparingly soluble phosphates has not been reported. The aim of this study was to investigate the ability of bacterial and fungal isolates isolated from periphytic biofilms and rice rhizosphere soil, as an origin of bacteria and fungi present in periphytic biofilms, to dissolve different sparingly soluble phosphate sources as single and in combination. In addition, effective phosphate-solubilizing bacterial and fungal isolates and the organic acids produced by these microbes were also identified. The isolated bacterial and fungal isolates had different ability to solubilize various P sources. The order of P dissolution from different P sources by bacterial and fungal isolates was as follows: Ca3PO4 ˃ Ca5(PO4)3(OH) ˃ mixed medium (a combination of 7 different P sources) ˃ Ca5(PO4)3Cl ̴ C6H17NaO24P6 ˃ Ca5(PO4)3F ˃ AlPO4 ˃ FePO4. The type and amount of the organic acids produced by these isolates depended on the type of microbe (bacterial isolate or fungal isolate), used P source, microbial inoculation (single or consortium), and type of the microbe used in consortium (bacterium-bacterium, bacterium-fungus, and fungus-fungus). Based on this assay, it was found that the bacterial and fungal consortiums, due to diversity in the production of different organic acids, showed a better ability to dissolve different forms of P. Based on the sequence of 16S rRNA gene and ITS region, the effective rhizosphere isolates of 8BR15, 8BR17, 2FR3, and 2FR18 and the effective periphytic biofilm isolates of 6BP11, 13BP25, 8FP12, and 6FP9 showed the most similar to Burkholderia vietnamiensis, Arthrobacter luteolus, Talaromyces minioluteus and Talaromyces pinophilus, and Acinetobacter calcoaceticus, Bacillus cereus, Talaromyces stipitatus and Talaromyces minioluteus, respectively. According to these results, periphytic biofilms and rice rhizosphere soil can be as a potential repository for isolating bacteria and fungi effective in solubilizing various P source. In general, the results suggest that the effective phosphate-solubilizing microorganisms have the potential to be used as microbial inoculants to release the bio-available P assimilated and stored in periphytic biofilms for sustainable rice cultivation in paddy lands.