Gnotobiotic growth and phosphorus limitation of Arabidopsis thaliana and co-occurring microbes on phosphated iron oxides

The macronutrient phosphorus is vital for sustaining cellular processes in all life forms. Due to its frequent adsorption on iron minerals, phosphorus bioavailability is low in many soils. While the abiotic adsorption of phosphate on iron minerals has been well studied, the direct effects of this process on bioavailability to plants and microbes has not been thoroughly investigated in a simplified laboratory system. We developed a hydroponic growth system that uses hydrous ferric oxide (HFO) to induce phosphorus limitation and can enable both plant and microbial cultivation as well as gnotobiotic co-culture. We demonstrate that this system can be used for phosphorus-limited growth of the model plant Arabidopsis thaliana as well as two root-associated bacterial isolates (from the genera Rhizobium and Pseudomonas). Elemental analysis of phosphorus and iron concentration in A. thaliana shoots reveals that the addition of increasing amounts of HFO leads to a progressive decrease in phosphorus concentration but does not affect iron quotas. We also report that phosphorus concentrations in both bacterial isolates decrease when cultivated in media supplemented with HFO. We further show that A. thaliana can be co-cultured with a Rhizobium isolate in our phosphorus-limited hydroponic system with bacteria relying on plant photosynthate as their sole carbon source. Our work provides a controlled demonstration of the effects of mineral adsorption on phosphorus bioavailability and a tool for further investigation of how plants and microbes access phosphorus in the environment.