Oxalotrophic bacterial assemblages in the ectomycorrhizosphere of forest trees and their effects on oxalate degradation and carbon fixation potential

Ectomycorrhizal (ECM) fungi can promote soil mineral weathering through acidification and complexation by secreting oxalic acid. However, the metabolism of oxalate in the ectomycorrhizosphere and identity of the bacteria involved are still poorly understood. Here, we investigated the abundance and community structure of oxalotrophic bacteria in the ectomycorrhizospheres of Pinus massoniana and Quercus serrata trees using quantitative PCR and high-throughput sequencing. The secondary minerals formed in the degradation of oxalate were analyzed using SEM, XRD and HRTEM. Our results showed that oxalotrophic bacteria are abundant in the ectomycorrhizosphere (2.60-5.03 x 10(8)/g soil) and that ECM fungi can influence oxalotrophic bacteria, resulting in communities that are compositionally distinct from those in non-mycorrhizosphere soils. The results also showed that approximately one third of these bacterial species were nitrogen-fixing, accounting for 43-60% of the total sequences of oxalotrophic bacteria. An oxalotrophic Streptomyces sp. NJ10 was isolated from the ectomycorrhizosphere of P. massoniana and shown to be able to degrade calcium oxalate and induce the formation of carbonate minerals (calcite or dolomite). This study provides novel evidence that ECM fungi can enrich specific oxalotrophic bacteria in the ectomycorrhizosphere, that degrade oxalate using the oxalate-carbonate pathway, representing a potential long-term sink for photosynthetically fixed carbon derived from the atmosphere. These findings improve our understanding of the possible ecological functioning and environmental effects of plant-fungal-bacterial interactions in forests.