Theoretical and practical aspects of the quantification of biodiversity among microorganisms.

The quantification of biodiversity among microorganisms has to address both theoretical and practical aspects. Species concepts are often at variance with those applied in macroorganisms, and satisfactory concepts suitable for general use in bacteria and fungi have yet to be formulated. Molecular approaches have not yet provided a universal solution to this key issue. Quantification in habitats such as soil is difficult as isolation procedures yield only a small and skewed selection of the microorganisms present. Indices of taxonomic or phylogenetic diversity have potential in the quantification of microbial diversity at a range of ranks, but the non-equivalence of ranks and representatives of the taxa detected have to be addressed. Chemical and molecular methods have immense potential in the quantification of microbial diversity in environmental samples; 16S rRNA has shown particular promise with bacteria, but as yet the fungi lack a universal probe. A greater awareness of the limitations of existing approaches and methodologies used by microbiologists is needed, but significant progress can be anticipated as new technologies are developed and become more widely adopted.