Unearthing ectomycorrhizal dynamics

We are in the midst of a revolution in field research of microbial ecology (Horton & Bruns, 2001). Historically, soil biologists have been hampered by difficulties inherent with working on microscopic organisms below ground. However, recent advances in molecular approaches have greatly facilitated our ability to survey microbial communities in situ. These techniques have become relatively inexpensive, and the capacity to process large numbers of samples has increased. Together, these two trends allow investigators to conduct field studies of ever-increasing scope. In this issue, Izzo et al. (pp. 619– 629) demonstrate how these new capabilities can be used to glean foundational knowledge regarding microbial community structure – their work addresses the question as to what extent ectomycorrhizal communities change over time in an otherwise stable ecosystem. The research is groundbreaking in terms of its comprehensive and simultaneous measurement of spatial and temporal variability in ectomycorrhizal community composition. Izzo et al. sampled at intervals ranging from 5 cm to 200 m at two depths over 3 yr. This combined approach was useful, because the authors found that the ectomycorrhizal community turns over frequently at smaller scales, but much less so than at larger scales. These results indicate that the pool of available ectomycorrhizal species within an ecosystem may remain more or less constant, whereas the exact location of individual species may shift over time. Had the authors focused on the contribution of either time or space, but not both, they would not have detected this pattern. The project of Izzo et al., which was initiated in 1999, required the genetic analysis of a total of 1300 ectomycorrhizal root tips in all. Current macroand microarray approaches now allow us to characterize thousands of samples at a time in runs lasting one month or less (e.g. Valinsky et al., 2002). Molecular techniques are now a tractable tool for in-depth study of mycorrhizal communities.