Macroecology of Microbes – Biogeography of the Glomeromycota

Arbuscular mycorrhizal (AM) fungi are among the most abundant soil microorganisms, associating with 95% of plant families and occurring on all continents of the globe (Smith and Read 1997; Trappe 1987; Read 1991). All AM fungi are members of the newly created phylum Glomeromycota (Schüßler 2001). They inhabit most latitudes and terrestrial ecosystems worldwide, including both natural and human impacted systems. Despite their prevalence in the environment and importance to plant productivity, much remains unknown about patterns of diversity and the biogeography of Glomeromycotan fungi. Biogeography is defined as the study of the geographic distributions of organisms and the mechanisms that drive these distributions. Traditionally, AM fungal diversity was thought to be locally high and globally low; up to 20 species can associate with an individual plant, but less than 250 species have been described worldwide (Morton et al. 1995; Bever et al. 2001). Furthermore, international germ collections have been established in North America and Europe where researchers from around the world can send soil samples to be cultured and archived. According to these collections, many communities from around the globe appear similar, with the same morphospecies such as Glomus intraradices seeming to occur globally (Morton and Bentivenga 1994). Over the years, the number of morphospecies in international germ collections has remained low while the number of accessions has increased, indicating low global biodiversity for AM fungi. Furthermore, many taxonomic species such as Glomus intraradices and Glomus mosseae have been observed in a variety of geographic locations in drastically different environmental conditions. Together, these observations have contributed to the notion that AM fungal species have global distributions. However, critics claim that much of the biogeographical inferences currently made about AM fungi are based on information gained from biased sampling and variable methods (Fitter 2005; Johnson and Wedin 1997). Indeed, as the number of scientists working with AM fungi increases and novel regions and ecosystems are sampled, new AM fungal taxa as well as novel morphological traits have been discovered (Bever et al. 2001; Kramadibrata et al. 2000). In addition, methods used to determine AM fungal diversity and species composition are shifting from morphological to DNA-based. New techniques, new species concepts, and collaborative research efforts have invigorated studies of AM fungal biogeography. Joining conceptual frameworks and quantitative models with empirical studies will greatly advance our knowledge of Glomeromycotan biogeography.