The hyphosphere effect: Do ectomycorrhizal fungi exert selective pressure on hyphosphere bacteria to increase nutrient mobilization?

Background/Question/Methods Ectomycorrhizal fungi (EMF) provide nutrients to their host trees by means of hyphal networks that extend beyond nutrient-depleted rhizosphere soil. Foraging hyphae access organic nutrients by means of extracellular enzymes and also solubilize inorganic nutrients through release of organic acids. Recent work has shown that bacteria isolated from the mycorrhizosphere (EM root tips) demonstrate a greater potential to mineralize phosphorus than the bacteria isolated from bulk soil, indicating that the EM symbiosis may exert a selective pressure on the functional diversity of nearby bacteria. To determine if the selective effect seen near EM root tips is also present in the hyphosphere, where foraging EMF hyphae are actively involved in nutrient acquisition, we incubated sand-filled mesh bags in a 60 year-old Douglas-fir / paper birch forest. Bacteria were isolated from three types of bags: root-exclusion, hyphal-exclusion, and total-exclusion. Additionally, a novel root window technique was used to visualize phosphatase-active micro-sites in soil insitu and to collect small soil samples. The fungal assemblages associated with phosphatase active soil micro-sites were compared to the assemblages found in samples taken from areas where no phosphatase activity was visualized using terminal restriction fragment length polymorphism (TRFLP).

Results/Conclusions Multi-response permutation procedure (MRPP) was used to evaluate differences between the total fungal assemblages of active and non-active micro-site soil samples from the root window technique. There was no significant difference in either total or ectomycorrhizal fungal assemblages between phosphatase-active and non-active micro-sites. Using the sand bag technique, soil fungal assemblages were not different in the root-exclusion bags from those found in the hyphae-exclusion or control (total exclusion) bags. However, the bacterial assemblages found in root-exclusion bags were different from those found in hyphae-exclusion or control bags. Root exclusion bags contained a higher proportion of actinomycetes than the other bags, suggesting an EMF structuring effect of bacteria in the hyphosphere.