Fierer and Jackson 2006

Fierer N, Jackson RB. 2006. The diversity and biogeography of soil bacterial communities. Proceedings of the National Academy of Sciences of the U.S.A. 103: 626-631.

These authors measured biodiversity of soil bacterial communities from 98 samples across North and South America. Rather than the expected variables that structure macroorganism biodiversity such as climate and latitude, the single most important factor for bacterial community diversity was soil pH. Soils with higher pH had greater biodiversity, regardless of geographic separation between sites or geographic position. Because soils with pH greater than 8.5 are rare, it is impossible to distinguish between a unimodal distribution, and a plateau at near-neutral pH.

Most surprising to me was the lack of correlation between bacterial diversity, as either species richness or Shannon-index diversity (richness and evenness) and plant diversity. While these authors did not assess plant diversity, they did express their own surprise that sites in the Peruvian Amazon, with acidic soils, had very low bacterial diversity despite having one of the highest measured plant biodiversities on Earth, and expanded their soil collecting sites to include other tropical and sub-tropical forests with very high plant diversity. Sites with near-neutral pH provided the highest estimates of bacterial diversity, and were primarily dry-grassland, dry-forest, and humid temperate forest sites, with low-to-intermediate plant diversity.

The method used to estimate diversity of bacterial communities was terminal-restriction fragment length polymorphism analysis, or T-RFLP. This technique relies on PCR to amplify the sequence of interest, in this case 16S rDNA, followed by digestion with restriction endonucleases and generation of “fingerprints” for each community. This is a fairly low-resolution method, with less than 100 bands per community, and no ability to distinguish between species with similar restriction sites. However, it does permit high throughput and costs much less per sample than does cloning and sequencing.

This paper is an excellent example of a recent study supporting the hypothesis that microbial global biodiversity is controlled by factors quite distinct from the factors controlling macroorganism (plant and animal) biodiversity. In this case, soil pH is the master control variable, rather than climate or climate factors.