University of British Columbia, Vancouver, Canada

A microbiologist wonders where diversity comes from.

Recent estimates indicate that the total number of bacteria in the biosphere approaches or exceeds 10 to the power 31. A major goal of microbiology is to understand what creates their diversity and how it is maintained.

Having trained as an organic chemist, I came to appreciate microbial diversity through the extravagance of small molecules that microbes produce. This reflects a diversity in microbial metabolism, which one might expect to have evolved as a result of the (organic) richness of the organisms' environments. But a couple of recent publications present findings that do not sit easily with this view.

Our first inkling of the huge diversity of the microbial world came from the use of ribosomal-RNA typing in the late 1980s. In the 1990s, this morphed into the expanding field of metagenomics, which is now providing catalogues of microbial communities from diverse terrestrial and marine environments.

One comparison of such catalogues showed that the seemingly bare and boring Arctic tundra exceeds fertile forest soils in phylogenetic content (J. D. Neufeld and W. W. Mohn Appl. Environ. Microbiol. 71, 5710–5718; 2005). A more recent study compared information from more than 100 different environments, finding that the microbial content of soils is generally less diverse than that of sediments and hypersaline environments (C. A. Lozupone and R. Knight Proc. Natl Acad. Sci. USA 104, 11436–11440; 2007).

I am looking forward to seeing what happens when the Human Microbiome Project gets under way. What variety of microbes is there to find living within us? What are they all doing? In what way will the population depend on diet? Given that we don't yet seem to understand the relationship between diversity and ecology, I am making no predictions.