Addition of fructose to the maize hyphosphere increases phosphatase activity by changing bacterial community structure

Abstract

Organic forms of phosphorus (P) tend to accumulate in soils with long-term fertilization, however they are generally not easily available to crops. Many crop plants are obligately arbuscular mycorrhizal (AM) and AM fungi are known for not being able to produce phosphatases, the enzyme involved in the mineralization of organic P. AM fungi are able to recruit bacteria in the hyphosphere which can produce phosphatases and access organic P. It is thought that fructose produced by AM fungi acts as a signal to select these bacteria. We designed a system to test whether fructose could stimulate phosphatase activity in the maize hyphosphere. The concentration of fructose produced by AM fungi and the large concentration of fructose equivalent to the entire concentration of hyphal exudate carbon, were added. Small concentrations of fructose did not stimulate phosphatase activity in soil, cause shifts in bacterial community structure or select phosphatase producing species. However, at large concentrations of fructose, phosphatase activity was increased and this was associated with a larger bacterial population and a shift in bacterial community structure towards species which specialized in using fructose and glucose for energy metabolism, such as Saccharibacteria. Nevertheless, there was no specific shift towards species which harbored alkaline phosphatase genes and it is suggested that the change in phosphatase activity was due to subtle shifts in the properties of the phosphatases being produced. These results suggest that fructose was unable to act as a signal to select phosphatase producing bacteria in soil, but could act as an energy source to stimulate phosphatase activity by shifting soil bacterial community structure.