Friend or Foe? Bacterial Nitrate and Iron Reduction in the Roots and Rhizosphere of Vaccinium corymbosum and Vaccinium arboreum

The rhizosphere surrounding roots is a dynamic ecosystem where plants and microorganisms can be partners or competitors for nutrient acquisition. Microbial partners can enrich the rhizosphere, enhance plant nutrient uptake or form mutualistic associations with plants. Conversely, microbial competitors limit the amount of nutrients available for plant uptake by taking them up in the same form and concentration ranges as plants. Vaccinium corymbosum L. hybrids (VC) and V. arboreum Marsh. (VA) differ in their soil adaptation responses and their ability to take up nitrate and iron. To determine the bacterial communities’ contribution to these differences in iron and nitrate uptake, we isolated and cultured bacterial colonies from the roots and rhizosphere of hydroponically grown clonal plants. Rooted cuttings of VA and VC were grown in a hydroponic system with a complete nutrient solution for five weeks. Representative root samples and nutrient solutions were then used to inoculate nutrient agar plates. All unique colony types were subcultured for enzymatic activity analysis. Seventy-two isolates were obtained, 23 from VA roots, 21 from VC roots, 17 from VA nutrient solution and 11 from VC nutrient solution. Ferric chelate reductase (FCR) activity and nitrate reductase (NR) activity assays were adapted for measuring the activity of these enzymes in bacterial suspensions. Bacteria exhibited extremely low FCR activity. FCR activity was significantly different from zero in only 20 of the isolates (13 from VA and 7 from VC), where it ranged between 0.29 and 3.81 nmol Fe²⁺ per million cells per hour. A ferrozine gel-based visual assay for FRC activity indicated minimal color change for bacterial suspensions compared with Vaccinium roots. NR activity was low as well, ranging between 0.007 and 0.905 nmol NO₂^- per million cells per hour. Direct comparison with root NR activity was not possible. These results indicate that bacterial communities in the rhizosphere of Vaccinium spp. are unlikely to enrich the rhizosphere with plant-available Fe²⁺ and NO₂^-. Additionally, bacterial communities in the roots and rhizosphere are unlikely to be the source of the uptake differences between VA and VC.