Rhizobia on top of the world
Do microbes have biogeography? Or, to put it another way, does dispersal limit their geographic distribution? This has been an open question for many decades, with the null hypothesis being “no”, as captured in the memorable phrase attributed to Baas Becking: “Everything is everywhere, but the environment selects” [1,2]. The idea is that microbes move so freely in dust in the air, and so on, that any species can reach any spot on the planet. If we do not find it there, it is because it did not thrive, not because it did not arrive. Quite a few studies in recent years have challenged this hypothesis by demonstrating that microbial communities become more different with distance even when environmental differences are factored out.
A new study shows that the Rhizobium leguminosarum genotypes nodulating pea in two adjacent Himalayan valleys are different . The valleys are separated by a high mountain pass, so there is a geographic obstacle that may have kept them apart, but it is also true that the environmental conditions differ between the valleys. In Spiti valley, the samples came from fields ranging from 3685 to 4408 metres above sea level (yes, these farms really are on top of the world), while altitudes in Lahaul valley are merely 2923-3305 m. There is no clear association between rhizobial types and altitude within each valley, though. While we cannot exclude an environmental explanation, it is intriguing that the rhizobia in Spiti resemble strains previously reported only from China, while those in Lahaul are similar to globally distributed genotypes. A look at the map reveals that roads from Lahaul lead into India, whereas the road from Spiti leads into Tibet. At least, it used to, though for some time now (half a century, I imagine) it has been closed for political reasons. Nevertheless, the people of Spiti have cultural affinities with Tibetans, and my guess is that they arrived from the east with their crops and the rhizobia came with them.
It is, of course, hard to disentangle the distribution of rhizobia from the effects of agriculture, and there have been previous suggestions that rhizobia have been transported unconsciously as humans have taken agricultural crops around the world [for example, 4].
Incidently, I am in the middle of reading a fascinating book  about the history of world trade from the fifteenth century, which describes how Europeans used South American silver and African slaves to buy Chinese silk and porcelain and create a global market for tobacco. This was part of the Colombian Exchange, which spread crops, animals, pests and diseases around the world and changed the biogeography of the planet for ever. In the seventeenth century, the single European currency (the peso) nearly collapsed, while the Chinese built up great reserves of silver. Spanish merchants complained that the Chinese were flooding the European market with textiles that sold at half the price of local products, and there were even shipments of Chinese-made statues of Jesus. Welcome to the modern world!
PS: Praveen Rahi, the first author of , is just finishing his PhD, so I guess he might be looking for a postdoc position. You can see from the paper that he is an excellent molecular ecologist, so get your job offers in quickly!
 Baas Becking LGM (1934) Geobiologie of Inleiding Tot de Milieukunde Van Stockum & Zoon, The Hague.
 De Wit R, Bouvier T (2006) ‘Everything is everywhere, but, the environment selects’; what did Baas Becking and Beijerinck really say? Environmental Microbiology 8, 755-758.
 Rahi P, Kapoor R, Young JPW, Gulati A (2012) A genetic discontinuity in root-nodulating bacteria of cultivated pea in the Indian trans-Himalayas. Molecular Ecology 21, 145-159.
 Alvarez-Martinez ER, Valverde A, Ramirez-Bahena MH et al. (2009) The analysis of core and symbiotic genes of rhizobia nodulating Vicia from different continents reveals their common phylogenetic origin and suggests the distribution of Rhizobium leguminosarum strains together with Vicia seeds. Archives of Microbiology 191, 659–668.
 Mann CC (2011) 1493: Uncovering the New World Columbus Created. Knopf. ISBN-10: 0307265722.