Characterizing and handling different kinds of AM fungal spores in the rhizosphere.

Spores are important propagules as well as the most reliable species-distinguishing traits of arbuscular mycorrhizal (AM) fungi. During surveys of AM fungal communities, spore enumeration and spore identification are frequently conducted, but generally little attention is given to the age and viability of the spores. In this study, AM fungal spores in the rhizosphere were characterized as live or dead by vital staining and by performing a germination assay. A considerable proportion of the spores in the rhizosphere were dead despite their intact appearance. Furthermore, morphological and molecular analyses of spores to determine species identity revealed that both viable spores and dead spores with contents were identified. The accurate identification of spores at different developmental stages on the basis of morphology requires considerable experience. Our findings suggest that surveys of AM fungal communities based on spore enumeration and morphological and molecular identification are likely to be inaccurate, primarily because of the large proportion of dead spores in the rhizosphere. A viability check is recommended prior to spore molecular identification, and the use of trap cultures would give more reliable morphological identification results. We show that the abundance and activity of AM fungi in the rhizosphere can be determined by calculating the density of viable spores and the density of spores that could germinate. The adoption of these methods should provide a more reliable basis for further AM fungal community analysis.